XR75 High RPM Misfire

[BSer]

I bought a 79' XR75 2 weeks ago. At 3/4 to full throttle the bike falls flat, spits and sputters.

I adjusted the timing, points, valve clearances, cleaned the carb, replaced the plug and the bike still does it. However, if I remove the air filter the bike runs much better with only a slight hesitation once in awhile during extended wide open runs.

I checked the compression, it was 85 psi. Added a bit of oil to the cylinder and the compression was 100 psi. The bike doesn't smoke though. The manual says the compression should be 170 psi, which seems high.

Could low compression be causing my problem? Or should I look elsewhere?

Thanks in advance.

 

[2strokerfun]

Low compression sounds like the culprit, assuming the carb is jetted correctly. These engines were not particularly finicky, but 70 psi is very very low.

 

[Dirtjunkie85]

I agree with 2 stroker. With compression that low it's definitely got to be the cause.

 

[SHSPVR]

Any time you add bit of oil to the cylinder and the psi gose up that good sign that it need be rebuild and 85 psi is a bit to low any way for it run rigth.
170 psi it about the rigth with fresh build motor I belive the stock piston had something like 9.0:1 or 9.5:1 compression back in day.
Sound the like you need get it bore out for new Piston & Rings and have head done rigth not half a@@.
How ever if want get some real performance parts
Check out
http://www.powroll.com/ or http://xrminiracer.com/
Back it day I help friend build up one we order ever thing from powroll we add
High Lift Camshafts
HD Valve Springs
Wiseco 11.0:1 High Compression Pistons
20mm Mikuni Carberators and Intake Manifolds
DG Performance Pipes
Barnett HD Clutch kit
Plus we made are own case savers for Stock XR frame