Compression Check

[dover338]

I bought a 78 XS650 Special a week ago. Reading through the forum, it was suggested you check the compression before you move on with any other mods. So, that is what I am trying to do.

I "borrowed" a compression tester from AutoZone.
The instructions on it say to warm up the engine, then take the plugs out. Thread in the adapter and hook it up to the compression reader/dial. Then crank the engine a couple of times to get the max compression

I did all of this, but it I am not getting a reading from the tester. Well, it jumped up to 25psi, but that's it. I did this on both sides with the same results.

What am I doing wrong? The bike drives fine, so I don't think the engine is bad, I am just doing something wrong in the test.

 

[dover338]

Ok, I figured out part of my problem. I was still in gear and I was NOT holding the throttle wide open.

So, I put it in neutral and tried it again with a WOT. The battery had gone dead (another issue I'm fixing with the Banshee upgrade) and the engine had cooled slightly. So, I was kicking it over.

This time the compression jumped up to 55psi-ish on both sides.

What should I try next?

 

[Jack]

You need to prop the carb slides in the upward position to draw in as much air as possible on the compression stroke to force the rings tight against the liner wall when performing a compression test.