Compression Test Question

[cp___32]

Hey everyone,

I've been having issues pulling fuel through the carbs and into cylinders 3 and 4. (1 and 2 are running well).

From what I've read using the search tool I need spark, fuel, and compression. Since I have spark on all 4 cylinders, free fuel flow to all 4 float bowls, the missing piece of the puzzle might be compression. I read that to do a proper compression test I need the bike to be warm. If only two cylinders are firing, will this be sufficient to conduct a proper compression test? Should the test be done with all the plugs out or just one at a time? I have the Haynes manual (I know I should get a copy of the Clymer manual but nobody locally has one so I have to order it) and I read through it but there doesn't seem to be a listing for compression readings. I remember reading that it should be somewhere between 120 and 170 or so (The main concern being that the cylinders are approx even).

While I'm at it, should I need to check the valve clearances what are the specs for the '80 GS 550L? (I don't know if it's listed in the Haynes manual and it might save me another post)

Thanks again!
CP

[Nessism]

Valve clearance spec will be in your manual. Look it up. And yes, you should check the valves, they will burn and damage the engine if not properly cared for.

After you have this done, compression can be checked. Checking cold is fine but the numers will be lower. Should tell you what you need to know though.

Regarding your bike, have you rebuild the carbs yet? Replaced all the o-rings? Removed the pilot jets and held them up to a light to see if the orifice is open?

[Steve]

If only two cylinders are firing, will this be sufficient to conduct a proper compression test? Should the test be done with all the plugs out or just one at a time?

It won't matter how many cylinders are firing, because there should not be any plugs installed.

You test compression by holding the throttle wide open and engaging the electric starter for a few compression strokes.
The only resistance to rotation should be the compression of that one cylinder.

.

[cp___32]

It won't matter how many cylinders are firing, because there should not be any plugs installed.


.

I know that for the actual testing portion you will need to have the plugs out, but since I had read that most people suggested getting the bike 'warm' before conducting the test it's difficult for the two cylinders that aren't working to get nearly as warm as the ones that are working. This could skew the results of the test since the cylinders aren't under the same conditions at the time of the test. That could possibly lead me to make a false conclusion about what is wrong with the bike, so I just wanted to confirm the optimal way to do the test in my situation.

Yes, I have rebuilt the carbs following the CV Carb guidelines. Then checked them again. Checked the float levels, and checked again. Petcock is flowing fuel as needed. Floats flow fuel until full, and draw once levels are low. Everything seems to be good other than I'm not getting fuel up the venturi tubes from the bowls into the chamber. (That's what currently has me stumped)

[hampshirehog]

You can do a cold compression test. As Ed says, your numbers will be lower but not way lower if everything is ok. And the differences between the cylinders will be near enough the same, in proportion.

If your carbs are clean, and I mean really clean, chances are you've got valve clearance issues if you're not sucking fuel in.

[tkent02]

It either has compression or it doesn't, what are your compression numbers?
It does not need to be perfect or even within limits to run.