HELP! 50cc Lifan Engine Cuts Out

Hi, my 50cc Lifan LF50QT-2A 2 Stroke seems to cut out when I reach 35mph or above, this only happens at full throttle, until I back of the throttle till about half way or even 1/4 throttle the engine will return to a smooth cruise.

I have upgraded the exhaust, the pipe is clear.
I have jetted the carb up to an 86.
However I still have the stock airbox. Could this problem to down to lack of air or to much air? Or too much petrol or too little petrol? Or could this be completely to do with my gearing?

Thank you!

 

gearing is not the issue here, so you can cross that from the list.

the carb needle sets the richness/lean at mid throttle to wide open. with the mods already preformed and running the stock airbox , its possible your getting a over rich setting on carb at wide open throttle with that jet upgrade.
more fuel in requires slightly more air in.

Now theres two ways to see if this is affected by the air intake for motor. one is to remove filter and replace with something less restrictive, but theres a risk you bring the same problem but reversed ie to much air to the fuel being drawn in and then means down jetting or replacing the new filter with the stock and returning to the over rich setting.

a more simple plan would be finding a second hand stock airfilter box. and modding this to allow for less restrictive flow of air.
with a cheap second hand box you would drill around 4-5 holes into the side of it and fit it to the bike. you would then duct tape over the holes and go for a ride, with all holes taped up you will find that wide open throttle bogging. so on return to home you would write down 5 holes blocked no change. each time you unblock and remove tape from a hole you write down the effect it had,( ie 3 holes unblocked wide open throttle is fine or three holes shows slight bogging/ over lean. you may end up with something like two and a half holes unblocked bike runs fine wide open is nice and pull is smooth and constant.)

next you would go for a second ride out but this time you have unblocked one of the holes you drilled out, what this is doing is allowing a little more air into the airbox than standard. you would have a little ride and at wide open throttle see if your bogging down again, you will rinse and repeat this removing tape off each hole until the wide open throttle problem stops this then shows the problem was a air starvation problem.
each hole uncovered allows a little more air to bypass the boxes restrictions and allow that flow to directly enter the carb via the foam filtering. you also may need to adjust your idle as your doing this as more air is allowed into the airbox that in stock form.

NOW. theres some drawbacks to this, your filter is going to get dirty quicker as there is a more direct path that air is taking to the filter itself so it will need regular checking - washing - drying and re-oiling ( simple to do if you have a spare foam filter pad otherwise you must wait for the filter foam to fully dry out) another drawback is possible water entering but its not going to be that bad like massive water floods. so a good oiled filter pad is a must this captures air and water stopping it entering the carb and engine.

its a bit of a cowboy fix but is the better way than forking out on aftermarket filters and then having to re-jet up or down again, you have more control over the amount of extra air your allowing into that carb than just throwing on an open filter and crossing fingers. As for those new hole sizes i would say around 8mm to 10mm and roughly 4 to 5 holes