2011 KTM 250SX compression advice

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5/26/2020 4:53 PM

I bought this bike new in fall of 2010. It rips. All I have ever done engine-wise has been to change filters somewhat regularly, clean the carb often, and change the oil occasionally with cheap 10w30 dinosaur oil. I also started using a BR8EIX plug for the past several years instead of my go-to BR8EG. Oh, fuel has been exclusively 93 pump gas with Motorex 50:1. The bike has seen hare-scramble type abuse and I never installed an hour meter so I have no idea on hours. Hundreds. Not sure if over a thousand, but would not be surprised.

4 years ago I put new rings on the piston and installed a 1-step thinner base gasket. It was out of the blue, the bike was running fine, it compression tested at 185 psi and I had no reason to do this. I just pulled the jug for the hell of it, looked at the cylinder, measured nothing, put it together and it made 195 psi and felt a bit stronger. Ran like a dream.

Last year I decided I needed a little more top end so I changed the front sprocket to a 14T. It added noticeable length between the gears and took some getting used to, but I think I like it. I still have the stock clutch too. Since I've been using the clutch a bit more now for obvious reasons, I decided I'll just purchase new steels/fibers.

This bike still rips. It will pull wheelies in 3rd easily and 4th if its not running too hot, but for some reason I feel like its due for another freshening-up. So I just did a compression test and got these numbers on a cold engine with throttle wide open - 90, 140, 170, 185, 195, 205, 215. I gave up at that point, but probably could have inched the thing higher.

My question -

Should I do a new top end? Surely this thing could benefit from new rings....probably even a new piston at this point, but these numbers look pretty damn good to me. And the bike wasn't running bad at all. I just feel like there is some extra pep in there that I could be getting (and doing proper preventative maintenance). But honestly I was hoping for worse numbers to justify it.

At this point I'm leaning toward just plopping in a Vertex B-Group piston and rings, but I've been bitten more than once trying to fix something that isn't broken.



5/26/2020 6:33 PM

I would replace the piston for sure, service the powervalves. Inspect reeds & crank/ rod play. The bike has treated you right, it deserves fresh top end... original piston? If yes, it’s cast & compression won’t mean anything when it explodes & ruins a bunch of other parts.


5/26/2020 7:22 PM

Good point