Help bike losing revs then picking up at full throttle

[pj67]

So I ride my bike fine but as soon as I am full throttle the revs drop and pick up and drop over and over without me loosening the throttle I don't know what it could be any help would be appreciated.

[FORMA MOTO]

bike

year

model

condition

tuning

puncuation

[pj67]

It's just a pit bike 125 manual so nothing special

[FORMA MOTO]

change your plug, check your plug wire

[pj67]

I checked my plug it was sparking fine, but I took the air filter off and it runs fine full throttle so is it a smaller main jet I need?

[FORMA MOTO]

I checked my plug it was sparking fine, but I took the air filter off and it runs fine full throttle so is it a smaller main jet I need?

 

Sounds like it

[ossagp]

I checked my plug it was sparking fine, but I took the air filter off and it runs fine full throttle so is it a smaller main jet I need?

 

If it was not doing that before it might just mean you should clean or change the filter.   I love the no brand no history included why go to the trouble,  but since it is a pit bike I understand more now.

[pj67]

I think it's a yx125 engine, as far as history goes I have only had it a couple of weeks, it had a rebuild so new piston and valves etc but because of the clearance there was no filter on it as the one the person had was too big, I not long got a smaller one to fit and not riding it full throttle until a few days ago didn't notice, I will clean the air filter but have also ordered a smaller main jet, would that enable more air into the mixture at full throttle?

[FORMA MOTO]

I think it's a yx125 engine, as far as history goes I have only had it a couple of weeks, it had a rebuild so new piston and valves etc but because of the clearance there was no filter on it as the one the person had was too big, I not long got a smaller one to fit and not riding it full throttle until a few days ago didn't notice, I will clean the air filter but have also ordered a smaller main jet, would that enable more air into the mixture at full throttle?

 

did you cut the valve seats for the new valves?

did you check the end gap of the piston rings?

[ossagp]

I think it's a yx125 engine, as far as history goes I have only had it a couple of weeks, it had a rebuild so new piston and valves etc but because of the clearance there was no filter on it as the one the person had was too big, I not long got a smaller one to fit and not riding it full throttle until a few days ago didn't notice, I will clean the air filter but have also ordered a smaller main jet, would that enable more air into the mixture at full throttle?

[pj67]

Yes I think the problem now sits with the air filter I'm using as it's blocking the bypass holes, would this affect full throttle with no choke on?