Knock or ping at 5k rpm

[darins]

Ok so here is the deal I transplanted a blaster motor to a ysr50 and the thing screams. Idles great, accelerates perfect. The only problem I have is when I let of the gas to 1/4 throttle (cruise) at around 5-6k it pings or knocks like crazy I messed with jetting going extreme from one end to the other. I fully rebuilt the motor with new bearings, seals, rod and piston. I took it apart to check things out and found the piston to cylinder is .013, that seems huge, shouldn't it be .004? the thing that confuses me is it seems like it would be all the time. I had a local machine shop do the cylinder. Did he mess it up? Thanks in advance I know it is not a quad anymore but figured you guys are the ones in the know on this motor.

 

[99LRDblaster]

Did you buy an oversized piston for the bore?

 

[darins]

Yes I bought a pro x piston and the machine shop was supposed to bore it to match. .060 over

 

[sicivicdude]

Are you measuring the piston at the proper location? You're supposed to check the piston diameter 10mm up from the bottom of the skirt from front to back.

A piston to cylinder clearance if .013" would be rocking inside of the bore. Wiseco recommends .0022", factory says .0015"

If you really have that much clearance, the bore should be checked for taper and out of round. If it passes those checks you could get a .070" oversize piston and drop it in that bore, that would be the proper size...

 

[darins]

but could this be the problem with the knock (ping) at around 6k at cruise?

 

[Awk08]

discarding the clearence issue, since we don't know if it was measured correctly....
sounds like a detonation problem, what octane fuel you runnin ????
was it leakdown tested for airleaks ????
and what pipe did you use for this ????

 

[99LRDblaster]

but could this be the problem with the knock (ping) at around 6k at cruise?

It's more than likely the piston skirt slapping the side of the cylinder.

 

[darins]

It has a TC exhaust, 93 octane, leak down fine, compression at 125. BTW has a new keihin pwk28

this was measured using a piece of solder between the piston and cylinder in the up position.

here are the actual measurements

10mm from bottom of piston 2.655

top of cylinder 2.657
bottom cylinder 2.658

 

[Awk08]

in the vid....looks like alot of side to side play, i'd say the local shop screwed it up

and that motor will never run right with that exhaust < always the biggest problem with hybrid transplants