ATC 90 lost compression.

[81ATC185S]

My 1977 ATC 90 lost compression after running for 15 minutes. The timing was good and the valve lash was right and the spark plug is good and seals well. I took the jug off and the rings are good and not stuck. It didnt burn any oil. So I am not sure the problem.

[ATC_Bros]

FIRST check BORE SIZE should be 50 mm or a TINY BIT UNDER like 49.80 or 49.90 for the stock(my 200 is like that)Measure at the top of the jug BUT CLEAN any and all junk in the way first and THEN check the ring end gaps and side groove clearance to see if there WORN, to do this if not sure you take the piston and use a feeler and stick it between the rings and the piston and check the play, Spec should be Top .004 in or .09 mm and second ring .004 in or .09 mm if that's OK take rings off VERY EASILY, don't break them like i did once, put them inside the jug about .5 - 1 cm away from the top(like TDC) and measure the gap with a feeler gauge, specs should be TOP .02 in or .5 mm Second ring .02 in or .5 mm,if that is fine try checking the size of the piston and see if it needs to be replaced use a caliper/micrometer and measure 1 cm from bottom and see if in spec somewhere around 49.90 mm-49.80 mm, im kinda new to atc's only got into them about 3 months ago but already own 3 and im LEARNING QUICK, the FREE service manuals this KILLER SITE provided HELPED HUGE same as with buying i run all numbers before i buy to know what im getting, works to get better prices when you know more about the bike then they do, i hope this info helps and if your close to me i will offer my place and tools to help figure it out if need be.

[John Tice]

You can purchase a quality dial bore gauge on EBay for less than 50 bux. A small price for a precision tool. Instead of you folks always scratching your heads for a solution; learn how to do it yourselves.
See “How it’s Done” this weekend


John Tice
503-593-2908 Alternate 541-508-3944
www.smallenginemachineworks.com & www.nwsleeve.com
Turning Custom Cylinder Sleeves Since 1971

[ATC_Bros]

i use a micrometer and NEVER have i scratched any of the heads i have measured, Maybe its because i'm Careful when i play with such items, i used to Race Nitro R/C and self modded my engines so i have a soft CAREFUL touch.....LOL, good idea to check the out of roundness of the bore though, that CANT be done properly with a micrometer, bore gauge required like stated above for that part of the work