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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!

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  • Do you aftermarket air filter(s) or an aftermarket exhaust system? Have you removed the airbox and replaced it with individual air filters?
    – DucatiKiller
    Commented Oct 31, 2016 at 21:30
  • @DucatiKiller I haven't done any work on it and I'm not exactly sure what the previous owner did. I could look at the parts I'm sure as I get into it and determine if they are aftermarket or not though.
    – MillerMedia
    Commented Nov 1, 2016 at 1:05
  • Can you post a picture? Something like this. i362.photobucket.com/albums/oo61/thammon5101/100_0109.jpg
    – DucatiKiller
    Commented Nov 1, 2016 at 1:22

2 Answers 2

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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

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    By the way, if it never ran very well it could also just be caused by a badly adjusted carb.
    – Bart
    Commented Oct 31, 2016 at 20:33
  • Great this is exactly what I needed, just trying to figure the direction to point myself in and this did it.
    – MillerMedia
    Commented Nov 1, 2016 at 1:04
  • Question on this, what is the best thing to use for detecting air leaks? I was doing some research and saw carb cleaner, propane tank (not lit obviously), etc. (didn't come across WD40 in any of the research). Are there any risks involved with each one? It seems like some of these may be flammable but Im guessing there's no risk around the carburetors?
    – MillerMedia
    Commented Nov 2, 2016 at 6:51
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    @MillerMedia most sprays are flamable, one more than the other. The more flamable the better in this case. Spraying some on an airleak means it'll get sucked into the engine, where it'll burn, causing a rise in rpm. You then know that you have an air leak where you just sprayed. To maximize the effect, it's best to set your engine lean. It'll respond the most to the spray then. There is some risk, because you're spraying flamable stuff onto a hot engine. So to avoid any problems, try not to let any spray come on hot spots. Carbs are cool because of the fuel vaporizing in it.
    – Bart
    Commented Nov 2, 2016 at 7:25
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    @MillerMedia Set as lean as possible by opening the choke or adjusting the carb? If you can, try to adjust the carb so that the bike runs without choke. From what i see in the picture, there's a short tube from the carbs to the cylinder. The potential air leak should be in that rubber tube so it can't really be hard to find. I recommend removing it and bend it a bit to see if cracks show up. If they do it's dried out, get a new one. If not, wrap some locktite tape over the carb where the tube fits, and refit it with the hose clamp fastened tighly, to minimize chance of leakage there.
    – Bart
    Commented Nov 21, 2016 at 8:13
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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.

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