So I got these KFX50 4Wheelers...

So I have two 2005 Kawi KFX50 4 wheelers and they use to run really good...that is until something happened.

KFX50 #1: This was my most reliable one. It started after a couple pulls but one day after trying to start it something happened. I was pulling and mid pull the engine cranking sound became lower. I think it might have to do with pressure as the air force coming out of the muffler is much less than the other 4 wheeler. I am new to engine work and can do some basic stuff but don't really understand how everything is working together.

KFX50 #2: My second most reliable one...this one was running good and my son was out riding one. He ran over a stump, the chain fell off, and the engine ran off. I pulled the plug and it still kept going...weird because I just heard that 2 strokes can do this the day before. Well it'll start only with the choke on and pumping the troddle.

Sort of at a loss. Don't really know what to do. I was thinking of doing a top end rebuild on both which might do it but I could just be wasting my time. Any help would be greatly appreciated.

#2 has an airleak. A bad one. Find it, and Fix it. Vacuum lines, etc.

#1 sounds like the rings went bad. Possibly oil starvation.

Another possibility is the Auto decompression mechanism failed, and is now keeping an exhaust valve open

So would a test like this need to be done on both of them?

v=6OxOx45eyUE[ame="https://www.youtube.com/watch?v=6OxOx45eyUE"]https://www.youtube.com/watch?v=6OxOx45eyUE[/ame]

Same test, different method.

Harbor freight has your back.... Or I've got one you can borrow... Lol

http://m.harborfreight.com/cylinder-leak-down-tester-94190.html?utm_referrer=direct/not provided

You need it to diagnose BOTH ATVs, and before you install the cr125 motor too. Always test 2t after rebuilds.

So I guess I should have taken pictures but I rebuild both engines quite easily. It was pretty fun experience and totaled about 15 bolts but here the results.

4 Wheeler #1: works great no problems and it pretty spunky. Fun watching my son rip around.

4 Wheeler #2: 2 stroke oil leaks from the exhaust when it's and blows a lot of white smoke. I don't know what it could be or why that would happen. Any advice would be appreciated.

1st mistake: Kawalski 2-.
2nd mistake: Kawalski 2-stroke.
3rd mistake: Did not memorize the Owner's Manual and follow the maintenance routine slavishly, as one must with a 2-stroke, especially a Kawalski.

Green is for when you can't afford a reasonable color.

Just IMO, of course.

Any better advice?