I think you are on the right track. Sounds like it is ready for a top end.

Quite likely. BTW, the top end wear could have been accelerated by a plugged or kinked vent hose. IIRC, the vent hose on that model is stuffed between the swingarm and the frame.

Hm, it must be well hidden, or I must be blind, because I can't seem to find any vent at all on it...

Oh, and out of curiosity. Is there any recommended source for parts, or is ebay the best bet? Since it's starting to look like I might be needing some gaskets and stuff. And hey, if bumping up to 110cc is actually that cheap, can I afford NOT to? :P

Here's one: http://www.ebay.ca/itm/110cc-ATV-Top...item1c241a8894 I'd use eBay.

Check on the left side of the engine, just behind the top motor mount bolt. Should be a black hose there that routes behind the engine.

Awesome! If you didn't tell me exactly where to look, I wouldn't have seen it. So covered in grime that it was basically hidden! I think this thing needs a good wash before I pull anything else off it hahaha.

Just as an idea, what should the compression be? I'm STILL waiting on that adapter, but considering it was supposed to be in yesterday, it shouldn't be too much longer. I hope!

If it tries to blow your thumb off of the spark plug hole, it has compression. :lol:

Seriously, I don't have a clue, but I'd guess around 140psi. If you intend to install a new cylinder and piston, it'll be optimized. That said, I'd be sure to hone the new cylinder and clean it thoroughly before installing it.

Did you find that the vent hose was trapped between the engine and swingarm? :?:

The vent hose appeared to be free and in good shape, other than being coated with mud and grease.

I did some checking online and found that while compression should be in the range you mentioned, it would be impossible to get a good reading with a 4 stroke.
I do have a leakdown tester in the toolbox as well though, so it might be a better indication of what's going on. Still need that pesky little adapter though... hahaha

Leakdown against a standardized 80 PSI is how we check airplane engines at annual inspection.

What's the issue with a regular compression test?

Check out this short article:

http://www.dansmc.com/compression_test.htm

I saw a few theories against it.
One being that the combustion chamber on these little bikes is so small that the hose on the tester can actually have a larger volume of air in it and absorb alot of the pressure, leading to a low reading.
Another said that most 4 strokes have auto decompression to make them easier to kick over. So it would vent alot of the pressure out.
And ANOTHER person said that it would be virtually impossible to get a good reading with a kickstart as you can't kick it all the way through a full compression stroke.

I'm not sure if any of these apply or don't apply here, but it seemed like an awful lot of reasons for a low reading

Thumb test always worked for me. I don't think that these bikes have a compression release and I would just kick it a few times in a row.

Good points.

One has to disable the compression release if the bike has one that's automatic.

I can see how the hose volume would matter.

Thanks for the thoughts. :idea:

Yeah, I have no idea if any of them are valid or not haha. I'll try to check compression either way and see what it says. Check leak down as well, since it makes it easy to pinpoint an issue before tearing it down, which I'm sure it'll probably need at this point

Relic, you sound very experienced with such things. There is no decompressor on this motor, and I agree with Kato on kicking it several times. If the gauge has a check valve in it, it will record the highest pressure achieved. Nevertheless, you'll get a sense of how it's doing.

I wonder if that vent hose is at least partially clogged with goo. On another note, these engines aren't equipped with an oil screen, so oil must be changed regularly.

I've got a fair bit of experience with cars, but none whatsoever with bikes.
i will give it a shot with the compression tester in the morning, since my fitting finally arrived! I'll be interested to see what #'s I come up with, and compare it to the leak down. Might be good for reference for someone else one day :)

I'll check the vent hose more closely too once I get the motor cleaned up a bit. Thanks for all the help so far!

Compression test is 50psi on my "good" tester. About 40psi on my cheap one