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?



Just for note:

The 4 stroking in high rpm is caused by too much fuel in the mixture (as Joe, Norman and Riding rich said), so you should lean it out. Smaller main jet, or moving the c-clip to the first or second position

Problem solved

[Bikeguy Joe]

Four stroking is cause by an over rich condition.

Your jet is too big.

[Norman]

Yep! main jet too big.

[Riding Rich]

Also if you have a gasket between the intake port and tube make sure it matches.
On my Grubee if there is any blockage it acts that way.

And i would remove the stock head gasket if you used a thin base gasket.

That'll get the compression back up.

Mine runs much better with super thin head gasket.

[DanielMaia]

if i put de c clip in the first or second position? (at the moment is on the 4th trace)

[Riding Rich]

Yep thatll lean it out.

mine actuall like to run super rich.

My clip is all the way down to the last notch.

Try all the positions and use the one that runs the best.

[DanielMaia]

But i saw yesterday the sparkplug and it was in good condition (brown, semi grey)

:\

[Riding Rich]

Did you start with a new plug after the modifications?

I would still try adjusting the clip to diff positions.

theres nothing to loose.

[Riding Rich]

Also how much did you take off the piston skirt?
That actually changes the Port Timing.
So dose the base gasket you have.

My motor became reall touchy after i modded it to death.

Literally to death.
My bottom bearing is very very tired.
Can't wait to get it apart and see the carnage.

[Bikeguy Joe]

Originally Posted by Riding Rich

Did you start with a new plug after the modifications?

I would still try adjusting the clip to diff positions.

theres nothing to loose.

Start with a new plug, and your clip where it is. Then go from there. Some run fine with the C clip at the second from the top, some will require a different position.

DON'T JUST MAKE IT A COUPLE OF POSITIONS LEANER without doing the above first, or you could ruin your engine with an over lean run!

Still it is most likely four stroking for the main jet being too large.