Topic: Similar problem to Imago's: Poor throttle-on response - bogging

[DoctorMuffn]

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?

[lucky]

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!!!

[DoctorMuffn]

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.

[lucky]

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.

[DoctorMuffn]

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.

[DoctorMuffn]

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

[lucky]

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.

[lucky]

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.

[TwoTired]

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.   

[lucky]

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

[DoctorMuffn]

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.