Compression test--what is the procedure

I an looking at buying a 82 XJ550. It is all stock, and a two owner bike. The present owner has had it for 20 years and it has less than 12,000 miles and looks to be in excellent condition--at least in the pictures. I want to do a compression test before buying it. I have a compression gauge and I know that I need to do something with the coils or TCI to avoid damaging them during the compression test.

Can I simply ground the plugs to the engine, or do I need to actually disconnect the coils? If so, where do I find the disconnect point?

 

Just grounding the plugs is fine, have a fully charged battery & hold the throttle wide open, you are looking for equal psi across all 4.

 

Thanks. Do you ground them bu just laying them on the head or do you use some sort of grounding clamp/wire?

 

It is easier to just unplug the TCI unit. Otherwise make sure the plugs are touching metal to allow the spark a ground path. Failure to ground the plugs could lead to the coils back feeding to the TCI causing it to blow.

Make sure you hold the throttle wide open when turning the engine over. You will get a faulty ready if you do not.

Ghost

 

Like I indicated, I'm not very confident on the electrical system. Where do I find the TCI?

 

On the 550s, it's under the seat (see pic below.) It's the box w/the blue label on it just above the fuse box. Unplug the two connectors.

***NOTE*** The 550s use 12mm spark plugs; you will probably need an adapter for your compression gauge.

 

Thanks for the picture. A picture is worth a thousand words. Also thanks for the heads up on the 12 mm adapter. Fortunately, my gauge came with one.