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Old 07-11-2014, 10:05 PM   #1
svejkovat OP
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Joined: Jun 2009
Oddometer: 856
Diagnosing 50cc Puch that will not idle?

What are generally the reasons why a two stroke won't idle?

I replaced the 12mm bing on this moped with a generic 16mm ebay carb similar to this...

That brass screw at 3:00 o'clock relative to the throat is the idle mix adjustment, correct? The adjustable needle in the round slide is the high speed adjustment?

The moped is unmodified except for grinding out the ports a little (only width and not affecting timing) and mounting a tuned pipe.

Replacing the stock pipe and 12mm bing on this little errand runner made a very satisfying difference. Just enough to bring out the fun without stressing the little engine's limits.

But I can't get this carb to idle. At a stop it will wind down to idle speed and slowly die. I have to keep popping the throttle. Maxing out the idle adjustment screw that raises the round slide does no good. It settles down to a higher idle, but still dies within seconds. And the idle mixture screw seems to have no effect on low idle or high rpm whatsoever. I start at 1 1/2 turns out. Slowly all the way in, slowly all the way out.......... no apparent affect. Maybe it's plugged?

I don't think there can be air leaks since I've taken apart, inspected, and reassembled the intake elbow and carb a few times now.

Otherwise, the setting of the needle, the main jet orifice, and the air cleaner produce an ideal performance curve from off idle on up to max rpm. No four stroking or burbling at any rpm and it pulls nicely up hills. Plug chops are perfect gray color.

I've played with the needle settings. It runs best in the middle of six settings. I went down (richer) two settings to see if it would idle. It was noticeably better but lasting only a few seconds longer before stalling. And this needle setting ruined the mid to high range performance inducing four stroking and spotty torque.

I was thinking of mounting the needle in my drill press and modifying it. Using a micrometer to thin (richen) the section of needle that is within the emulsion tube at idle while leaving the section beyond that (mid to max rpm) unchanged.

Any other tips? Does it sound like I might have an air leak after all? How do you check? Which is better? Starting fluid or propane? Does it make the engine rev or slow down when sprayed near a leak?

svejkovat screwed with this post 07-11-2014 at 10:12 PM
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Old 07-13-2014, 12:16 PM   #2
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Location: Clifton Springs NY
Oddometer: 902
For the cheapo carbs off ebay you want to always disassemble and clean them before using them.... I have had 2 that still had casting sand in the bowls.... its a good thing I saw more sand on the exterior that alerted me to look further...I would guess the pilot jet is clogged...
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Old 07-15-2014, 07:45 AM   #3
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Joined: Jan 2011
Location: Summer: Kemiö, Finland; Winter: North Germany
Oddometer: 872
16 mm is quite big for a 50cc engine when left in standard set up. When there is no big increase in rpm the air flow will go slower through the carb so you might look up for a different main jet. I would try a smaller one.

I have a Tunturi Pappa with an fan aircooled Puch engine, very reliable, but needs some major service now as hasn't been touched from the mid 60's to now. I have an adjustable main installed in my Bing carb which is milled uo to 14mm, worked very nice but now power gain, the engine just used more fuel.
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Old 07-15-2014, 02:04 PM   #4
Joined: Jul 2012
Oddometer: 7
Just put the 12 Bing back on or find a 14mm Bing. It will make your life way easier.

If you really want a larger carb, don't mess with that clone business. Go legit Mikuni or Dellorto.
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