This simple 4-stroke top end rebuild is killing me. One problem after another.
2014 RMZ 250
Bike would not start after rebuild. Had spark, had fuel, timing was checked and good.
Compression tester said there was only 50-60 PSI after testing it (with throttle wide open)
Took it all apart to see why
Either leaking through valves or rings.
Pretty sure valves are good - testing them now. They were all cleaned along with the seats.
I looked at the compression ring on the piston and saw it was compressed and stuck in the groove 
Would not expand. Was tight in the groove. End gap was tight. Never seen anything like that before.
Piston and rings are new (Wiseco - factory 77mm replacement - stock)
Anyways - I'm thinking of replacing the piston and rings again - and double checking end gap again. Should be about .012"
Unless someone can tell me that my piston and rings are good (brand new) - and I can still fix this without getting a new one ???
I "lightly" honed the cylinder just to deglaze it. About 30 sec worth. I cleaned it pretty good with a new shop towel afterwards. Maybe I left honing grit on the cylinder walls and it got stuck in the ring groove........?
What's going on??
Very strange. After honing you need to wash with soap and water. Dry, then wipe with white paper towels and WD40 or penetrating fluid on it. Keep wiping until no dark residue can be seen on towel. I no longer hone after the Nikisal chipped off cylinder after 3rd top end. Appears to be a lot of scratches on piston that never started. Grit might be the culprit. I would replace compression ring after cleaning everything. When you insert the piston in the cylinder check for ring binding by rocking piston forward and backwards at TDC. Crazy how much they can move. No rocking, rings are binding. Good luck and let us know how it turns out.
Take the ring out and try to see where it's getting hung up in the ringland. They make small files for filing rings and such, maybe you could touch up that one spot? Or call Weisco but they'll probably just tell you to pound salt. All I've ever used are CP Carillo pistons for that reason
I'm thinking I didn't clean the cylinder good enough after honing. I never washed it. Just wiped it down. I'm thinking grit is stuck in the ring lands and now it's seized up. Rookie mistake.
The ring was nice and loose when I installed it.
How can I get the ring out without scratching the piston?
It's seized up in there
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Maybe try a pick and see if you can separate it? Dude you gotta clean up the bike and work area a bit more before you tear it down like that.
I keep a clean work area for sure, but I never washed the cylinder after honing. Now I got grit stuck on both sides of that compression ring and it's seized up good. Looking like I'll never get it out. Been working on it. No luck. Probably have to buy a new one.
I just got it out without damaging piston. Used .003" feeler gauge to get in the ring groove and clean it enough to loosen the ring.
I'll hone the cylinder again and then WASH THE CRAP OUT OF EVERYTHING BEFORE REASSEMBLING
I’ve always been told to never hone a Nikasil cylinder. I just go with Scotchbrite and WD40, good wash and wipe down after.
If the crosshatch is good DO NOT hone the cylinder. If the crosshatch is gone it needs to be replated. Rickyisms is right. Just going to cause you problems down the road.
Pretty sure I ruined it. 3rd rebuild in last 2 weeks. Just finished.
Will try to start it Tomorrow
Pretty sure compression is still to low. Initial test was 60 PSI
Probably have to get new bike
Compression test isn’t going to be accurate on a 4 stroke. You need to do a leak down test to check for ring and valve sealing condition.
Thats correct. Your bike has an automatic decompressor. It bleeds off some cylinder pressure through an exhaust valve to make it easier to kick start
where is this "automatic decompressor" ?
I don't see it anywhere and I've taken the motor apart 3X.
not mentioned anywhere in the manual either
My bike is a 2014 RMZ 250
how do i do a leak down test and how do i determine where the leak is coming from?
could be valves, rings or head gasket.
You need a leak down tool. Similar to a compression tester. Threads into spark plug hole, has a set of gauges one to regulate air and one that tells you how much pressure is lost. You hook it up to a compressor while engine is at TDC and you listen for air leaking. If it’s coming out of your exhaust, probably an ex valve leaking. Coming out of airbox, in. valve. Etc…
Your decompression system is built into the Ex. Cam. Has a little weight on it that works by opening a little pin on one of the lobes that slightly opens a valve to release compression.
I understand that - but i would think it's impossible to hear a tiny leak coming through a valve or head gasket or blowing past the rings - especially if going by the rings...?
so without a "leak down" test - there is no way to accurately check compression on my engine if it has a decompression device on the exhaust camshaft?
Leak down tester from Harbor Freight.
Ok fine - I just looked at it. Seems decent and good instructions.
Easy to use and has all adapters etc.
sounds like you have to "listen" for the leak.
that's the part i was confused about. Didnt really make sense.
If all the tool does is blow compressed air into the cylinder (at TDC) and you have to listen for leaks - I can probably just connect my air compressor tank somehow into the spark plug threads and blow air in there and listen for leaks and save myself $80 ?
Anyone have any experience with cylinder leak testing? I don't.
Pit Row
The benefit of the tool is the gauges. 1 to regulate air into your cylinder, you don’t want too much pressure. And the second gauge shows you the percentage of air lost (IE if you blow 100 psi in and it’s holding at 90 you’re at 10% leak down which is fine and hard to hear.) If something’s really leaking you won’t have trouble hearing the air come out, and you’ll easily be able to hold the crank at TDC. It’s easier with 2 people that way one can hold the crank with a breaker bar and a socket and the other can hook up the compressed air to the gauge.
Did u ever find the solution to this I’m having the same problem at the moment
Yes...
I failed to completely clean and wash the cylinder, piston and rings after I honed the cylinder. I initially only sprayed and wiped down the cylinder with WD40.
Consequently the cylinder had thousands of microscopic dirt and metal filing particles that got caught in the piston rings and resulted in a loss of compression and there was no ignition because no compression. The air fuel mixture won't ignite unless it's compressed.
I took it all apart, washed it all with soap and water and reassembled and it fired up.
Ideal cheers mate will try this next 👍
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