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50cc 2 Stroke Tuning? Answered

I have an old moped that I got for cheap and I am trying to fix it up. I replaced the carburetor and the new one is a little bigger even though it is off of another 50cc. It insisted on running extremely lean, but I was able to get the fuel/air mix balanced by putting some foil tape over part of the intake to get more vacuum behind it.

It will only run right if the exhaust is on so that it is not lined up perfectly with the exhaust port on the cylinder. If the exhaust is taken off it will start first kick, but if it is lined up the way it should be it will not start. I took it off today and when I put it back on as close to the way it was before it would start but is now running very lean, beyond what I can tune out with the carburetor. If I give it full throttle it bogs and has no power, but it has fair power at 1/3 to 1/2 throttle. It also is getting really hot and has absolutely no power once it gets very hot.

Yesterday I hat it all just right and it ran with none of these problems except that full throttle was still not where it got full power.

Does any one have any ideas about what I have to do to make it run right? Maybe there is no option but careful misalignment of the muffler?


It turns out that the problems were that the muffler was clogged, and that it was running too lean at first, but too rich once it warmed up. Thank you for you suggestions.


wow... that sounds really wierd. I would check the magneto and spark plug. Replacing the spark plug might help a little bit, but your discription sounds like the mixture is way to rich, not lean. If it bogs down when you give it more gas instead of less, that probably means that your getting to much fuel and not enough air. to solve the heat problem, you need to get more airflow to the motor. Another thing you might try would be choking the carburater. try using the foil tape to start it, and then removing it when you need to give it more throttle to go. other than that I would either just leave the exhaust off or "carefully missalighn" it as you said. If the problem with the carburater is that it's running too lean, try tuning your throttle cable so it gives the motor a little more gas at idle. hope this helps, im just trying to throw some ideas out there. iv'e tuned and fixed quite a few 2 strokes and these are some very good starting points.


6 years ago

It sounds to me as though you aren't getting enough back pressure in the exhaust system. Some of those old two stroke mufflers had to be repacked periodically with steel wool. Or if baffle type, its possible some of the baffles have rusted or been blown out.
Partially block the end muffler exhaust with a heat resistant material and see if that improves performance. I always used my hand covered with a heavy welder's glove. If that works you'll either need to repair or replace the muffler.

Do you know how to clean the baffle type muffler really well, because taking the cap of the end and scraping out a bunch of soot really helped. I think it was too much back pressure instead of too little. I cant get in there to clean it out better though.

Put the exhaust on properly - it's an important part of the whole engine function.
Note that running lean and hot can cause engine damage, e.g. burning a hole in the piston-crown (I've seen it done)



6 years ago

Could the carb and the exhaust be exchanged on a 2 stroke ?