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I have a 1972 Honda CB750 that I have been working on and have a question about running the engine with the choke off. I originally posted this question (1972 Honda CB750 Motorcycle Choke) and it solved some of my problems but now that the bike is running I'm having an issue where it never runs with the choke fully off. Here's what I've found:

• With the choke on, the bike starts up great. No problems there.

• With the choke off, I can't get it started.

• After the bike heats up a bit, I can start it and run it with the choke about halfway in and it runs fine and I'm able to ride it around indefinitely with no issue.

• Even after having it run and riding it for 30-45 minutes, if I push the choke all the way in (turning it off), the bike will run but it doesn't seem to be inputting any gas to the engine. If I rev the engine, the bike doesn't speed up and it just gradually begins to die until the bike is just rolling and not on (it shuts off). I try to give it gas and nothing really happens that I can tell.

I don't know if this has to do with the carburetors but I would guess it maybe doesn't since the bike is running fine with the choke halfway on. I will say that with the choke halfway on, it seems to feel a lot 'heavier' than with the choke all the way on (probably not the right term but it just tends to feel a bit rougher and louder with the choke halfway on as opposed to the choke all the way on).

Basically, I'm trying to figure out why the bike doesn't run ever with the choke off no matter how heated up it is.

Any ideas on where I can begin to look? Thanks!

Community
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MillerMedia
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2 Answers2

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My bet would be an air leak in you intake, this causes the mixture to go excessively lean. Consequently, it only runs decently when you choke it, and even then it may run badly because the mixture is not consistent. You can detect air leaks by spraying air leak finder or just wd-40, on places where there could be an air leak. If there's a leak, you'll hear a change in rpm and you'll also see the spray disappearing. There are several ways to fix it, a quick google should help you out. All the best

Bart
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I will tell you how i checked for rubber manifold leaks with carburetors off get a spray bottle of soapy water and with a small Shop-Vac use the blowing end and fit a hose in the carburetor side of manifold put pressure on the hose ture it on the spray the soapy water around back of manifold and if its leaking you will see soap bubbles appear around of the back of the manifold.

Butch
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