Compression Test Help Needed

[jbgore]

Going to look at a non-running bike tomorrow morning. Went by Autozone and borrowed a compression tester. Thought I'd try it out on our XS that runs very well (but is currently almost completely disassembled). Pulled plugs, coil is already removed so no need to ground wires, held carb slides open and kicked many times. Highest reading I got was about 50psi in each cylinder.
Based on the target numbers I've seen - 135 -150, I'm wondering what I'm doing wrong. Surely a well-running motor couldn't have only 50psi per cylinder.
I'd like to test the compression on the bike I'm looking at in the morning, but I'd like to get readings that actually mean something.
Any advice would be welcome.

 

[jamesgs4]

You arent going to get good readings using the kicker pedal. Use jumper cables attatched directly to the starter to test the engine you are going to buy. Engines that have been sitting are going to read lower because of the lack of the oil film present on running engines.

 

[BigGeorge]

On the bike you are looking to buy, squirt about 1/2 oz of oil into each cylinder especially if the bike has been sitting a long time. Turn engine over several times to distribute oil evenly on the cyl walls. Hold carb slides wide open. Take reading on each cylinder using the starter motor. Do it several times on each cylinder and record highest readings then compare them. I think the rule of thumb is cylinder readings should be within 10% of each other...

 

[gggGary]

I have kicked a compression gauge to 165 pounds (both sides) on an XS that hadn't run in a bunch of years. Might be my massive 170 pound 6' 3" body though. Helps to leave the other plug out. Watch the gauge, is it leaking down between kicks? My gauge used to do that till I took it apart and put a little oil on all the valves and fittings.
Stick it on your car see what it says?
Typical of sitting XSs is 100 psi more or less, spray some PB blaster in the plug holes and try again. As always both sides close to the same is more critical that super high and there are lots of reasons for low compression that don't involve engine overhauls. bring it up to compression and kick it through slow have a buddy tell you where the air is going.

 

[jbgore]

Thanks for all the replies so far. I'm starting to think it might be the gauge I borrowed from Autozone. I went back out to my good engine, pulled both plugs, screwed in compression gauge, held carb slide open and connected battery directly to starter. It read a big fat 0 on both cylinders.

 

[XSLeo]

Don't worry so much about the slides being up. Hold the throttle wide open, so the butterflies are wide open. The slides will do there thing ok.

 

[olddude13]

Kicked my 8,700 mile find not ridden in years to 150

 

[XSLeo]

The book spec is 145 lbs 10% between cylinders. Any thing above 125 cold and even will run ok. A squirt can of oil so you can do a dry test and a wet test gives you more info.