I have a midnight star 2004 1700 with 60000 mi. I just adjusted the valves and while doing so bought a compression tester from auto zone and checked the compression. I'm not sure how accurate the tester is. With the engine hot psi is 120 in the front cylinder and 118 in the rear. The standard is 175 and minimium is 145 psi. I am not burning any oil and have no compalint with the performance of the motorcycle. Since this is quite a bit below specs I'm not sure what the next step should be?
I have thought about replacing the rings this winter and have looked at the test articles for tearing down the top end of the engine, I don't see that to be a problem for me. I have also thought about installing high compression pistons while I have the engine apart. If I do this is it necessary to replace the ignition system or have any thing done to the valves?
I guess to sum it up with two qustions:
How concerned should I be about the low psi?
If I go with high compression pistons what all else should be done?

Sundance

Sundance
Sure others will get in here in a bit. With that much drop in compression, there should be some kind of symptoms.

I would check the easy stuff first. Are the 2nd plugs in and tight? Is the o-ring on the tester and is it tight? Is the clearance on the valves correct? Too tight could cause this problem.

Sounds like you are familiar with working on engines, so just some third eye suggestions.

jvcexc

sundance, checking the rings sounds like a good idea. Also, I would think if you're going to install high compression pistons you will also need stronger valve springs, and a beefier starter wouldn't hurt, like a VMAX starter, 4 ga starter wires, etc. If I'm wrong, others will point it out soon enough.

Sundance,

I am not sure on this, but I think that you have to hold the throttle wide open when checking the compression. If this is correct, I believe that the carb slide needs to be held open also. Someone correct me if I am wrong.

Stan

You need to disconnect the decompression solenoid.

Doc

Make sure you test wet and dry....here is a good link that does some explaining. Personally, I as long as both cylinders were about equal, it ran OK without burning oil, I'd leave it alone.

http://www.imperialclub.com/Repair/Engine/tests.htm

DocShadow wrote:
You need to disconnect the decompression solenoid.

Doc

+1

I tested before and after adjusting the valves, very little difference in that respect, also checked Hot and Cold, about 5 psi difference there. I did check plugs and O ring on the tester to insure they were sealing properly. I opened the throttle on the carb as well. I did not disconnect the decompression solenoid. Where is it and what is the proceedure? that sounds like something that would cause this issue.

Sundance

sundance wrote:
Since this is quite a bit below specs I'm not sure what the next step should be?
Sundance

What I would do...
Both cylinders spec'ed out that close together and if there were no other issues (burning oil, etc.)... I'd keep on ridin' the snot out of it just like always!

A compression test without dissableing the decompression solenoid is guarenteed to result in low readings! On the right side of the engine right under the air filter a is a kind of triangular cover that covers the cam & cam followers. just behind that cover is the decomopression solenoid cover.

Clifford