Hello, I am new here.
I just bought a atc90 and a 110. They are both in terrible shape but I got the 90 running. The 110 needs rings, clutch and the shifter moves but it doesnt shift. So I probably am not putting any money in it. The 90 has been painted very poorly and the fiberglass fender cracked almost compeletly off. It starts first pull and it runs very strong and seems to have compression but it smokes worse then a two stroke. The rear grab bar gets oil on it from the smokin. I am kinda puzzled with the smoke because of the power and compression it has. My brother has a nice running 110 that the 90 has no problem keeping up with and the compression on the 90 seem stronger. It also seem to have more grunt getting over logs then the 110. The 110 dont smoke. The 110 does have 22 inch tires and the 90 has 21" but I dont think that would take that much grunt from the 110. It could be just the oil ring is bad but would seem weird. Does anybody know how to determine if it is a ring problem or valve seal problem? Others possibly problem. Thank you

 

Get a compression gauge, and do the test on a dry cylinder. Then put a bit of oil in the spark plug hole,(this helps the rings to seal better), then do another compression test. If the second test is better, then that amount is usually due the the rings. If theres no difference, then its probably the valves.

 

Well did compression test, 100lb without oil, 150 with. Took motor apart yesterday and rings look good. The piston was skuffed around skirt but didnt think it was bad enough to smoke like it did. Then I measured piston, 53.34mm. Bore was 53.95mm. Stock is 50. I think somebody bored it and through a piston in it. Neither was that worn looking and it has little more then .50mm clearance. There wasnt even a ridge on top of cylinder. I dont know the tolerance but that seems like alot to me. I didnt think it could be bored out that much either and cylinder is very thin. It is a 90 cylinder because it says 89cc on it. Hopefully it is usable yet. Taking it in to machine shop tuesday I hope.

 

After researching found out that it can be bored out to 53.95 but that is all I found on that. Dont know about what kind of piston to use.
Also found out if bored out to 54mm can use a honda xr100 piston with 4th oversize and it will work. That would bring it to a 104.5cc. Just thought I would post it for those who have a honda atc90.

 

Does your 90 have the original balloon tires on it? If it does, and they hold air, they are worth a lot of money.

 

I the cylinder didnt have a ridge, why not just get it deglazed and run a new set of rings....Also check the valve guides while you have the head off and be sure to replace the valve guide seals! If the valve seats look iffy...then I always lap them in while it is apart!

 

Hey Thrash,

When you say deglazed, could you just hone the cylinder and put new rings on? Do you think that would be sufficient?

 

yea.....its the same thing [img]i/expressions/face-icon-small-happy.gif[/img]

 

The piston is not the proper size. Somebody just through it in I guess. So I need piston rings. I just got a quote at about 130 for piston and another 30 for rings. Ouch.
The later models didnt have the balloons. Mine are regular knobby.

 

that price is pretty steap..........I just did a 110 a few weeks ago.....the bore job was $45.....and the piston/rings/pin/clips was $88