Author Topic: Similar problem to Imago's: Poor throttle-on response - bogging  (Read 9111 times)

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Offline DoctorMuffn

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http://forums.sohc4.net/index.php?topic=106555.0

I finished looking over Imago's thread and concurred that I have a similar problem:
My bike bogs excessively when the throttle is goosed from idle. It idles perfectly once warmed and there's no fouling of the plugs when checked at idle. The pilot screws have been at 2 turns out and are presently 1 3/4 turns out with no change to the symptoms. Leaving the bike standing but pinning the throttle from an idle I can cause the engine to cut off completely starving it for something (either air or fuel not sure which hence this thread). This translates to incredible hesitation at steady 1/4-1/3 throttle application such as that used for cruising on the freeway, occasional lean pops, and "hiccups" at accel on to pass/speed up. Pinned while running she seems to fire well after some sputtering, but I'm left wanting for more.

My bike:
1978CB750K with an F motor
Wiseco 836 pistons (bored and honed by Curt)
4-1 header (no muffler)
K&N pod filters (expensive).
1978 PD carbs with accelerator pump (been rebuilt, cleaned and re-cleaned; and will be cleaned again this time around when I get into them to find out what jets I have)
Webcam: Mild Performance Camshaft from Curt (I think it's the #41 grind, but I can't seem to find the literature at the moment)
KZ1000 retainers (Curt's suggestion to replace the trash I kept getting from Ken)
Great Compression last I checked:
1. 145
2. 140
3. 145
4. 145
Or at least if the compression is not great (due to a fault of my own when building/machining the head no doubt) it's generally even across the cylinders.

What should my slow speed and main jets be for stoichiometric running throughout the powerband? How do I get rid of this annoying sputtering and hesitation? What other information do you need? I just want it to run reliably. Please help.

Offline dylboss

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #1 on: May 16, 2012, 03:58:08 AM »
With those pods and amount you have the air screw out, I would say you're running way too lean.  If you want to keep running those pods I would think rejetting is in order.  Also, try turning that screw in so it is 3/4 out from fully seated, and if that makes any difference.  That'll at least tell you if it is on the lean side.
Current

- 1972 CB750 K2 - Brier Brown Metallic
- 1973 CB750 K3 - Flake Sunrise Orange
- 1974 CB750 K4 - Flake Sunrise Orange
- 1976 CB750 K6 - Candy Antares Red

Previous
- 1978 Honda CB550K
- 1972 Honda CL350 Scrambler
- 1972 Honda CB450
- 1998 Suzuki TL1000R
- 1993 Yamaha FZR 600

Offline Lumbee

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #2 on: May 16, 2012, 07:09:52 AM »
What size are your main and idle jets. Air screws are for fine tuning, but you need to be in the ball park first, and thats what the jet will do. You have very modified setup, so you are not going to find your correct settings in a manual somewhere. You are going to need to experiment with different configurations until you find the right one for your bike.

I have a 77 750F. I ran the stock setup for years. I just put on a vance and hines 4-1 this past spring and stayed with the stock air box. Mine was not back at idle and off the line, but above 3k rpm I could tell it was lean. Stock mains are 105's, and I went to 120's which I am happy with.
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Offline lucky

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #3 on: May 16, 2012, 07:59:01 AM »
 I don't think IMAGO's bike jetting is finished yet.
Last I heard he was working on other issues.


I would recommend #120 mains and #42mm idle jets for your bike.
3/4 turn out on the mixture screw.

I do not know if the F model had adjustable needles.
If it does, I would raise the needle one notch by lowering the clip after you change jets. Wait and see what the jet change will get you.

I would go with just the main jet and idle jet change to start.

If it is still not rich enough then you could raise the needle (if it is adjustable.)

That cam is not going to b much of a factor.



« Last Edit: May 16, 2012, 08:04:09 AM by lucky »

Offline lucky

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #4 on: May 16, 2012, 08:00:06 AM »
With those pods and amount you have the air screw out, I would say you're running way too lean.  If you want to keep running those pods I would think rejetting is in order.  Also, try turning that screw in so it is 3/4 out from fully seated, and if that makes any difference.  That'll at least tell you if it is on the lean side.

I think 3/4 turn out would really make it leaner.

Offline Imago

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #5 on: May 16, 2012, 08:04:54 AM »
The biggest improvement occurred after I followed Two Tired's advice and took a small drill to the "Mickey Mouse" ear holes on the accelerator pump diaphragm. One hole simply allows the accelerator rod to passthrough, but the other is critical to the operation of the pump. Night and day. Correction learned from Two Tired: One hole feeds gas from bowl to the pump, the other hole is the discharge path from the pump to squirt jets.

This was after I cleaned the PD carbs twice and ensured the two small brass blocks that hide the tiny spring and ball valves were passing air in only one direction.

My set up is stock except for a MAC 4 into 1 exhaust. 105 main, 35 slow idle jets. Mixture screws @ exactly 1.5 turns from seated.

Another improvement came after setting the points and adjusting timing.

I still plan to sync the carbs with my old manometer and adjusting each mixture screw as per the Honda manual.

Lucky's advice to me regarding larger mains may very well have application with the number of mods on your bike, although in my case they were not required... good luck!
 
« Last Edit: May 16, 2012, 08:57:29 PM by Imago »

Offline DoctorMuffn

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #6 on: May 16, 2012, 10:20:37 AM »
Thanks to all for responding so quickly. And those were exactly the responses I was looking for, Lucky and Imago. I've worked those mickey mouse holes in one of my previous cleanings, Imago, but I'll be sure to look at them again. 3/4 turn out from fully seated seems really lean when the Clymer manual recommends 1.5 for the K and 1.75 for the F; I'll double check that information though. And I'll be back later to confirm the jets I'm running (as poorly as this bike was maintained before me there's absolutely no reason to suspect they'd be different than stock K PD carbs). And I'll be back as well to ask where I can buy those replacement jets.

Thanks again, Everyone.

Offline lucky

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #7 on: May 16, 2012, 10:32:13 AM »
The biggest improvement occurred after I followed Two Tired's advice and took a small drill to the "Mickey Mouse" ear holes on the accelerator pump diaphragm. One hole simply allows the accelerator rod to passthrough, but the other is critical to the operation of the pump. Night and day.

This was after I cleaned the PD carbs twice and ensured the two small brass blocks that hide the tiny spring and ball valve were passing air in only one direction.

My set up is stock except for a MAC 4 into 1 exhaust. 105 main, 35 slow idle jets. Mixture screws @ exactly 1.5 turns from seated.

Another improvement came after setting the points and adjusting timing.

I still plan to sync the carbs with my old manometer and adjusting each mixture screw as per the Honda manual.

Lucky's advice to me regarding larger mains may very well have application with the number of mods on your bike, although in my case they were not required... good luck!

Thanks for checking in IMAGO!!
I was wanting to hear back from you.
I can ad this to my data base.
You must have the MAC 4 into 1 muffler?
That muffler is quiet and restrictive and that would certainly account for the fact that you were able to retain your stock jets.
CONGRATULATIONS!!

bollingball

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #8 on: May 16, 2012, 11:02:28 AM »
The biggest improvement occurred after I followed Two Tired's advice and took a small drill to the "Mickey Mouse" ear holes on the accelerator pump diaphragm. One hole simply allows the accelerator rod to passthrough, but the other is critical to the operation of the pump. Night and day.

This was after I cleaned the PD carbs twice and ensured the two small brass blocks that hide the tiny spring and ball valve were passing air in only one direction.

My set up is stock except for a MAC 4 into 1 exhaust. 105 main, 35 slow idle jets. Mixture screws @ exactly 1.5 turns from seated.

Another improvement came after setting the points and adjusting timing.

I still plan to sync the carbs with my old manometer and adjusting each mixture screw as per the Honda manual.

Lucky's advice to me regarding larger mains may very well have application with the number of mods on your bike, although in my case they were not required... good luck!

Imago
Same here stock jets 4 into 2 with baffles big improvement when I got the pump working and set the fast idle cam correctly. Starts instantly at cold starts. I love these PD42b carbs. The people that don't like them are usually not stock or they just don't know how to set then up. Got to be the easiest carbs to work on. IMO You may still see some improvement when you sync the carbs I sure did. Unless you got lucky on the bench I never measure the float setting. The clear tube method is a great bonus on these Pd's

Ken
« Last Edit: May 16, 2012, 02:46:44 PM by bollingball »

Offline lostmykeys

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #9 on: May 16, 2012, 12:07:32 PM »
 My bike had the same symptoms as yours for the most part.It would bog from first gear run good till about 4500 rpm's and not pull to redline from that point.
 When I cleaned my carbs for the second time I changed the three hoses between the carb bowls that feed the acc pump nozzels.I sprayed carb cleaner into the nipples on the carbs until I had a strong jet comming from the 4 nozzels.#two and three carbs will have two nipples so block one off to get more preasure to the nozzel.I changed all these hoses to clear tubing and inserted the original brass tubes back in the clear tubing.
 No bore bog and it pulls strong all the way to redline.My ams screws are set at the factory setting of 1-3/4.
 I hope this helps as your bike is not stock.

Offline lucky

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #10 on: May 16, 2012, 01:25:32 PM »
My bike had the same symptoms as yours for the most part.It would bog from first gear run good till about 4500 rpm's and not pull to redline from that point.
 When I cleaned my carbs for the second time I changed the three hoses between the carb bowls that feed the acc pump nozzels.I sprayed carb cleaner into the nipples on the carbs until I had a strong jet comming from the 4 nozzels.#two and three carbs will have two nipples so block one off to get more preasure to the nozzel.I changed all these hoses to clear tubing and inserted the original brass tubes back in the clear tubing.
 No bore bog and it pulls strong all the way to redline.My ams screws are set at the factory setting of 1-3/4.
 I hope this helps as your bike is not stock.

Original brass tubes in the clear tubing??
I have never seen that before. What do they look like???

How long are the tubes. First you said restrictors????

Sure would like to see a photo.

NEVER mind i had replaced my black hoses with clear tubing and did not see the brass restrictors.

I saved all but one of the hoses and went and retrieved the brass tubes.
It makes sense that they would increase the pressure to get the gas all the way over to the #4 carb since it is a long way from the #2 carb pump.

Two of the hoses had short tubes (two tubes per hose), And one of my hoses the restrictors were slightly longer.

Did the hose to #1 carb have the restrictors too? Were they short or long??
« Last Edit: May 16, 2012, 01:40:03 PM by lucky »

Offline lucky

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #11 on: May 16, 2012, 02:00:07 PM »
What a great discovery!!!! WOW...

Did anyone else know about these restrictors???

Offline TwoTired

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #12 on: May 16, 2012, 02:26:50 PM »
What should my slow speed and main jets be for stoichiometric running throughout the powerband?
For power/acceleration, you don't want stoichiometric, you want about 12:1.  For idle and cruise, you want stoichiometric for optimum air density, temperature and humidity.  Since all of the latter are variable, you have to compromise with carburetors.  For ideals, you'll need a closed loop injection system.

How do I get rid of this annoying sputtering and hesitation? What other information do you need?
The IMS screws need to be opened, perhaps beyond 2 turns to attempt compensation for Pods (shudder).

I don't really see the requirement to "pin the throttle" (it's not like a there is a computer on the bike to compensate for brash human behavior).  But, to get proper throttle twist response, that accelerator pump system must work very powerfully, dribbling, sputtering, drooling, just won't do.  The two holes on the pump diaphragm ears must be size matched to the mating surface holes. One hole feeds gas from bowl to the pump, the other hole is the discharge path from the pump to squirt jets.   The two check valves must open for full flow in only one direction and completely block flow in the opposite direction.  The lines between pump and squirt jet must hold fuel and not leak, maintaining full fluid retention between pump pressure cycles.  The squirt jet exit orifices must be clear and clean but never enlarged beyond factory dimensions.  Finally, the actuating arm must have correct clearance to the pump rod.

When all is proper, those squirt jets will shoot a fuel stream about eight inches from the nozzles.  Even if your pilot and IMS setting is lean, the accel pump should make the engine pick up briskly.  That is precisely what it is for.

Have you taken the actuating arm mechanism apart?  I haven't need to.

I just want it to run reliably.
Then return it to stock.    ;D  They did exactly what you are asking for when brand new.  ;D

With the frickin' pods and no back pressure exhaust, the slide needles will have to be lifted and the main jet orifice sized larger.
Certainly, the bowl fuel level must be within 1-3mm from the bowl's gasket when full.

FYI
Here is why your bike "bogs".  (<---hate that description, btw.  How the engine recovers tells you why it had "hesitation".  "Bogs" just says the operator ain't happy.)  Anyway...
Mechanical slides, when nearly closed, route all the vacuum from the motor to the pilot jet exit orifice.  This vacuum pulls fuel from the circuit and drags it into the carb throat. 
Engine says "yum". 
Suddenly snap the slides open, and all that atmospheric pressure coming into the carb throat chases the vacuum away from ALL the fuel exit ports into the carb throat.  The fuel circuits either stop delivering fuel or reduce volume to a mixture level that can't support combustion. 
The engine says "aaaawwwe".

The only way to get the exit port pressure back to effective vacuum is to have higher air velocity in the carb throat, so the venturi can provide the required pressure drop needed to suck fuel.
Can't get air velocity without the engine revving, but won't rev without fuel.  Chicken vs egg.

So, to get fuel into the engine with the slide's sudden opening, a separate pump shoots raw gas into the carb throat, without a care about actual carb throat vacuum.  With lots of air from the open slide, and fuel to mix with it, the engine says "varroom".

Cheers,
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline lucky

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #13 on: May 16, 2012, 02:41:20 PM »
Twotired....  Did you know about these brass tubes inside of the accelerator distribution hoses between the carbs???

Do you happen to know which carb gets the longer restrictor tubes??? Thanks.

bollingball

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #14 on: May 16, 2012, 03:04:56 PM »
Quote from Lucky
NEVER mind i had replaced my black hoses with clear tubing and did not see the brass restrictors.

Imago
Same here stock jets 4 into 2 with baffles big improvement when I got the pump working and set the fast idle cam correctly. Starts instantly at cold starts. I love these PD42b carbs. The people that don't like them are usually not stock or they just don't know how to set then up. Got to be the easiest carbs to work on. IMO You may still see some improvement when you sync the carbs I sure did. Unless you got lucky on the bench I never measure the float setting. The clear tube method is a great bonus on these Pd's

Lucky  this is exactly what I was talking about You have changed your bike from stock and are just now finding missing parts you didn't even know were missing. It is all in the little small details.

Ken



Offline lucky

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #15 on: May 16, 2012, 04:32:46 PM »
Quote from Lucky
NEVER mind i had replaced my black hoses with clear tubing and did not see the brass restrictors.

Imago
Same here stock jets 4 into 2 with baffles big improvement when I got the pump working and set the fast idle cam correctly. Starts instantly at cold starts. I love these PD42b carbs. The people that don't like them are usually not stock or they just don't know how to set then up. Got to be the easiest carbs to work on. IMO You may still see some improvement when you sync the carbs I sure did. Unless you got lucky on the bench I never measure the float setting. The clear tube method is a great bonus on these Pd's

Lucky  this is exactly what I was talking about You have changed your bike from stock and are just now finding missing parts you didn't even know were missing. It is all in the little small details.

Ken

I have changed many stock parts.
I have Dyna ignition.
I have Dyna coils.
I have a 4 into 2 into 1 exhaust.
I have all new hoses.
i have custom gauges.
i have alloy rims 6lbs lighter on eash one.
I have a high performance cam.
I have  Cycle -X super bars.
I have cycle -X steering head nut.
I have Cycle -X larger brighter headlight.
I have custom turn signals.

I love all that stuff.

Offline TwoTired

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #16 on: May 16, 2012, 06:01:01 PM »
Twotired....  Did you know about these brass tubes inside of the accelerator distribution hoses between the carbs???

Do you happen to know which carb gets the longer restrictor tubes??? Thanks.

I don't remember.  I'd reason it out with with physics, learning first if the internal orifice size was different among them.
No doubt they are to even the pressure delivered at each post, from the log type distribution.  Perhaps even to limit drain back.

The nearest outlets to the pump in the log would get the highest pressure, unless an orifice was used to restrict flow.

Come to think of it, I don't think I've ever taken those off the carbs.
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline DoctorMuffn

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #17 on: May 16, 2012, 07:05:08 PM »
I got your pm, Lucky, and I appreciate everyone's responses.

And your explanation helps, TwoTired. My apologies for using the word "bog;" I'll post a vid to show you what I mean and maybe you can help me describe it better. With 15,000 posts I can respect the passion you have for these vehicles to continue to counsel and consol those of us snot-nosed youths picking up bikes we know nothing about. I understand reliability is usually engineered into the vehicle from the factory (especially when it comes to Honda), and this is why I've kept my 2005 RC51 completely stock.

With my CB I've made mistakes in the care of the K engine taking it for granted and resulting in the swap to the F and since have acquired "shidy" parts resulting in several rebuilds and replacements of internals to follow; not to mention my investment went from a $600 non-runner "just needing a carb rebuild" to a $6K+ cafe project that still doesn't run right and looks like the ugly side of a barn. Mind you, I've not wanted this to be much more than a looker and 65mph commuter since the Wiseco installation at the last rebuild and I just finished a rewire since I needed this bike back on the road after being hit on my RC. <---Read snot-nosed kid with high expectations and in spite of the constant work (I put myself through school to learn to do myself by the way) maybe still not enough patience for such a legend as the CB. So thanks for your responses.

I'll post the vid and some pics of what I'm looking at tonight when I get it apart. A note, I've changed out the leaking, black (possibly vacuum) hoses for the accel pump distribution gallery with transparent, fuel appropriate hoses and don't remember any restrictors. However, some pics might be helpful in jogging my prematurely failing memory as I did most the carb work 2-3 years ago and have only since tuned the idle screw by spark plug reads and synchonized the pistons using a proper synchronizer.

Thanks again. See everyone back on the board soon.

Offline lucky

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #18 on: May 16, 2012, 08:47:47 PM »
Twotired....  Did you know about these brass tubes inside of the accelerator distribution hoses between the carbs???

Do you happen to know which carb gets the longer restrictor tubes??? Thanks.

I don't remember.  I'd reason it out with with physics, learning first if the internal orifice size was different among them.
No doubt they are to even the pressure delivered at each post, from the log type distribution.  Perhaps even to limit drain back.

The nearest outlets to the pump in the log would get the highest pressure, unless an orifice was used to restrict flow.

Come to think of it, I don't think I've ever taken those off the carbs.

I think I figured it out.
The little brass tubes were cut off in a lathe and the longer pieces were probably the end piece on a longer tube. A left over you might say.
Anyway you cannot put two long ones together between any of the carbs,won't fit, because it exceeds the distance between the carb brass inlets. That is why on the original installation one hose has a kink in it because the two long brass tubes inside the rubber hose wont fit properly. So I just put one long and one short one two of them and the third one has two short brass tubes. They just raise the pressure.

Offline DoctorMuffn

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #19 on: May 16, 2012, 11:16:06 PM »
Wouldn't those brass tubes just act like stainless steel lines in the sense that they don't allow the pressure to bulge the hoses?

And here are some pics that show my setup. Note the K&N pods, PD 42b carbs, the F motor, and the 4-1 unmuffled header. The diameter measures 2 1/2".

Offline DoctorMuffn

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #20 on: May 16, 2012, 11:18:28 PM »
See above descriptions...

Offline DoctorMuffn

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #21 on: May 16, 2012, 11:22:40 PM »
The throttle stop/idle adjustment as seen on the removed carbs from above and the kink that would normally be in the line from the petcock to the purolator automotive filter I purchase to replace the mesh screen I used to buy from Cycle Gear.

Offline DoctorMuffn

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #22 on: May 16, 2012, 11:24:40 PM »
Engine/port side of the carbs once they were removed. They look pretty clean.

Offline DoctorMuffn

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #23 on: May 16, 2012, 11:25:59 PM »
Filter side.

Offline DoctorMuffn

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #24 on: May 16, 2012, 11:29:58 PM »
The accel pump shaft and actuator lever. The actuator is spring loaded so it takes up the slack and always just rests against the pump shaft it seems. And the second pic is likely where the problem is. There's gunk in the float bowls. The one to the left has been cleaned out with brake clean and the one to the right shows the sooty rust like deposits. I wiped a finger through another float bowl to show the color and grit of the deposits.

This makes sense 'cause when I pulled the carbs with fuel in the bowls I rotated the throttle shaft to see the accel pump circuit in operation. Unfortunately I saw no fuel in the lines connecting the carbs and none squirting or even dripping/oozing from the nozzles.
« Last Edit: May 16, 2012, 11:34:33 PM by DoctorMuffn »

Offline DoctorMuffn

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #25 on: May 17, 2012, 12:06:45 AM »
I think my video is too big so I'll play around with it tomorrow to see if I can load it. My bike really behaves like Imago's except that my engine is noisier and hesitates longer and frequently when I roll on throttle aggressively the engine will cut out completely.

As for my previous post and the problems I began to notice with the gunk in the float bowls I began to use brake clean to see if the accel pump passages were patent or not. They were not. Specifically the passage from the pressure side of the diaphragm up to the circuit, hoses, and finally the nozzles. I spent some time cleaning every jet, nozzle, and orifice out then assembled and installed the carbs for a late night test ride. It ran a lot better, but I had the choke on so it didn't feel like an accurate test. When I shut the choke off after a few blocks I immediately noticed hesitation again at steady throttle and hiccups at aggressive accel on; the problem had returned or had never gone away. When I pulled into the garage I noticed the accel pump hoses connecting the carbs and feeding the nozzles were dry once again. I predict that the finer emulsifying orifice from the float bowl to the carb body of carb number 2 is once again plugged and my culprit now is the tank. I'll get a picture of the interior of the tank, but I'm certain it's shedding all kinds of detritus that formerly clogged up the mesh screens I'd get from Cycle Gear and somehow is still getting passed the fuel filter I have installed presently.

I did find out what kind of jets I have (the main jet is 115 and the slow jet is 35) even though I'll leave them alone until I can get my accel pump behaving normally by starting with a new tank. Any suggestions?

Offline lucky

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #26 on: May 17, 2012, 12:27:04 AM »
The #2 carb has an accelerator pump diaphram in the bottom of the float bowl.
There is a check ball in the bottom of that bowl.
That MUST work.

There is also a check ball in the top edge of the #2 float bowl that keeps fuel
from draining backwards down into the accelerator pump..
It must only allow gas to go up but not down.

The hoses that connects all of the carbs must be cleat and the nozzles must be clear.

Use carb spray but PROTECT YOUR EYES!!!

Offline DoctorMuffn

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #27 on: May 17, 2012, 09:46:09 AM »
Too late... I was spraying myself like crazy last night. It's a really weird feeling getting that cleaner in your eyes. I'm pulling them apart again in a moment. I'll look for the check valves... I really just think it's a cleanliness issue; that reminds me to take pics of the interior of the tank too.

Thanks for sticking with me on this, Lucky. I'll post again soon.

Offline lucky

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #28 on: May 17, 2012, 02:03:29 PM »
Facts of the case so far:


1978 CB750F with 1978 PD carbs.
4-1 header (no muffler)
K&N pod filters .
1978 PD carbs with accelerator pump (been rebuilt, cleaned and re-cleaned; and will be cleaned again this time around when I get into them to find out what jets I have)
Cam will not matter in this case.
Idle jets #35
Main jets #115


LUCKY's recommendation.

Main jets need to be larger.#120's.
The F model came stock with #105's leanest of all the CB750's
But these are 1978 PD carbs.
Idle jets #42's
Get some 1977 needles so you can adjust them easily.
Put the 1977 and 1978 needles side by side and then raise the 1977 needle
one increment over the stock 1978 position, by lowering the clip.

I would leave the 115 mains for now and just change the idle jet to a #42.
ONE CHANGE AT A TIME.

Mixture screws 3/4 turns out. Maybe 7/8 turn. ONE turn will be too much possibly.

There is another member on this form in a similar situation now and is running a 112.5 mainjet. One .020 shim on the needle, and #42 idle jet. A/F ratio meter looks good.
But he has a baffle.
« Last Edit: May 17, 2012, 02:08:32 PM by lucky »

Offline DoctorMuffn

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #29 on: May 21, 2012, 12:15:40 AM »
I found a problem with the diaphragm when I got into the carbs the second time around. The accelerator circuit diaphragm is busted. This might have happened because the one way check/emulsifying valve from the float bowl body to the carb body and hoses was clogged up. While I was in there I shot brake clean through the orifice and confirmed the function of the one way check valve leading from the bowl to the accel pump chamber; it's clean and clear.

I'm also writing to clarify some of the details you've listed above.

It's a CB750K Bike (and carbs) mated to a CB750F engine. This might be why the mains are 115 and not 105 as you stated would come stock on the F. I'm in touch with Vickie at CB750supply.com, but it seems that they don't have the pilot (slow speed) jets I want - the 40 or 42 to upgrade from the 35s I have - for my particular carbs. Furthermore, I'll check the Clymer manual, but I think for my particular carbs the turn out on the idle screws are preliminary 1 1/2 turns for the K and 1 3/4 for the F. Anyway, here are some pics of the accelerator pump. Oh and I'm doing the Kreem treatment on the tank as I write this. Thanks for hanging with me on this, Lucky.

Offline DoctorMuffn

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #30 on: May 21, 2012, 12:55:27 PM »
I've been in touch with Vickie at cb750supply.com, and it seems she might have the idle jets I need sized 35, 40, 42, 45, and 48. I was thinking about picking up three sets - 40, 42, and 45 - just to "hear" the burn and see the plug color for each to determine the best runner. Are these press fit jets for the PD42B carbs? And if so I know how to get press fit components in but for this particular application, namely carburetors, how do I get them out?

And I've been told the main jet needles on my carbs are not adjustable, Lucky. How would I utilize 1977 needles to adjust my 1978s? I can't even get the risers out without separating all the carbs, and frankly I'm not interested in ever doing that again. I did it during my first carb rebuild (being uninformed) whereupon everyone promptly told me I shouldn't have separated them and they didn't have those felt parts and soft seals I was looking for. Meh...

Thanks

Offline lucky

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #31 on: May 21, 2012, 02:07:51 PM »
You do not need to separate the carbs. Read that again.
Make only ONE CAHNGE AT A TIME!!
Do not worry about changing the needles right now.

IF you install a new jet and change something else at the same time you will not know what made the difference.
The temptation is,is if you have the carbs off you might as well change everything. WRONG!! It never works. The result is you have wasted your time because then you have to take the carbs off to take one part out. See what i mean?

To remove press in jets:
Remove the carbs from the bike.
Remove the float bowl. One at a time.
Remove the float and float needle and set them
aside to protect them from damage.

Use a very small pair of vice grips and clamp onto the lower
portion of the jet and pull and twist at the same time.
Like pulling a tooth.

When you put in the new jet put it in carefully and then using a small block of metal GENTLY tap it into position.

BTW...Never rely on technical information from a parts person.
Just my opinion. Many of them are armchair mechanics. ;)

You would get 1977 needles instead of the 1978's because the 1977's WERE adjustable.

About that accelerator pump check ball. With the top check ball in the float bowl you can just blow into it with a plastic straw from a spray can. Air should only go ONE WAY.
The check ball in the bottom of the pump plate has to let fuel pass down ward but not upwards.
The main problem with those pumps is the check ball in that bottom plate because debris and water can get in there and you have a steel ball and a brass keeper holding the spring down so there can be corrosion on the check ball causing it to not seal. Late on Honda changed it to a plastic keeper instead of brass.
The check ball in the float bowl rim usually was not a problem.
« Last Edit: May 21, 2012, 02:19:00 PM by lucky »

Offline lucky

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #32 on: May 21, 2012, 02:25:37 PM »
This is the check ball and spring in the bottom plate of the accelerator pump.
You can get the small brass plug out of the hole by CAREFULLY prying with a tiny electronics type screwdriver. Sometimes the plug is plastic.
IF the ball has junk on it you can roll it around on a piece of sand paper to clean it.
But if you look at it with a magnifying glass and it is pitted you will need to get a new complete bottom plate.

CLICK on photo to get larger image.

Offline TwoTired

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #33 on: May 21, 2012, 04:02:39 PM »
CLICK on photo to get larger image.

That's a nice picture.  I've actually never seen that check ball.  I just clean or soak the assembly in the carb cleaner, which dissolves and washes away whatever is making it stick. It's never taken longer than overnight to get it free.   I use the aerosol carb cleaner with the red tube it comes with, to prove it will flow through the passage one in one direction, and prevent any flow in the opposite direction.

...and I've worn the aerosol carb cleaner splash back to prove it!  It sure stings in the eyes.   ;)
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline lucky

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #34 on: May 21, 2012, 05:50:21 PM »
CLICK on photo to get larger image.

That's a nice picture.  I've actually never seen that check ball.  I just clean or soak the assembly in the carb cleaner, which dissolves and washes away whatever is making it stick. It's never taken longer than overnight to get it free.   I use the aerosol carb cleaner with the red tube it comes with, to prove it will flow through the passage one in one direction, and prevent any flow in the opposite direction.

...and I've worn the aerosol carb cleaner splash back to prove it!  It sure stings in the eyes.   ;)

I did not have such good fortune.
The first one I ran into I cleaned twice, and it would not work.
So the third time i took it all apart and found out the steel ball was so corroded it was not even round anymore,
The hole where the ball goes into is very deep and hard to inspect even with a LED flashlight.

You win some and you lose some.
I ended up buying a new OEM bottom plate complete-$85.00 ouch!!
I think now I have seen some for $65.00

Offline DoctorMuffn

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Re: Similar problem to Imago's: Poor throttle-on response - bogging
« Reply #35 on: May 21, 2012, 06:19:14 PM »
Excellent! Excellent! Excellent! I got it.

So I'll check on that check ball, and I'll order the diaphragm and idle jets but only install the diaphram first to see how she runs with that problem addressed. If I still get hesitation at 1/4 throttle or throttle-on I'll replace the idle jets starting with the 40s. And thanks for the walk through on replacing those, Lucky. This forum is definitely "lucky" to have you.
Sorry if that was lame or played out already.