Low compression, pics of my piston

peonpete · 09-08-2015, 02:08 AM

Hello. I have had trouble with my scooter for the past month, the issue began around a month ago. The problem is two-fold, for one, it began to hit a high RPM right off the bat [7500], then it would within seconds drop down to 5k and slowly build up from there to 6200 rpm and stay there. Secondly, it began to "burp", where it would lose power then gain it back again repeatedly. Tt sounded like a air - fuel issue and i checked the carb and it was fine. Now we come to a few days ago, the problem got worse and the scooter refused to start up. I checked for spark, check. Cleaned the carb out and the jets, and also checked gas.

All were good, so i bought a compression tester and the compression was awful [52 psi]. I pulled it apart tongiht and looked at the piston, it has black carbon deposits all on the top of it, but the worse bit are the scratches all over its side...Is this the reason for the bad compression ? I admit that once or twice it did run out of oil due to a leak, which probably caused the scratches on its side. I have pics attached to. If it is the piston should i just get another kit [I have a 100cc kit currently].

peonpete · 09-08-2015, 03:44 AM

Here are pics of the cylinder.

kz1000st · 09-08-2015, 07:57 AM

The piston is scoured, the cylinder might clean up with a light hone. The carbon on top of the piston is a non-issue. No oil, screeching rpm killed the engine. What caused the high rpm was probably a loose manifold, open vacuum line or air getting in someplace. No oil finished the job.

Pistons, rings and a new barrel are cheap. Fix the oil leak and find out what caused the rpm to go berserk.

peonpete · 09-09-2015, 06:40 PM

Thanks for the reply. So far I have ordered a new piston + rings and new cylinder. I did notice when taking the engine apart, that the gasket that goes ontop of the cylinder and connects to the rubber riser for the air intake manifold was pretty much destroyed. I think that might be why air was getting into the engine. Other than that issue, there is a small amount of oil getting into the CVT and im not sure how to fix that. Its not a huge amount of oil [id say 6 tablespoons worth per month] but its still a leak.

As far as the piston goes, I looked at it more and the middle ring on it was siezed up. It looks like carbon got in there and jammed it when it ran low on oil. Im going to try adding a small amount of sea-foam for now on when i fill up the tank and hopefully that will prevent that in the future [aswell as fixing leaks and adding oil obviously].

blueboy5000 · 09-10-2015, 06:40 AM

The crankshaft oil seal leaking is 9 times out of 10 due to excessive play in the crankshaft bearing. This is a very common problem, found when engines oil starve, especially on the 139. You can attempt to replace the crankshaft seal, but I highly doubt it will fix the leaking.

Your engine is most like ruined, crank wobble, of even 1/1000" is enough to ruin the engine case bearing journals

09-09-2015, 06:40 PM	#4
peonpete Join Date: Sep 2015 Posts: 13	Thanks for the reply. So far I have ordered a new piston + rings and new cylinder. I did notice when taking the engine apart, that the gasket that goes ontop of the cylinder and connects to the rubber riser for the air intake manifold was pretty much destroyed. I think that might be why air was getting into the engine. Other than that issue, there is a small amount of oil getting into the CVT and im not sure how to fix that. Its not a huge amount of oil [id say 6 tablespoons worth per month] but its still a leak. As far as the piston goes, I looked at it more and the middle ring on it was siezed up. It looks like carbon got in there and jammed it when it ran low on oil. Im going to try adding a small amount of sea-foam for now on when i fill up the tank and hopefully that will prevent that in the future [aswell as fixing leaks and adding oil obviously].
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