1980 Yamaha QT50

I'm having problems with my spark plug fouling on my QT50.

I have checked and cleaned the carb correctly and still have a problem with gas/oil fouling plugs. Bike starts and stalls within seconds with wet film on plug. I also replaced the ignition coil and cap and have blue spark but it seems somewhat weak. I tested compression and it's about 105psi... One problem that has me worried is the oil that is comming from the exhaust.... Ugh, Crank Seal???? I drained oil to see if the amount of oil comming from exhaust would be reduced. Didn't change a thing. I removed pipe from head and attempted to start the bike and no change. Start/Stall with a wet plug... No head/base gasket leaks... I still haven't checked the reed valve and I'm also going to relpace the needle in carb. I disconnected the oil injector and plugged openings, using a 32:1 premix. Going to try a new plug to see what happens... Any ideas??? Thanks! Mark

Re: 1980 Yamaha QT50

At first sight, it sounds like a simple case of the carb delivering too much fuel .. and the plug fouls.

Things like a high float level can do it.. If the float level is adjusted properly, something like a leaking float-needle can cause that.

Another thing that can cause a high float level is a float body that doesn't "float" like it should .. It could be punctured (metal can-type float) or it might have internally absorbed fuel (solid plastic foam type) .

Some floats have a number stamped on them.. this is their weight in grams. If a float has absorbed fuel it would weigh more and would no longer float properly.


Oil out the exhaust might just be an accumulation of unburned fuel/oil that has been exhausted into the pipe. This is likely with many failed attempts to start and run with a very rich mixture.


A blue spark is a pretty good spark.. weak spark is more of a faint red or yellow.. Since the plug is so often wet-fouled that plug's poor condition might make the ignition appear weaker than it actually is.


Crankcases normally experience only about 6 PSI during operation. Seals are gonna hold up to maybe about 8 or 9 PSI before they are damaged... So if you even think about testing the crankcase for seal leaks, be very careful about how much pressure you feed into it. Use a hand pump and a accurate gauge.

IMO you're most likely trace the wet-fouling problem to carburetor trouble.

Re: 1980 Yamaha QT50

The needle seems to have been bent. I'm going to replace it and see what happens. The float needle valve has been replaced with a new one. As far as the float I tested it for holes and also set it in a glass of water, it's ok. The float is molded plastic and it has no tab for adjustment. I would say that the float level seems fairly high, no fuel comming out of overflow.

1980 QT50 Carb Question Plez Help!

Plug Still getting wet and fouling within 5 seconds of engine start. I replaced needle valve with no luck yet. The carb is 100% clean, I used solvent and compressed air and cleaned it properly and even checked all passages with compressed air from my lungs! The float... Floats! No leaks... The float is a molded plastic type with no tang for level adjustment... If I hold the carb in upright position with hose on fuel barb on carb and blow gently blow on the hose and raise the float by hand the float doesn't seem to operate and close till float in just about in full (raised) position. So being that no adjustment tang is on float any suggestions???? The needle valve silps on float arm in a groove. Also ordered new reed pedal. Thank you for any help!!!! Happy Riding!

Tonight I'm having an exhaust BBQ!l

Re: 1980 Yamaha QT50

i dunno.. "the needle seems to have been bent" would seems to say the thing was badly mishandled by someone.. and how the hell does a needle get bent?

What else mighta been messed up when that happened? Maybe the needle jet is now deformed?

Other than looking for reasons that air flow is blocked (and so a rich mixture is delivered) I'd take a real close look at the carb with an eye towards detecting further damage.

Floats that have no adjustment tab are usually not in need of adjustment... they can't really go out of adjustment unless the plastic is cracked or something. If the float body is near level with the surface of the unside down bowl, its close enough.

Re: 1980 Yamaha QT50

ming was doing the almost exact same i put a new needle valve. the wet flim is the carb is getting flooded out

Re: 1980 Yamaha QT50

Getting it going... Replaced Reed Valve and Needle Jet. Waiting in Jet Needle as it has slight damage... But it runs...

Re: 1980 Yamaha QT50

jake worley /

Have you checked your two cycle mix? i had the same problem and it turned out to be that, its the smaller rubber cap located under the seat

Re: 1980 Yamaha QT50

He is running pre-mix, no injection.

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