Sputters when outside temperature is hot, runs fine in the cold

[Dom]

I have a well used 50cc Chinese Bronc Rally and have maintained it myself for the last 5 years. This spring I got a new battery, cleaned the carb, replaced the drive belt, and replaced a rusty clutch. It ran like new.

A few months ago it began to sputter a high speeds and has been getting slowly worse, top speed of only 25-30. This only happens when the outside temperature is above 60F or so. It runs fine in colder weather, like in the morning, and always starts fine in all conditions. I can ride it to work fine in the morning 5 miles away but the ride home is awful and I slow down traffic.

I replaced the spark plug, coil, and bystarter (autochoke) with almost no improvement. The gas tank seems to be vented properly. I have no idea what could be causing this to happen. My current best guess would be something funny with the exhaust that acts up when it's hot outside.

Any ideas?

[kz1000st]

Check the intake tract for clogs. Cold it's like a choke and the engine likes it. Hot it's running rich. On the other hand check the valves. If they're tight the engine is losing compression when hot.

[blueboy5000]

You should experience no difference in performance at 60f. The difference between freezing and 60f is only a factor of initial cold start up. The gy6 becomes fully warmed in approximately 3 minutes.

What you are describing sounds like a dirty carb. I'd start with a full fuel flush and carb cleaning.

It is also possible your carb is adjusted incorrectly. Correct mixture should be approx. 2 1/2 to 2 3/4 turns from seating.

[Dom]

I replace the leaky fuel filter and in the process drained the carb and decided to flush the whole system with a few ounces of Sea Foam. I took it to work as normal this AM and it ran fine as usual. After sitting in the sun all day I took it home at 5pm. It sputtered a bit at full throttle and sounded a bit rougher than the previous sputtering and was maybe 5 mph shy of what has been top speed, but it did run better. So it appears the problem has been improved. Maybe the Sea Foam loosened some junk in the carb. It still is a bit puzzling why the performance only decreases when it sits in the sun or is hot outside but atleast it works well enough to me to use daily again.

Thanks!