I think I've got a vacuum leak!

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Just finished putting a Grubee GT5 on a friend's mountain bike. The engine runs like dog unless you switch on the choke, just a little bit. I'm thinking i might have a vacuum leak to locate. With the choke part way on, the engine accellerates much better, even when up to operating temperature. Turn the choke off and it instantly bogs down. It's a fresh engine so it might need a little more break in. I've got two good heat cycles in it right now. Will try another one tomorrow.

Andy help is appreciated.
 
The guys were talking about spraying something around the likely culprets and seeing if the engiine rpm and tone changes.
 
Aerosol carb cleaner, brake cleaner etc. Start where the carb mounts on the inlet, then where the inlet bolts to the cylinder.
Check that the nuts are tight and visibly check the inlet gasket for apparent damage.
... Steve
 
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Before you go chasing any leaks, raise the jet needle in the slide throttle to enrich the mixture and see if that helps. (lower the little c-clip a notch or two) You can also raise the float level a couple of mm to enrich the mixture too.

Too many people start looking for vacuum leaks first, rather than last. If there was a vacuum leak that leaned out the mixture enough to cause bogging, it would be large enough to severely upset the idle mixture - enough to cause the engine to stall or to have a high idle even if the throttle stop screw is backed out all the way.
 
Too many people start looking for vacuum leaks first, rather than last.

Where I come from, I was taught to ensure that there were no leaks, the fuel filter was not clogged and that the air filter was clean before tuning, not after. This is especially important when there is a problem.
There's no effort in, or harm done by, a quick squirt of aerosol to double-check the joints, before re-tuning.

... Steve
 
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Where I come from, I was taught to ensure that there were no leaks, the fuel filter was not clogged and that the air filter was clean before tuning, not after. This is especially important when there is a problem.
There's no effort in, or harm done by, a quick squirt of aerosol to double-check the joints, before re-tuning.

... Steve

I'm with you Steve, a spray or two is by far as the easiest check. Don't forget the carb cap where the throttle cable goes through. I'd be a little concerned as to using brake cleaner. That is what I used to remove ALL the paint from a "BLACK" painted engine, after the paint started to come off.
 
Well, you guys do things your way and I'll do things my way. Like I said before, if there was evidence of a vacuum leak (stalling, high, unadjustable idle) then of course that would be the first place I would look. But the symptoms are not consistent with an air leak, but with a carb that is in need of adjustment. Where I come from, you troubleshoot based on symptoms and not waste time with things that are low probability cures. ;)
 
Motorbike Wanabe said that he thinks he has a vacuum leak and asked for help. We merely told him how to check for it. And the choke thing is a good sign that there may be a leak.

Ron, good point. I didn't think about brake cleaner as paint remover. Mine's an unpainted engine. I forgot that some are black.

... Steve
 
Motorbike Wanabe said that he thinks he has a vacuum leak and asked for help. We merely told him how to check for it. And the choke thing is a good sign that there may be a leak......

Well, if the engine doesn't idle unless the choke is on, then yes, there may be a pretty large air leak. If the engine simply performs better with the choke slightly "on", then it is most likely a carb tuning issue.

I've seen too many people in this forum chase their tails for "air leaks" or "vacuum leaks" to find out that their issue was something totally unrelated.
 
Well, spent some time tinkering with it. I tried moving the needle, didn't benefit at all. I had some teflon tape wrapped like a rope around the outside of the threads but, as soon as we removed the top, the tape went south and we've not been able to seal it properly again. It runs better with the choke halfway closed. My understanding is that the additional fuel from choking it is compensating for the additional un-metered air that is coming through the various leaks.

What have others used to seal up these little carbs? I used a little teflon tape on my engine and it runs great with the clip on the highest groove of the needle, just sipping the fuel.

I got some O-rings and put one between the cap and the carb body and put one inside the carb, between the intake and the carb. I'm hesitant to keep one in there for fear it might get injested inside the engine. Maybe I worry too much. :confused:

Anywho, with the 0-rings in place, I can get it to run somewhat okay. Mine is running much smoother than this one but, mine is broken in pretty well and that may make the difference. Still trying to work out the bugs on this one.
 
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