I'm stumped, perhaps someone has had this issue before

Jan 21, 2015
610
21
18
Portland, Oregon
Chinagirl engine, got sick of fiddling with cns carb which wouldn't run, swapped out for NT carb, engine will idle, but only at full choke. The moment I turn off the choke, or twist the throttle at all, the engine dies. It also seems to be idling erratically, it will rev up really high for a few seconds seemingly at random. Also it is easiest to start on the leanest carb setting. I'm stumped as to what could be causing this, and am starting to regret agreeing to look at this guy's bike, anyone got any ideas?
 

crassius

Well-Known Member
Sep 30, 2012
4,032
156
63
USA
revving at random is almost always an air leak - check intake and head for leaks, but if motor sat a long time, it could also be base or case gasket that has dried out & deteriorated (sometimes soaking case gasket in penetrating oil along the seam will refresh it, then tighten case bolts well))
 

crassius

Well-Known Member
Sep 30, 2012
4,032
156
63
USA
I check for air leaks with a spray bottle of water - try spraying bottom of motor and around base gasket to see if you get an rpm change.
 

Tyler6357

Well-Known Member
Mar 15, 2012
1,143
170
63
Santa Barbara, CA
Sure sounds like an air leak, usually you can feel any leaks around the head gasket because you can feel air shooting out around the head when it's running. Make sure the head nuts are torqued down at around 15 foot pounds. Hmmmmm, no visible oil leaks? How about the seals? See any oil in with the magno or on the clutch side?
 

Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
6,616
422
83
Los Angeles, CA.
Might be the CDI?

I had one the other day that was acting like that... (& it was new from the kit)
It sounded great at low RPMs, but it would sputter & pop at higher RPMs.

Changing the CDI fixed it.
 
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Jan 21, 2015
610
21
18
Portland, Oregon
Finally figured it out, and now feel stupid for not realizing what it was. After fixing all the air leaks, as well as a host of other issues that this bike had, and the engine performing better, but still not great, I tried enlarging the main jet because it was the only thing I hadn't done, and what do you know, after that and moving the needle clip to the second lowest setting, it started right up and ran like a charm. Every NT carb I've come across runs super rich right out of the box, even at the leanest clip setting, so it never occurred to me that the carb might be running too lean. Live and learn, I guess.