A question pertaining to a compression test

So later today i will be compression testing my xj 550, and i wanted to ask a quick question before i did it. What should i set my fuel petcock to? (Prime, Reserve, On)

I will not be compression testing for probably atleast 4-5 hours because i have class in a little while. Any and all advice is appreciated. Thanks everyone.

 

I'd set it to ON and slip the vaccum line off. You don't need the fuel since you're not sparking. Just make sure your throttle is wide open and your battery is good and fully charged.

I did mine with the carbs off but that's because I was cleaning them. I got between 151 and 156 psi in all 4 on my 81 650.

 

Alright. Thank you gameover. Will have to get out my manual and find the vacuum line

 

Fuel should be "on" , but doesn't matter.

What matters is - - all plugs out, IGN disabled by pulling the plug on the TCI box, throttle held open, battery boosted using a 10/2 charger, or jumpers to a non-running car. This gives the highest readings possible.

Let the compression gauge take 8-9 "hits" or until the needle stops moving.

If you follow up with a "wet" test using oil, use the EXACT same amount of oil in each hole (about 15 CC). No improvement indicates bad valve seating.

 

Is it the smaller plug on the TCI or the larger plug? And i am also not sure what you mean by IGN. Ignition maybe?

 

Yes, IGNition.
I think the smaller plug feeds the power, but go ahead and pull them both.

 

Great to know. Thanks timetoride and gameover for your quick responses. The biggest reason why i keep coming back here is because of the quick response time.

 

The vaccum line is the small line running from the petcock to the vacuum nipple on the #2 intake boot (carb-to-head.) By disconnecting that, you keep the petcock from feeding fuel when you're spinning the motor.

You DON'T want to put it on PRI.