High rpms = 4 stroking

[DanielMaia]

Hi

I did some mods, repaired a broken thread with a heli coil, i did some port matching in the intake, i think it's great.

But i have one problem: until i get 34 or 35km/h, the bike have the typic sound of 2 stroke. Up than 35km/h starts to 4 stroking

I have a NGK B7HS, stock carb, base gasket is aluminium papper (0.05mm or so), head gasket stock, and a little skirt mod in the piston.

Can be the carburetor that restricts the air and starts the 4 stroking?

 

[DJEEPER]

running a B7HS plug? why so cold?

you should run a B5HS plug if it is getting cool, or just stick to a B6HS if its 80-100*.

try a different plug....

 

[DanielMaia]

B7HS was the recommended one for my motor, i think is runs very good

I tried a B5HS before, but i run only 10 or 20km, i will try that tomorrow.

But i think it's not the spark plug...i think that's the main jet or the air restricted :\

 

[kerf]

I believe the four stroking sound can be symptom of an over rich mixture (air/fuel - not oil/gas). I have also had this occur due to some exhaust restriction which probably effected the mixture at WOT.

 

[Ghost0]

Yes air fuel are the main culprit of 4 stroking. You would do well to go to a smaller jet. Also you said that you went to a thin aluminum base gasket. This has probably changed your port timing. You have essentially lowered your intake and exhaust ports. This will improve your low end torque but will hurt your top end performance probably also contributing to your 4 stroking at high rpm.