04 Rm65 Low Compression After Rebuild

Adam Neff

Member
Jun 13, 2016
5
0
So i bought a used 04 rm65 last year. Not running. I tore down sent cylinder off to langcort. They sized to numra kit i bought. I put in wiseco crank and seal kit. Bought new head as previouse had dremal marks in it. Put it together and ran it. Never started easy, never did compression test, figuredit was good. This year started racing, tested compression to see where it was at. 115. Multiple testers. Bought exact same piston kit as last year, 44.47. Put in had same 115 compression. Note i live in colorado about 5000 feet. Bike runs fine, just have to kick 5 6 times to get low idle for 30 sec then can get to rev up and go. Manual says 109 to 170 is spec. I thought compression loss might be due to altitude. 3% per 1000 feet. But talked to local others and say it sould have solid 135 to 140. Took apart again, measured bore its at what lancourt finished it at. Ring in gap good. Cylinder very clean. Pressure test engine, holds pressure. My only thought now is could i have the wrong cylinder on this bike. It was used and had been apart before. I even put high compression gasket kit in it and got 118. Wet test and only get 5 pounds more. Not sure what else to check. Thanks
 

Adam Neff

Member
Jun 13, 2016
5
0
Also want to note that compression test has been done with throttle wide open and carb off. Also piston goes to top of cylinder.
 

Okiewan

Admin
Dec 31, 1969
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I'll move this to a better forum for questions like this.
 

Rich Rohrich

Moderator / BioHazard
LIFETIME SPONSOR
Jul 27, 1999
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Has the exhaust port been modified? Trapped compression ratio and the cranking compression value on a two-stroke is a function the volume difference between the piston at Exhaust Port Height from TDC and the piston at TDC. If you raise the exhaust port height (a common mod on minis) you will lower the cranking compression value. Just something to consider.
 

Adam Neff

Member
Jun 13, 2016
5
0
I do not think so. Based on everything looks factory to me. Nothing sanded smooth. This bike still had factory decals on it when I bought it. It did not strike me as being raced. But who knows. One thing that has changed since i posted this is that its been 90 plus out this week, and the bike now fires on first kick from sitting over night dead cold. No choke. I would think this means lean condition, but why does the choke not help when colder out. These little bikes are causing me grey hair.
 
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