I just got an ATC 110 for my kid. It runs really well for it's age. The previous owner said he replaced one of the crankcase halves because the old one had a leak in it. He honed out the cylinder and put on new rings. It has pretty good compression. I've noticed after about 20 minutes of riding if you take the dipstick out, a rush of air blows out. I don't remember this happening on my old ones I had as a kid. I also noticed that if you have the dipstick loose in the hole, it will pop up when you pull the starter rope. Normally I would think it is blowing by the piston and putting pressure into the crank case, but the compression is pretty good. Is there any way to remedy this problem? And what is causing it?

 

Check the crankcase breather to make sure that it isn't plugged. The piston as it comes down from TDC to BDC pressurizes the crankcase(what makes the reeds close and pressurize the case and force the mix thru the transfers in 2 strokes, but still applies to 4 strokes). If the breather tube is plugged/clogged/obstructed then the case will pressurize and exhibit what you're seeing.

 

I can't see that there is a breather. I've looked all over the thing and can't find one. It's hard to believe but I think it doesn't have one.

 

Is there a vent in the head cover...

 

I'm pretty sure honda 110 and 90 motors don't have crankcase breathers. I remember seeing aftermarket dipsticks with a built in breather. In fact, try:

http://cgi.ebay.com/ebaymotors/aw-cg...mp;r=0&t=0

I've never used it or seen it used, so I don't recommend it. But I've never noticed built up pressure on my 110 or 90.

 

Believe it or not...I was the high bidder on that dipstick thing AND I got it in the mail today! I'll be trying it out over the next few days and I'll keep you posted as to how it works.

92LT if you still have your 90/110 can you loosen the dipstick so that it is just sitting in the hole and then pull the cord and see if the stick jumps. I haven't checked yet if mine jumps on the compression stroke or the power stroke. It makes sense that there would be back pressure when the piston goes down.
thanks