Need some help from you 2 stroke guys

[wvspeedfreak]

I have a 250r with a CT 350 Pro-x powervalve cylinder.I just bought this from a guy who said the engine was just freshened up from CT.I brought it home rode it around for a while and it seemed to run pretty good.I ran a compression test just out of curiosity and it only had 145 psi.It has the 100 octane dome in it and according to Allan Knowles at CT is should have 200 psi of compression.I pulled the top end apart and the piston looked new and you can see some crosshatch on the cylinder walls from honing still.I checked the ring end gap and it is .020 of an inch.However,there are a couple scratches on the cylinder wall that I can just catch my fingernail on.My question is do you think the scratches are enough to lose that amount of compression?Could the rings not have seated properly?Here are a couple pics of the cylinder.Let me know what you think.

[wvspeedfreak]

another

[wvspeedfreak]

piston

[wvspeedfreak]

another piston pic

[MIKE400EX]

Looks like a really tired cylinder that someone broke the glaze on to get the rings to seal some. It needs to be bored and re-finished to clean up the grooves, also check the bore for roundness, straightness etc.. Might be the lighting, but that piston does not look new, looks cleaned up. One thing to note when using a compression gage on these relatively small engines; use a hose between the gage and sparkplug adaptor that is as short and small of volume as possible to get the most accurate compression reading. Most of the long fat rubber ones commonly used for car/truck engines will yield a significantly lower and incorrect reading. Good Luck!

[Tom TRX250R]

The cylinder def. needs rebuilt because the scratches in the cylinder will significantly reduce compression. The piston doesn't look too bad, I've seen much worse, but I would rebuild the top end. Also as stated above do NOT use automotive style compression tester. I checked my compression on my R and my buddies after rebuild and it said we had 100lbs!!! After realizing we were using a crap tester, went out to Sears and bought their $50 Craftsman compression tester than comes with different size hoses and tips to accomodate for different engines. Then using the quality tester my engine (after rebuild) was right around 180. After CR gasket about 205-210.

[wvspeedfreak]

Thanks for the input.The compression guage I was using is an automotive style with a relatively long hose.It was a new guage and came with all the small adapters for small engine plug sizes so I just assumed it was ok for testing these things.I will try to locate a smaller hose for future use though.

[2muchquad]

yeah that cylinder doesnt look too bad,it could be cleaned up though.you could also have a bad crank seal as well.compression tests are just used as a baseline more or less to determine the condition of the motor.if it ran good and pulled like you thought it should,then run it.temp,elevation etc can make a difference in compression,it could have been different when CT did it in their shop..:ermm:

[wilkin250r]

I highly HIGHLY recommend a good compression tester.

The grooves don't look good, but it's very difficult to say from just a picture whether that was the real culprit. The difference between 145psi and 200psi could easily have been your compression tester, OR the grooves in your cylinder.

Now that it's apart, I would go the next bore size to eliminate those grooves, and take that variable out of the equation.

[zlam27]

shouldnt he try the "cheap fix first" method? dont rebuild the thing just because, get a better tester (unless you dont think thats the problem) and make sure you are getting an accurate reading. you might also run a leakdown test to make sure all the seals are good.

just a thought. unless you are sold on the idea that the compression is low and its not running right- dont spend $150 on a rebuild just yet.

zach

[wilkin250r]

Originally posted by zlam27
shouldnt he try the "cheap fix first" method? dont rebuild the thing just because, get a better tester (unless you dont think thats the problem) and make sure you are getting an accurate reading. you might also run a leakdown test to make sure all the seals are good.

I think that would apply IF he hadn't already torn the top end apart. If it was still assembled and running, then yes, I would certainly recommend the easy fix first.

However, since he HAS taken the top end off, there's no point in him replacing the rings, gaskets, doing the entire assembly, and then going through all the heat cycles of break-in, only to find that the cylinder really is bad or not, and then have to disassemble it again, blah blah blah. Since it's already torn apart, just get it fixed right the first time.

[wvspeedfreak]

Now I have something else to throw in here.I took the cylinder to work with me today(I work at an automotive repair shop) and I spent a few minutes on the cylinder with a hone and it turned out pretty nice.Now my question is do I put rings in it and put it back together or does it actually need bored.According to my micrometer the cylinder is .010 larger than the actual piston measurement.There is no cylinder wall taper or out of round.What is an acceptable piston-to-wall clearance?If this is ok I will just re-ring it and put it back.

Here is a pic of how the cylinder looks now.

[wilkin250r]

:eek2: :eek2: :eek2:

0.010 is HUGE. Typically you want around 0.002. You're definitely going to need a new bore and piston.

[wvspeedfreak]

Thanks Wilkin.........that would explain the compression loss.:eek:

[wilkin250r]

If you work at an auto shop, you may have the capabilities to bore it yourself, but don't be tempted to. Leave it to a shop with experience with ATV overbores. It's not that pricey, usually $45 to $65 for an overbore.

Typically, your clearance should be 0.001 to 0.003 (larger clearance for extreme conditions, like racing, where rebuilds are frequent).

[wvspeedfreak]

I work as a mechanic at an auto shop.We don't do any machine work.I do know of a reputable machinist about 35 miles from here that specializes in motorcycle/atv work.He will be the one doing it.Thanks for your help.

[MIKE400EX]

You're almost at the next bore size already, you probably won't even have to bore it. Buy a piston that is .25 mm larger than the one you currently have and then hone the cylinder to the correct size. Before you finish hone it, make sure that all the port chamfers are good and smooth.

[wilkin250r]

I wouldn't hone out the cylinder to the next size. Way too much possibility of getting out of round. For a $50 bore job, just have a shop bore it and forget about.