250SX milling the head

[wildonewest]

I may go to the whitewash sand dunes near Moab, UT in April but for know I'm getting my trike ready to go. I found a 200X front end and I'm also going to do a engine upper end tune up. First I'll do a engine hot compression test and if the compression test is weak I can perform other test that will determine if the weakness is in the valves or piston rings or both. It runs good but a little week on the power side so I'm hoping I just need to lap the valves, check valve springs, install new valve stem seal and so on.

My question for all of you is; Can I mill down the head just a little to bring up the compression with out taking away clearance of the valves or piston smacking into the head. I don't want to bring it up to much. Has something like this ever done to these engines? How about porting the intake and exhaust.

I don't want to invest in a high compression piston because then I'll have to get new cam, double valve spring and everything else that will have to go with doing a mod like that. I just want to to boost the power a little.

[Yamaha_Rules69]

Milling the head to gain compression would not be a good idea for this engine, they make aftermarket high compression pistons for a reason, and that would be your best bet. I believe a 10:1 would be a great hop up, otherwise 12:1, but then you have to run a race gas mix, and that can get expensive, plus it wouldnt last as long, but the 10:1 would run fine on premium from the pump. The major downfall is the shaft drive rear end - you will never be able to go faster top speed with motor mods, only get there quicker. The only was to gain faster top end is to run larger rear tires. The other downfall is the auto type clutch. You modify that engine enough, and the clutch wont be able to transfer the power, and start slipping.

[Dirtcrasher]

If you wanted to mill the head you would have to do a clay test on top of the piston and check clearances. IDK how much you wanna mill off but you could also remove the base gasket and use 3bond instead. May gain .020 - not sure, you'd have to mic the gasket.

I call BS on the clutch, once the centrifugal clutch gets you rolling, the main clutch takes over. My 250SX has 11:1, Web camshaft, Cobra exhaust, no ab lid and a 350X carb. My only issue was bending the swingarm. The clutch will take the power without issue............