what octane fuel?

[ESR250R]

i just rebuilt my engine on the 250r after losing a crank bearing. had a fresh bore put on the cylinder and put everything back together. ran the quad for about 2 minutes to start the breakin and shut it off. compression tested the motor and came up with 248#'s of compression. now i know thats alot and im thinking about ripping the thing apart to try to lose some compression, but maybe i should give it some time to breakin and recheck it. i was anticipating about 220-230#'s but not 250. btw the distance between the head and the piston is .0033", which i was told was in the safe zone.

[Aceman]

That's quite a bit of compression. I'll bet that 248 psi jumps to around 255 psi or so after break-in, my 310 gained 7 psi. I run 110 leaded in mine at 222 psi, but I don't know how much higher the cutoff is for 110, maybe it's alright at 250 psi, I don't know.

I'm not very knowledgable with stock cylinders as I bought my R with a pro-x. But I know there are different combinations of gaskets that will lower your compression. Running that much compression would make me worry about premature crank bearing failure unless you replace them on a regular basis, but that could get expensive.

You could always run it as is and see how long it lasts.......:eek:

[ESR250R]

ok, the crank bearing is what just failed on me, with nearly the same setup as i have now (never compression tested it before). i have a shop around here that sells 116 octane, so i'll just pickup some fuel from them and hope that she holds up for awhile. just curious but if i added a extra base gasket into the mix (about .0021 thick) about how much compression do you think i would loose.

[machwon]

I think you have a typo in the clearances you listed. .0033 would be more like your piston to bore tolerance than the piston to head clearance. Even at .033" piston to head clearance, that would be super tight. I've run them closer before, but you will have trouble making it survive and keeping the coolant from boiling out of the radiator.

The base gaskets are commonly .020" thick, adding one to an assumed set up of .033" would give you a .053" squish tolerance. It would definitely survive on race fuel here, but likely dropping your compression by 20 lbs or so.

yeah gaskets will help either lose or gain compression

[ESR250R]

ok when i pull this thing apart (next few weeks) i will throw two base gaskets in it to lower the compression, plus it will open up my port timing by 3-4 degrees. it just cost me $600 because my crank bearing went bad, i dont want to go through that again.

[ESR250R]

Originally posted by machwon
I think you have a typo in the clearances you listed. .0033 would be more like your piston to bore tolerance than the piston to head clearance. Even at .033" piston to head clearance, that would be super tight. I've run them closer before, but you will have trouble making it survive and keeping the coolant from boiling out of the radiator.

The base gaskets are commonly .020" thick, adding one to an assumed set up of .033" would give you a .053" squish tolerance. It would definitely survive on race fuel here, but likely dropping your compression by 20 lbs or so.


ya i meant .033 piston to head clearence. my extra base gaskets measured out at .021 so im gonna put two in and im hoping to get around 220-230 compression.

[k265r]

i would try a head gasket before a base gasket

[ESR250R]

Originally posted by k265r
i would try a head gasket before a base gasket


i have a o-ringed head, gotta go base.

[k265r]

sorry didn't know you were running a o-ringed cylinder.