As the title states, my 84' moto4 80cc sputters when under load, but more specifically, and strangely, runs great on full throttle, or when you are just resting your thumb on the throttle, with just enough pressure to get off idle. Another thing is whenever I choke it, the roles seem to flip. Runs like **** under hardly any throttle or full throttle, but runs great under partial throttle. I'm assuming its something carb related, but I've never seen this before, and I'm pretty experienced when it comes to diagnosing problems and fixing them. No amount of tuning has helped. I'm just stumped. For some extra info, its a brand new carb, and has fresh gas. I believe it still has the original coil and CDI, so im not sure if those are what may be causing the problem. Thanks in advance for any help or tips! I'll try to post a video of it for a better representation when I get home.

 

My guess would be your brand new carb has the wrong jets. Clean the old carb out and put it back. If it is too far gone to re-fit, swap the old jets and needle into the new carb.

 

I would clean the old carb out but it’s way to far gone. Do you think I should drill out the jets?

 

No, once you do that you can't go back. Clean the old jets out and fit them into the new carb, if it ran right with those jets before, it should run right with them now. Try a new plug just in case it is electrical.

 

Okay, thank you.