Compression on a 1984 Big Red

[Hunter79]

My dad and I were checking the compression on my 1984 Big Red last night and when we pushed the starter button, the compression would jump to 20 on the first pull of the motor, then 30, then 45, then 60, then 70, then 80, and SOMETIMES then it would get to 90. We left the compression tester on for about 5 minutes (it would have been longer but it started to rain) and it held the poundage.

My question is: What is the compression supposed to be? Thanks in advance!

It is a 1984 ATC 200ES Big Red.

[Hunter79]

I just found that it's supposed to be between 140 & 170 psi. Is this correct? If so, what in the heck could be wrong with this bike? I have no idea where to start. Thanks again!

[Jmoozy27]

Did you have the throttle wide open. If so, and 90psi is your highest mark then chances are that you need to pull the top end. Although, adjusting your valves to spec may be your first move before you pull the cylinder.

[Hunter79]

Did you have the throttle wide open. If so, and 90psi is your highest mark then chances are that you need to pull the top end. Although, adjusting your valves to spec may be your first move before you pull the cylinder.

I didn't have the throttle wide open. I hooked up the compression tester and then just hit the starter. Am I supposed to have the throttle wide open?

[oscarmayer]

you HAVE to have the throttle open. it cannot suck enough air through a closed slider. hold the throttle WOT and disconnect the spark plug wire also turn the fuel petcock to off and drain the fuel bowl of fuel. Doing all this ensures a proper reading.
I imagine you will get into the 100s somewhere. I would say a "minimum" of 160. any less and you have a lot of wear.