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Discussion Starter #1
Hello guys!

I need help figuring this out before I take my motorcycle apart.

My motorcycle suddenly developed unstable idle. It's difficult to set the idle since idle is going up and down. It's not erratic. It will level out for a few seconds then flicks up then falls again. It does this in a sort of pattern and it's very annoying. Turned the idle up just to keep my MC from dying in the morning.

It's a 150cc Suzuki with a CV carb if this helps. Is the carb just dirty? Air filter is clean.
 

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Unstable idle is sometimes a result of air leaks around the intake manifold. You can check with starter fluid, spraying on the manifold where it joins the head and carb. If there is a leak, it will rev higher. You can also use WD40 spray in the same manner, but what it will do is stabilize the idle or make it drop for a few seconds while the fluid temporarily fills the gaps. If that does not help, the next thing to check is the pilot jet and circuit on the carb. Even a small amount of blockage in the pilot will make it hard to find a good idle. Probably just a dirty carb, but mentioned the air leaks because it is easy to check that first, without taking anything apart.
 

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Discussion Starter #3
Okay, so we got the motorcycle running. The carb was definitely the problem. Swapped out my Mikuni CV carb to a Keihin mechanical carb (I hate diaphragms!). A friend of mine had one lying around with the same size as the mikuni carb. We figured out most of the tuning except for one.

The motorcycle idles strong, revs are good. Problems comes when we're trying to run the motorcycle at high RPMs (about 1/2 to 3/4 throttle). The engine sputters and rate of acceleration decreases. Problem spot is around 5,000 RPM at high speed. I thought my ignition was loose, but it's not. We've been comparing spark plugs and think we're running lean. Do we increase the main jet? Does sputtering mean we're lean? Everything is clean: air filter, carb, etc. And we made sure no vacuum leaks this time.
 

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Flat spots in the middle of the revs usually means the main jet needle is lean (sputtering means lean, bogging means rich.) Try moving the needle clip down a couple notches toward the tip and give it a test. If you can't move the needle clip down, then you need to go up on the main jet size. Before messing with any clip changes, make sure your jet and needle are super clean. A glob of residue on either will cause it to run lean as well. if it runs well at top speed, usually that means your main jet is the correct size. Glad to hear you got it running and I hate diaphragm carbs too!
 
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