Ok, problem is that about 400KM into the life of scooter (2 stroke People 50, 2010 model), the top end speed of the scoot went down about 5mph on flat surfaces and up hills. The acceleration to this top speed has also decreased, but I don't have an exact figure for that measurement. I did a wet compression test and I got 125psi. A week later (a few days ago from this post) it was at 120psi consistently. Air filter is clean, even tested without air filter installed. Fuel filter appears to be clean, however I have not yet replaced this yet. Tech Manual for this bike states probable cause for low compression associated with symptom of "engine lacks power" are the following:
1. Faulty reed valves
2. Faulty piston springs
3. Faulty cylinder head gasket
I wasn't convinced that the carburetor was being a good boy, so I took it out, took it apart, everything looked clean, but I cleaned it anyway. Removed reed valve assembly and the reeds looked good, nicely aligned, tight fit. So item 1 on the list, plus the carb seems good.
Took apart the top-end and right now I'm thinking this might be the culprit. The cylinder head has small oil stains and oil built up around the outside of where the gasket goes. The gasket itself looks ok, so I'm wondering if the head itself is warped on the surface the gasket sits on. Also getting a bit of oil up the spark plug thread area in the head. Piston rings seem ok, but the top ring was a little bent such that one end of the ring was higher than the other end. I just bent it back, but wondering how it got bent. Hmmm. The edges of the rings are nicely square, not rounded. The inside of the cylinder does have some sratches, but not deep, I woudn't even call it "scoring" and you can't feel them with your finger, however I'm wondering how typical these markings are for a cylinder with 1400KM on it. Piston itself looks good to go, only a little carbon build-up on the top of it. There is some carbon build up in the exhaust port of the cylinder and I plan to clean that out. Right now my brain is fixated on the head. Should this have any oil on it whatsoever for a scooter that has 1400KM on it? It is clearly leaking some oil, and I'm curious that if a new head and/or gasket were to be installed would this stop and would the compression increase, and would that bring the performance back to where it was when it was new? Thoughts anyone?
Cylinder compression for this scooter per the Kymco Tech Manual is supposed to be 11.8Kg/cm2. Someone please check my math, but I get that to be 168psi. That sounds pretty high compared to what I hear from scooter shop techs. Kymco Tech support told me that 125psi was "on the low side of the acceptable range". Also, it seems my entire assumption on this is that the alleged low compression and the lower performance problem are related. Are they? I dunno, seems reasonable, but I could be way off and perhaps 120psi compression on this model has nothing to do with this performance degradation. Thoughts?
