Topic: 78 CB750A

[Bodain]

Ok this one has been hashed over a few times. I've read all the threads but this one has a new twist. I've not had an old CB in a number of years. I really prefer the old two stroke street bikes, but when I had a chance to pick up a 1978 CB750A for a pitance I couldn't pass. So I have it. Ran a compression check. 160 PSI across all four, after all it only has 5000 original miles. As usual the carbs are totally gummed up do the work and mount the carbs back up. Bike starts right up and idles fine. Twist the throttle and it dies.. Hmmmm. Not even a large twist. The slightest twist instantly kills engine. Ok I can deal with that. I look to the accelerator pump. Clearly it's not doing its job. I pull that float bowl and fill it up with gas. I depress the plunger nothing. I would have expected gas to shoot out the hole. It doesn't.. Remove the rubber diaphram. It has a hole. Fine. I buy new diaphram plunger unit and install it on float bowl. Fill float bowl and press the plunger. Gas squirts two feet into the air.. This is good! Reassemble carbs and mount on bike. It starts right up and idles fine. The slightest twist of the throttle and the bike still dies. Hmmm.... I visually confirm gas is squirting from the vertical brass tube a good inch into all four carbs. Start bike again. Still the slightest twist of the throttle kills the engine. Hmmm Clearly it's not getting the gas it needs. The choke on this unit is a butterfly, so the choke on this unit does not provide additional gas from a seperate circuit. It essentially increases vacuum. With the choke close engine revs fine. Open choke and the slightest twist of throttle kills engine.  Hmmm... This just keeps getting better.
Still there are times when it works perfectly. I start bike it idles fine. It revs fine. I tested it last night 4 different times. Work as it should perfect. Next morning. Test again.. Slightest twist of throtle and it instantly dies. It's starved for gase. Close the choke ( increasing vacuum) and it revs fine. Close the choke and the slightest twist of the throttle kills the engine.

So sometime it works perfect. The next day Not. The slightest turn of throttle kills engine. Iginition is not an issure here. With choke on it always revs. Timing is not an issue here.
So I've considered rubber boot intake leak. That's not it.
I've considered carb venting to equalize air pressure inside carb. Thats not it.
I've considered float level...  I've set it high. I've set it low. It's set at 12.5 mm as it should be.
I've visually confirmed float level height buy using a tube on outside of carb.
The back side of carb has 1 vertical brass fitting. I shoots gas just fine via the accelerator pump.
The two horizontal brass fitting feed air to the main jet and the other to pilot jet. Both are clear.
I've considered fuel flow to carbs. No issue.

So one time it work perfect and engine reves as it should. Next day any twist of throttle kills the engine. The engine is starved for gas. Close the choke (increase the vacuum) and the engine always revs fine....

So at some points in time. It appears to not be able to pull gas via the main, when the throttle is opened. YEs the emulsion tube holes are clear. And other times it work perfect as it should..
Results are same with stock air box on or off.

I've done lots of carb work. This one flat has me stumped. There is some inconsistant, changing varible in this equation and I can't see it.

[salukispeed]

I might be missing something here as I read.

 Did you disassemble the carb bodies and completely clean them in a good dip or ultrasonic cleaner. You may be chasing a restricted passage but not plugged. These era bikes were built terribly lean from the factory to meet emissions.
 
 Does fuel squirt from all four carbs from the accelerator pump nozzles or only the one? It must come from all of them somewhat equally. Or it will act lean if even one is still plugged


 Have you tried to drive the bike with the choke pulled slightly to see if it will run down the road  and then drop the choke to see if it will continue without the choke. This may help you find the Area in the fuel delivery circuit that is most at fault.

Something to verify, is the ignition advance mechanism free  and working properly as it will cause a similar effect to being lean if it does not advance properly. Mine was frozen at the base setting and the bike was nearly un-ridable without some choke  to mask the lack of proper spark timing . Lean mixture is much more difficult to ignite than a rich mixture.

[Bodain]

 Ok.. So I had this quirk. Didn't really expect to find a magic answer here where a peticular thing is cause this oddity. To restate the problem. Bike starts and idles fine. The slightest turn of the throttle kills the engine. It baffled me for many weeks, and it's not the accelerator pump.

Since I posted the original message I've had those carbs off and back on probably 6 times. Reconsidering everything. The fact is today it all works fine and I don't really have an explanation of why. One thing I do know. You don't need a working accelerator pump to rev the engine from idle. Typically the accelerator pump delivers nothing with very,very small twists of throttle. It does deliver it's goods with larger twists of throttle. The mechanical action that activates the  accelerator pump action is spring dampened, so it doesn't follow throttle action of the throttle 100% in the beginning. I actually made that spring dampening tighter! as a side note.

My problem was clear. While on idle I'm on pilot jet. All is well. Twitst throttle and needle jet raises a little allowing more fuel from the main jet. Yet it appears I couldn't get the flow from the main jet or would intermittently. Sometimes it worked perfect and other not... For now it seems to work perfect always. So I rode the beast for the first time tonight.   Wow... That was weird!

[salukispeed]

Glad to hear it is working for now?? Intermitent problems are the pits and the hardest to diagnose.

[Bodain]

So the intermittent problem returns.. It's not fixed. Sometime revs. Next day it doesn't. So I ultimately found a solution that does work everytime. Toss those 78 carbs in the corner and forget them. Install a set of 75 carbs with proper rubber intakes and it's now revs perfect all the time.

[salukispeed]

Glad you found a good solution. Intermitant problems are the hardest to cure. I also have a set of 75 round tops and 75 cam to try on mine and was hoping for your opinion on yours?

Did it seem to improve the performance?
Did you run the stock aircleaner and jetting
I have the 75 intake rubbers but it looks like the air cleaner rubber/stacks are smaller also.
Any issues with the throttle cables.
Thanks

[dave the welder]

saluki  I posted in another thread?about switching to round tops.the rubber boots from the carbs to air box are to small in dia to fit the round tops.you must also use the throttle cables from the early 70s(72or3-76).I am also running115 main jets because that was what was in the carbs,needle in middle slot.basicly all stock settings.much better performance,easier to start,idles great even in gear almost 40mpg,and it will still go faster than I want to go