Author Topic: Head against wall- Midrange problems  (Read 4809 times)

0 Members and 2 Guests are viewing this topic.

Offline josuepdx

  • Full Member
  • *
  • Posts: 38
  • BRAAAAAAAP!
Head against wall- Midrange problems
« on: October 11, 2013, 11:20:57 PM »
Hey everyone, I've been digging around the forum a bit and have found a ton of useful information but now I need some help!
I purchased a 1973 CB750 a few months ago with 20,000 miles and have been trying really hard to get it running like a top but have been hitting my head against a wall the whole f'in time! Who ever said 4 cylinders was a good idea? :P
On to my problem, I'm getting a lot of popping/misfiring/missing from the bike. Started out missing at all speeds. Particularly on cylinders 2/3. I was getting inconsistent spark so I just sucked it up and bought the ultimate pamco ignition (after replacing my points and condenser and getting the timing nearly perfect). The reason I got the ultimate kit was so I could get new coils along with the new ignition system, and plug caps. That on top of a complete tear down of my carbs. I put all new float needles and seats in the carbs and adjusted the float heights to 26mm. Synced the carbs. I adjusted the valve tappet clearance, 1 turn out on the air fuel mixture, inspected the jet sizes (40/110) and slide needle positioning (2nd from bottom) and everything is within stock spec. I have a stock air box with a brand new air filter from 750supply.com. Stock pipes, new plugs.

After doing all of this service, I eliminated backfires/ popping that was happening at idle and it pulls strong (at least compared to my tiny 360 twin). Where my problems are is in the mid-range. The damn thing sounds like a machine gun in the mid-range when holding the throttle constant or on deceleration. The top end isn't perfect but it's definitely working well. I pulled my plugs after riding it around town for a bit and 1/3/4 are a bit sooty which is to be expected since I was idling a lot, but 2 looked normal/lean relative to everything (a bit tan). I'm fairly certain that most of my problems are coming from cylinder 2 but I don't know why or how to fix it. That was the cylinder that was giving me the most problems at idle with backfiring, but now I suspect its only doing it in the midrange now. I sprayed the carb boots/manifolds with carb cleaner and couldn't find any air leaks, I'm stumped.

Offline trueblue

  • A person who has had many interesting experiences, some of which are true, is known as an
  • Old Timer
  • ******
  • Posts: 3,124
Re: Head against wall- Midrange problems
« Reply #1 on: October 12, 2013, 04:58:08 AM »
Did you clean the emulsion tubes when you did your carbs?  The other thing to check is that your spark advancer is working properly ;D
1979 CB650Z
Nothing can be idiot proofed, the world keeps producing better idiots.
Electronic Guages for your SOHC 4

Offline pamcopete

  • Enthusiast
  • **
  • Posts: 153
  • Ride. Enjoy. Life is simple
    • CB750Ignition
Re: Head against wall- Midrange problems
« Reply #2 on: October 12, 2013, 05:02:21 AM »
Well, I don't know if this will help, but when I first got my '74 CB750 I had similar issues. When I took the carbs apart I found that all four of the main jet towers had a neat rubber sleeve over them. Not being familiar with these carbs, I assumed that was normal, but after repeated attempts to get the carbs to behave, I finally removed the sleeves from the main jet towers and this is what I found underneath:



So, the PO had cracked all four of the towers by tightening the jets too much into the towers and he "repaired" the cracked towers with sleeves. Now, you may not have this exact condition, but I think it is common for these towers to get cracked due to over tightening the jets into the towers. Check for cracks, perhaps not as huge as this one, but even a hairline crack will allow fuel bleed into the main jet tower and enrichen the mixture. Depending on where the crack is, it could also do the opposite and allow air to leak in.

I went on Ebay and found a good set of carbs by only looking at carbs that were on sale with very good pictures of the innards. I used stock jets and the stock air box and the bike has run perfectly ever since.
« Last Edit: October 12, 2013, 05:34:09 AM by pamcopete »

Offline lucky

  • Really Old Timer ...
  • *******
  • Posts: 6,717
Re: Head against wall- Midrange problems
« Reply #3 on: October 12, 2013, 05:51:13 PM »
A real shame about those carbs.
They can be repaired or at least all the parts salvaged.

Makes me sick. :'(

Offline Don R

  • My Sandcast is a
  • Really Old Timer ...
  • *******
  • Posts: 19,967
  • Saver of unloved motorcycles.
Re: Head against wall- Midrange problems
« Reply #4 on: October 12, 2013, 06:01:18 PM »
Check the factory wiring and connectors if you haven't. Have you done a leak down or compression test?
No matter how many times you paint over a shadow, it's still there.
 CEO at the no kill motorcycle shop.
 You don't need a weatherman to know which way the wind blows.

Offline TwoTired

  • Really Old Timer ...
  • *******
  • Posts: 21,805
Re: Head against wall- Midrange problems
« Reply #5 on: October 12, 2013, 07:28:57 PM »
What method did you use for carb sync?
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 crazypj

  • I'm brill, me
  • Old Timer
  • ******
  • Posts: 4,467
  • first 100,000 miles. 1977 CB550F
Re: Head against wall- Midrange problems
« Reply #6 on: October 12, 2013, 07:34:56 PM »
It sounds like your checking something wrong and timing is over advanced plus lean mixture.
Do you have air screws with holes in? (bleed holes)
I fake being smart pretty good
'you can take my word for it or argue until you find out I'm right'

Offline HondaMan

  • Someone took this pic of me before I became a
  • Really Old Timer ...
  • *******
  • Posts: 13,882
  • ...not my choice, I was nicknamed...
    • Getting 'em Back on the Road
Re: Head against wall- Midrange problems
« Reply #7 on: October 12, 2013, 09:27:31 PM »
Hey everyone, I've been digging around the forum a bit and have found a ton of useful information but now I need some help!
I purchased a 1973 CB750 a few months ago with 20,000 miles and have been trying really hard to get it running like a top but have been hitting my head against a wall the whole f'in time! Who ever said 4 cylinders was a good idea? :P
On to my problem, I'm getting a lot of popping/misfiring/missing from the bike. Started out missing at all speeds. Particularly on cylinders 2/3. I was getting inconsistent spark so I just sucked it up and bought the ultimate pamco ignition (after replacing my points and condenser and getting the timing nearly perfect). The reason I got the ultimate kit was so I could get new coils along with the new ignition system, and plug caps. That on top of a complete tear down of my carbs. I put all new float needles and seats in the carbs and adjusted the float heights to 26mm. Synced the carbs. I adjusted the valve tappet clearance, 1 turn out on the air fuel mixture, inspected the jet sizes (40/110) and slide needle positioning (2nd from bottom) and everything is within stock spec. I have a stock air box with a brand new air filter from 750supply.com. Stock pipes, new plugs.

After doing all of this service, I eliminated backfires/ popping that was happening at idle and it pulls strong (at least compared to my tiny 360 twin). Where my problems are is in the mid-range. The damn thing sounds like a machine gun in the mid-range when holding the throttle constant or on deceleration. The top end isn't perfect but it's definitely working well. I pulled my plugs after riding it around town for a bit and 1/3/4 are a bit sooty which is to be expected since I was idling a lot, but 2 looked normal/lean relative to everything (a bit tan). I'm fairly certain that most of my problems are coming from cylinder 2 but I don't know why or how to fix it. That was the cylinder that was giving me the most problems at idle with backfiring, but now I suspect its only doing it in the midrange now. I sprayed the carb boots/manifolds with carb cleaner and couldn't find any air leaks, I'm stumped.

Things to continue checking:
1. The clamps on the rubber carb mounts: are they stretched, like with the screws almost pulled through the clamps? If so, go get 8 of the 2.5" size radiator hose clamps and use them: your hoses are very hard and ARE leaking, despite what you might think. ;)
2. If you used the typical Keyster aftermarket fuel valves in the carbs, the 26mm float depth is too shallow. Try 25mm instead. While this seems counter-intuitive, it will make dark plugs happen (takes a much longer post to explain the reasons, but 'tis true...).
3. The air screws: try setting them to 7/8 turn, especially with #110 mainjets: those mainjets are mighty rich for a K3. More K3 bikes came with #105 than with #110, as the K2 proved the #110 was too rich by about serial number 2007000, and it became #105 instead. Almost all of the 750s after that came with #105 until the "F" series bikes started, but many got "improved" over the years by the owners, in pursuit of K0-like power (the K0 had #120 and #115 jets, but also had many other different things to go with them).
4. Trueblue and Pamco Pete also have it right: they are aged (mature?) carbs, so check for odd defects and corroded air passages that narrow the air holes or let unauthorized fuel slip through. The pilot jets get this malady, too.

What is your compression? Have you adjusted the valve lash? Tight valves on cylinder#4 will cause the #2 plug to look leaner than the rest of them, when all would otherwise be too rich (as one example).

4 cylinders really IS a good idea: when 1, 2, or even 3 of them quit, you can still ride it home. (Been there, done it! :D  )
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

Link to My CB750 Book: https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book

Link to website: www.SOHC4shop.com

Offline thirsty 1

  • Hot Shot
  • ***
  • Posts: 512
Re: Head against wall- Midrange problems
« Reply #8 on: October 12, 2013, 09:37:35 PM »
73' CL125, 75' CB400F, 16' KTM 1190R, 05' KTM 525EXC

75' CB400F  -  http://forums.sohc4.net/index.php?topic=127295.0

My 79 CB750F for fun   ----   http://www.dotheton.com/forum/index.php?topic=19923.0

Offline josuepdx

  • Full Member
  • *
  • Posts: 38
  • BRAAAAAAAP!
Re: Head against wall- Midrange problems
« Reply #9 on: October 13, 2013, 02:17:25 PM »
Trueblue, emulsion tubes are clear. I've pulled them multiple times and they have always looked great. Still soaked in carb cleaner and scrubbed down just in case each time. I've checked my timing multiple times with a timing light and have it just right. Like I said, I'm running the Ultimate Pamco ignition so it's kind of hard to set it incorrectly. Advancer is working fine and I checked the springs when I had it out when installing the new ignition.

Pete, I was concerned about possible carb damage but even after close scrutiny, I couldn't find a damn thing wrong with them. 

Totired, I used a sync tool like this one ->http://www.amazon.com/vacuum-carburetor-synchronizer-carb-gauge/dp/B004MSJ7E6/ref=sr_1_cc_2?s=aps&ie=UTF8&qid=1381698232&sr=1-2-catcorr&keywords=carb+sync

Information that could maybe be useful... When I synced my carbs, all of them were basically perfect relative to each other except for cylinder 2. It was way off. Adjusted the screws on top and got it dialed in and that's what fixed my idling issue. I wonder why that one would be so far off and no other ones?

CrazyPJ, Not sure what you mean by air screws with bleed holes? If you are referring to my air/fuel mixture screw, then I don't believe that they have any "holes" in them? I feel like I'm completely off on what you are trying to say, sorry.

Hondaman, that float height change sounds promising to me. I did notice that the float needle spring seemed to sit further away from the seat that the stock needles did. I suspected that it could have just been from wear, but it seems that you are suggesting that it requires the floats to be set differently? This sounds like it could likely cause some of my problems. I'll also replace my hose clamps with those plumbing clamps ASAP since it is quite possible that it is leaking just a tiny bit from there. The stock clamps look a bit tired... Your suggested air/fuel mixture sounds like a good idea as well. I did adjust my valves recently, but I'll double check cylinder 4 to see if that could be causing my problem.

You guys are all awesome! Thanks for the help. I'll check back in after I test these things out.

Offline Xnavylfr

  • Expert
  • ****
  • Posts: 1,102
  • Beers, Babes and bikes since 1965
Re: Head against wall- Midrange problems
« Reply #10 on: October 13, 2013, 02:48:37 PM »
A 45 cal. shell casing fits perfect around the jet tube!!! I have a buddy that has repaired many Round top carbs with thwm.



Xnavylfr(CHUCK)

Offline HondaMan

  • Someone took this pic of me before I became a
  • Really Old Timer ...
  • *******
  • Posts: 13,882
  • ...not my choice, I was nicknamed...
    • Getting 'em Back on the Road
Re: Head against wall- Midrange problems
« Reply #11 on: October 13, 2013, 08:25:19 PM »
A 45 cal. shell casing fits perfect around the jet tube!!! I have a buddy that has repaired many Round top carbs with thwm.



Xnavylfr(CHUCK)

I gotta remember that, when I go to the range...the ex-SEALs here opened a 1st-class, top-notch club here, and lots of ex-military folks come by to leave their .45 shells lying about!
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

Link to My CB750 Book: https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book

Link to website: www.SOHC4shop.com

Offline brandEn

  • Old Timer
  • ******
  • Posts: 3,205
Re: Head against wall- Midrange problems
« Reply #12 on: October 13, 2013, 08:38:09 PM »
HondaMan.... could you explain the Keyster float valve situation a bit more? Is it because the springs are stiffer?

Offline trueblue

  • A person who has had many interesting experiences, some of which are true, is known as an
  • Old Timer
  • ******
  • Posts: 3,124
Re: Head against wall- Midrange problems
« Reply #13 on: October 14, 2013, 02:02:40 AM »
Not sure what you mean by air screws with bleed holes? If you are referring to my air/fuel mixture screw, then I don't believe that they have any "holes" in them? I feel like I'm completely off on what you are trying to say, sorry.


Some of the primary mixture screws had a small hole in the end of them to prevent excessive richening of the idle circuit.  I have heard a few times where they have been the cause of running issues ;D.  If you don't have the holes you don't have the problem ;).  When I first synced my carbs on my 650 #3 was way off compared to the rest, dunno why.  I just figured some moron had been playing with it.
1979 CB650Z
Nothing can be idiot proofed, the world keeps producing better idiots.
Electronic Guages for your SOHC 4

Offline josuepdx

  • Full Member
  • *
  • Posts: 38
  • BRAAAAAAAP!
Re: Head against wall- Midrange problems
« Reply #14 on: October 15, 2013, 09:16:21 PM »
Uhg. So I changed the float height to 25mm and swapped out all of the hose clamps for the ones that infinitely tighten. No cigar. Still missing around 3-4k rpm for some reason. Just for the heck of it, I swapped out my spark plugs for new ones again to see if I had any luck with the new settings. NOPE

I do have other speculations as to what may be the problem, but they are merely speculations. Could a leaky boot from the air box cause these any issues? What about a charging issue? I did switch to the Ultimate Pamco electronic ignition and notice that my headlight is a bit dim at idle and brightens up with RPMs. I really don't know what to do at this point at all. UHHGG. 

Offline trueblue

  • A person who has had many interesting experiences, some of which are true, is known as an
  • Old Timer
  • ******
  • Posts: 3,124
Re: Head against wall- Midrange problems
« Reply #15 on: October 16, 2013, 01:12:07 AM »
Do a plug chop at the revs it is giving you the issues and see how they look, might tell more of the story ;D
1979 CB650Z
Nothing can be idiot proofed, the world keeps producing better idiots.
Electronic Guages for your SOHC 4

Offline josuepdx

  • Full Member
  • *
  • Posts: 38
  • BRAAAAAAAP!
Re: Head against wall- Midrange problems
« Reply #16 on: October 16, 2013, 04:54:17 PM »
Alright, did the plug chop and got similar results as before. 1,3, and 4 are on the rich side of things (a bit sooty, blackish/brown) and 2 is white. I'm assuming that there has to be an vacuum leak of some sort, but I can't find anything. I sprayed the whole thing down with carb cleaner practically and couldn't find anything. Also, if there was a vacuum leak going on then I'm sure I would have noticed when syncing the carbs. Right?
I triple checked timing for the hell of it and it's spot on. If that was an issue anyways, I would be experiencing problems on both cylinders 2/3.
I checked around the exhaust port and couldn't feel any signs of an exhaust leak.

Offline Medyo Bastos

  • Master
  • *****
  • Posts: 1,591
  • Gusto mo titi ko?
    • project
Re: Head against wall- Midrange problems
« Reply #17 on: October 16, 2013, 05:13:45 PM »
try flipping your vacuum gauges for #$%*s and giggles, hooking them up in reverse order, and see if 2 clears up and 3 takes a #$%*.  i gave up on that type gauge years ago.  you essentially have to calibrate those all to 1 carb before you start syncing your rack...

Offline TwoTired

  • Really Old Timer ...
  • *******
  • Posts: 21,805
Re: Head against wall- Midrange problems
« Reply #18 on: October 16, 2013, 05:15:08 PM »
Perhaps it is time to do a compression check?  A leak down test would give good info about engine cylinder health.  You just can't tune out problems with a leaky cylinder with carb adjustments.

I recall in my early wrench days trying to tune a carter carb 4bl to a v8 with a rounded cam lobe.  Didn't work out very well.  AND I had to do the carb adjustments over again when the new cam went in.  Time consuming lesson, that.
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 josuepdx

  • Full Member
  • *
  • Posts: 38
  • BRAAAAAAAP!
Re: Head against wall- Midrange problems
« Reply #19 on: October 16, 2013, 05:32:16 PM »
Any recommendations on a compression tester?

Offline TwoTired

  • Really Old Timer ...
  • *******
  • Posts: 21,805
Re: Head against wall- Midrange problems
« Reply #20 on: October 16, 2013, 05:36:18 PM »
Any recommendations on a compression tester?

Just about any will work with the correct spark hole adapter.  You just have to adjust your expectations when using an automotive tester.  Check the engine FAQ.  You just want to see if they are equivalent across the bank of cylinders.  You just want to know if #2 is breathing the same as the rest in this case.
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 70CB750

  • Labor omnia vincit improbus.
  • Really Old Timer ...
  • *******
  • Posts: 14,805
  • Northern Virginia
Re: Head against wall- Midrange problems
« Reply #21 on: October 17, 2013, 03:34:35 AM »
Any recommendations on a compression tester?

Car parts stores often have compression testers for rent, I usually get one from Autozone; you charge it to your card and if you return it in 90 days you get your money back.  And I have my own adapter to CB750.
Prokop
_______________
Pure Gas - find ethanol free gas station near you

I love it when parts come together.

Dorothy - my CB750
CB750K3F - The Red
Sidecar


CB900C

2006 KLR650

Offline josuepdx

  • Full Member
  • *
  • Posts: 38
  • BRAAAAAAAP!
Re: Head against wall- Midrange problems
« Reply #22 on: October 19, 2013, 11:15:46 PM »
Alright, ran my compression tests. I did the test COLD, with wide open throttle, and no air filter. I have heard some people say cold tests are fine, not sure what your opinions are about the matter but this still seems telling.
Cylinder 1: 140
Cylinder 2: 120
Cylinder 3: 130
Cylinder 4: 135

Tried doing the drip down test and I think I clogged the valve in my compression tester... Haha. Maybe I put too much oil in the cylinder? Is there a trick to doing this test?
Anyways, what do you guys think about those numbers? 120 doesn't seem THAT bad, right? Should still be high enough to be working well. My understanding is that you're fine as long as it's MOSTLY above 100. There is a considerable amount of difference between the cylinders though.

Offline TwoTired

  • Really Old Timer ...
  • *******
  • Posts: 21,805
Re: Head against wall- Midrange problems
« Reply #23 on: October 20, 2013, 10:59:54 AM »
A cold test gives a good idea of cylinder health.  A bad valve or a holed piston with show equally bad cold or hot.
Although the numbers you show are not ideal, they totally unreasonable, particularly if the bike has been sitting a good long while in disuse.  The major difference is between high and low cylinders is 20lb. 20/140 = 14%.  You want like to see under 10%, but the test was cold and there is possibly some measurement error, and as you say the cylinder hasn't been firing, so ring seat and mechanical health is understandable if not on par with the other.  I have never heard of a drip down test.  And, if you read the FAQ, you would have learned not to add more than a few drops of oil, because of volumetric issues.  You'll need a compressor and a different set of gauges to do a leak down test properly, but this test will tell you what about the cylinder is not holding peak pressure.

At any rate, the lower cylinder pressure, it not so low as to prevent it from firing.  So you are back to spark or carb issues to properly diagnose.
A spark plug that has no combustion deposits, is either not getting spark or not getting fuel. (You've now ruled out compression as a cause.)

Does the exhaust pipe get as hot as the others?
Have you tried swapping plugs with another cylinder that is known to fire?
Can your lay the #2 spark plug against the engine and see it spark between the electrodes?

Can you get the plug wet with fuel by using full choke and cranking the engine for a few compression cycles, while the kill switch is in off position?  Then pull the plug and check (look and smell) the tip for gasoline.

If you indeed have spark and the plug stays dry, you have a carb issue to correct.

 


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 josuepdx

  • Full Member
  • *
  • Posts: 38
  • BRAAAAAAAP!
Re: Head against wall- Midrange problems
« Reply #24 on: October 23, 2013, 01:54:31 PM »
Alright, I pulled my pipes to check for any obstructions in pipe 2. Seems clear but I did find some small holes and cracks in the pipe, think that could cause some of my issues? Also, I took a quick look at my exhaust valve. Do you guys think my valve is burnt? The black valve is cylinder 1, the yellowish brown one is cylinder 2 which is where my problem is. I know I'm getting spark for sure. I've done all of those tests.

I've pulled the float bowl and made sure that I was getting gas into the carb. All good. I've pulled the mains and pilot jets and they were clear. I still cleaned them anyways.