Too much Compression on a YZ250

[del250]

Here is an odd problem, I recently purchased a 2003 Yamaha YZ250 in pieces from an individual. The piston litterally exploded and put a couple of cracks in the cases. I had a local shop repair the cases, bought a hot rod crank assembly, Wiseco std bore pro-lite piston, new crank bearings and seals, and all new gaskets and seals in the motor. After final assembly, I threw a compression gauge on it and it read 260 pounds of compression. I pulled the top end off, and reassembled it thinking I didn't line a ring up right and double checked the ring end gap and all were fine. I then put it back together and retested with the same results. I had the ports on the the cylinder smoothed out and the exaust port polished were the only other mods done to it. I pulled the head and brought it to a local engine builder and he machined the squish hoping to bring down the compression a little. It came down to 240. Something else has to be going on here and I need some ideas. Any help would be great.

 

[76GMC1500]

You need to start taking some measurements and check the squish clearance and such. Measure the clearance volume and do a corrected compression ratio calculation. The cylinder may have been machined down. Maybe the cases were trimmed during the repairs? Try double stacking head gaskets. How do you feel about running race gas? Do you know the compression tester to be accurate?

 

[1BAD250]

if you happend to use a little to much oil on the piston & ring lands it would couse a hydrolic effect at first. if you think this is the reson your compression is a bit high then no need to worry, i sugjest useing some good fuel i use 100 rocket brand race fuel its vary good stuff. hope this helps.. :cool:

 

[KX02]

Did the person you bought it from have the head milled?

 

[adam728]

Blindly milling or stacking gaskets to try and get a cranking PSI you want is not the way to go. I would measure your squish (lead solder through the spark plug hole, roll the engine and measure it where it gets flattened in the squish band). Then pull the head and find the volume, but don't forget about the volume added or taken away by the piston height at top dead center.

Now if all that looks good, just run it! Like 1BAD250 said, lots of oil can give false numbers, run it a little and then test. Compression numbers vary from engine to engine and gauge to gauge. I had an old Toyota with a rebuilt engine that put up almost 200 psi on each cylinder, about 60 psi more than the manual called for (if I remember right). I was told the head was milled too far, the block was over decked, all sorts of things. Truck ran great for the 30,000 miles I owned it, all on 87 octane gas. Point is, cranking compression is great for monitoring engine condition when checked fairly regularly. It's not great for determining how the engine is put together.

 

[hot125mod]

can the comp jump up like that if you have a bad ring i doesnt really make sense how a bad ring can make the compression go up could someone explain

 

[snb73]

My 2003 RM250 has 220 lbs.

 

[johnnyl]

Compression is maybe a touch high but that may be a good thing. The key to this is how it runs. Stick a decent octane fuel in it and ride. If it runs well and there are no signs of pinging or detonation then alls well and good.

 

[del250]

Thanks for the suggestions......can't double stack head gaskets, o-rings are used. I am leaning to one of the previous owners milling the head. We have used to compression guage on all sorts of engines and seems to be accurate. I haven't run it for any extended periods of time, there is a good chance that there is oil on the piston and rings "sealing" it more than normal. It sure kicks over like a son of a gun. I plan on using race gas for the most part anyway, but would like it to be pump gas friendly when we get out in the middle of nowhere here in SD. Any other suggestions would be cool. Thanks again.

 

[76GMC1500]

See if you can get a used head for cheap on ecove.

 

[Dekester]

That compression is actually normal for 03-07 motor. I have 2 and they both blew between 255-257 after some riding. I think that is the upper end of what you should see on that motor. I would have left it alone. My bikes run on crappy 91 octane just fine even on warmer days. I was impressed with the numbers when I first saw them, though.

 

[jason33]

have you blown a gasket??? whats wrong with it again?
whats it run like-

 

[yz80nbeast]

i want to know if the bike starts