Can not get 77cb 750 running properly. Please help.

[Thatperson]

Hi I just joined the forum. My 77 cb750a is not running properly. I have taken the carb apart, cleaned everything and re assembled it. The bike does start but from what I have noticed cylinder 2 is not running properly. I believe there is an issue with carb 2. I have checked the spark plugs and wiring, they're both good and I am getting spark. When I start the bike it runs great on the rite side, but the left is choppy. I sprayed some carb cleaner into carb  2 to while the bike was running and then the bike runs fine.

Any suggestions would be great. Thanks

[Lucky 1]

The number 2 carb is the carb with the accelerator pump and two check valves.

Try this:
Take the air filters off.
With the engine OFF, gas on look into the intake with a bright flashlight.
Turn the throttle a couple of times and you should see a squirt of gas from the brass spray jet sticking up .
If you do not see a squirt of gas then you need to take the carbs off again and remove the accelerator pump and clean it.
There is a SMALL O ring between the float bowl and the carb body,do not lose it.

Also notice the black fuel hoses that connect each carb to the accelerator pump.
Take a can of carb cleaner (safety glasses on) and spray through the hoses and make sure you see carb cleaner come out of the spray nozzle in the opening of EACH carb.

If the one way check valve in the bottom of the accelerator pump will not work you will have to replace it.
Do  search on accelerator pump CB750 and you will find photos and instruction about cleaning it etc.,.


ALSO....Make sure that you remove the slow jets and pull them straight out with a pulling twisting motion. Make sure you can see daylight through them.
Clean them with carb clean and a #10 electric guitar string wire.
This is very important.
When putting them in make sure you put them in straight and that you push them in all the way.

[Thatperson]

I believe I found the problem. There is no accelerator pump!
All of my float bowls are exactly the same. Maybe the person who had the bike before took it off
Should I buy a new car?

And thanks a lot for your help

[Lucky 1]

CB750 A . Car??? HUH?

[Lucky 1]

We need a photo of the carbs. End of story.

[Thatperson]

I meant I might just buy a new carburetor not car. I took
The floats off many times and when you mentioned that accelerator
Pump. I looked it up and it does not exist on my carburetor. I believe
That the person who had the bike before lost it, and just installed a regarding
Bowl on it.

Anyways I found a set of round top carburetor near by and I might
Pick it up tonight.
Will that work fine?

[Lucky 1]

No it will not work right ,you will need the carbs for the CB750A.

QUOTE:  "and just installed a regarding
Bowl on it. " regarding????????  I think you mean regular bowl

[shinyribs]

Some of the later model carbs use a smaller boot to attach the carbs to the engine. I know the keytops on the 78 models are like this. Not sute about your 77. Anyway,the roundtops will operate just fine on your engine,but you might need different carb boots to install them. Hidden costs :) Just a heads up

[Piute]

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[Thatperson]

I picked up the round top carburetor and the bike started rite
Up. I need to sync  the carbs now, but the bike is running
Rich. Any advice anyone?

[shinyribs]

You could try the idle mixture screws. 1.5 turns from lightly seated is a good starting point. But don't expect all fuel smell to disappear. That's part of the.fun of these older bikes...walking around smelling like one :)

[Hellbilly429]

Do the CB750A carbs have smaller jets? Possibly might have to rejet to smaller jets since you are running different carbs?

[Beekeeper]

I believe the DOHC 750F had carb VB42B, which had different jets (68 and 102?) and a different air cutoff.  You can use the VB42A on the F model by simply changing the jets.  I have them on mine right now and so far, can't tell any difference

[Lucky 1]

Doesn't the CB750 have some special considerations that work with the automatic trans or was that just pollution control valves?

Doesn't the CB750a have a solenoid valve,check valve and external accelerator pump valve?

[shinyribs]

Lucky, I was curious about the same thing. The roundtops he put on might not be A model carbs,though. This would be a good question for Tools-the resident 'matic guru

[Thatperson]

Thanks I am going to try that out today! Also about the jets, I am running 110 jets.
My throttle cables Are to long as well. So I need some shorter ones.