Argh, stupid carbs.

Bike: '92 Yamaha Jog 50cc two stroke scooter.
Carb: Simple slide carb with oil injection.

The bike idles and responds to very small throttle inputs fine. Once I crack the throttle open to take off, it dies like it's lean. It is lean. The problem is, I've checked the tank, fuel lines, filter, pump, vacuum line for the pump, carb, chased float levels around, adjusted the slide needle all over the place, and the goddamn thing still dies when I give it gas. Also, if I manage to get it revved up a few thousand RPM, it dies in about two seconds.

The only things I haven't checked yet are the manifold for any leaks (highly doubtful), and the reeds. But... it doesn't seem like a reed problem! Then again, it's been my experience that if something's up and you've checked all the usual suspects, the reeds are more than likely the culprit. If this is the case, I guess it'd be a good time to upgrade to some Boyensen reeds. ^_^

*mutter* Stupid reed valved two strokes...

 

Its not a vacuum slide is it?
ie: where the slide isnt directly attached to the throttle cable, and
instead rely on vacuum in the airbox to open the slide.


[Modified by rioninja, 12:26 AM 3/16/2003]

 

No, it's not a vacuum slide. Cable is attached directly to the slide.

 

You said you played w/ float levels... so the bowls are full and flowing when it runs lean?

 

did you try overhauling your carb?
could be that your MJ is plugged

 

One bowl feeding a monsterous 12mm carb. And yes, it's flowing.

I've had the carb apart quite a few times tonight playing with crap, recleaning, ect. The thing is spotless.


[Modified by Loco Honkey, 6:17 AM 3/16/2003]

 

then it seems like the reed valve to me

was it running fine before? or did you just get it? you might have a cylinder that is scratched some

 

New findings from todays 'tard-a-thon in the hanger...

Nope. Reeds are perfect. I inspected them today. Also, the intake manifold is immaculate- no leaks anywhere.

Well, it has 8900 miles on it, so I speculate it's run fine at one point in the past 11 years. The guy who gave it to me said he took it apart to clean the carb and never put it back together and it sat for four years. I'm assuming he had the same problems I'm having.

Got it a few weeks ago.



Surprisingly, the cylinder is perfect. A friend with an Aprilia SR50 gave me his old 50cc piston to use, and it only has 3200 miles on it. I installed that after giving the jug the 'ole ghetto hone. Even if the cylinder was scratched, it wouldn't cause a lean condition at throttle tip in. Interestingly- it only does it under load. I can rev it all night on the stand, but once I'm on it, it bogs. I'm going to try a carb kit and see what happens. I'm at the stage now where I'm going to start throwing parts at it and see what happens.


[Modified by Loco Honkey, 12:52 AM 3/17/2003]

 

parts monkey tech is never a good sign... good luck

 

Hey, it was free, so even if it costs $500 to get it going, I'm still ahead of the game. The bike is in great condition other than this problem.

 

dont tell me thats the pistons in your bike, cause that is pretty fucked up piston

if the pistons are fucked, it will bog under load (it will run rich though).
the other thing im thinking could be the wheight rollers?


[Modified by Raz1983, 1:22 AM 3/17/2003]

 

Yeah it is! And it came out of the bike. The one that's in there isn't scored at all.


Piston isn't fucked anymore. The flyweights aren't the problem; the tranny shifts normally. I inspected it last week.

 

Does the piston you just installed also have a windowed skirt?
If it doesnt, that could be your problem.

 

It does. It's exactly the same in every dimention.

 

possible crankcase leak, or basegasket leak.... but it would have to be pretty bad.