Compression test

[Upyourkiltbatman]

Anyone know when to change piston or rings. What would be a good compression reading ?

[bowhunter007]

Preferably before failure. I check mine at 30-40 hrs...re-ring. Generally, on a trail ridden bike, I'll install a new piston assembly at 80-100 hrs. If the bike is raced, I shorten those intervals quite a bit. My 2ts are mixed on the heavier side @32:1(it's what I do...no particular reason). I've yet to do any compression tests.

[(F5)]

compression tests are bull on a 2 stroke

Just pull the lid if the bike is new to you & clean the PV etc, measure free end ring gap while you are there, done in an evening & 2 beers. (Just read the instructions on melting the nylock nut on the PV arm, its all in the help bits on this forum).

Order parts if required & slap it back together, 4 beers (gotta stand round afterwards & congratulate yourself, but go upstairs before you have any more and decide its a good idea to test ride it).

[Zman]

You can pull off the pipe and look at the piston through the exhaust port and see if you are having ring seal issues. If you are seeing dark carbon deposits below both rings you are ready for a new top end.

[Mattnzl]

Last year I changed the piston & rings at 100 hours (2007 ec250) and they looked fine, so this year I waited till 120 hours.... still looked ok, just some brown marks below the rings.

However! I'm fully just a trail rider. Lots of puttering about with just the odd full throttle hill climb/drag race each ride.
Bike is definately much crisper with the new gear, so will only go 100 hours again this time.

[twowheels]

Originally Posted by Upyourkiltbatman

Anyone know when to change piston or rings. What would be a good compression reading ?

A smart ass might say when they need it but you're likely at the outer limits at 120 hours. A piston rep once told me the issue is not so much the rings (you can always slap a new set of those in) but the piston skirt gradually collapsing.

The GG250s run about 185psi iirc, but the 300s will fool you with a super low reading because of the compression bleed hole (I recording 135psi on a new 300 and just about died before I knew this).

Some would say leak-down is a better measure of health, but at 7000 rpm, how much time is there for leakdown anyway? Besides that you miss the dynamic sealing action of the rings ... best to take compression numbers periodically and change the top-end gear when they start to slip.

[honuki250]

I just did a compression check on a cold motor. Kicked it 10 times and got a read of 175-180. Release and did it again and got the same reading. Bike runs great and I dualsport or trail ride.
Do you guys that know more than I do about engines, should I do a top end on it now?
Thanks.

PS bought the bike used and had carb issues but since I got that dialed in its a great bike. 2007 ec 300

[Leo]

Originally Posted by honuki250

I just did a compression check on a cold motor. Kicked it 10 times and got a read of 175-180. Release and did it again and got the same reading. Bike runs great and I dualsport or trail ride.
Do you guys that know more than I do about engines, should I do a top end on it now?
Thanks.

PS bought the bike used and had carb issues but since I got that dialed in its a great bike. 2007 ec 300

175 is greath!

[honuki250]

Thanks, with all the different reading I just wanted to get a second or third opinion. Twisting it this weekend is snow keeps coming down.

[motopsycho87]

Compression tests are very worthwhile on a 2 stroke, but you must remember a low reading can be something other than rings, cranks seals, reeds, head gasket... You need to know what your compression ratio is also before you start making judgements based on numbers