Compression issues have me a little confused

[lehrin]

OK, so I went today to work on my gs1100e, after having let Marvel Mystery Oil sit in the cylinders for 2 weeks. 2 weeks ago compression was 120-90-70-120 or so. Today, I tested but due to weak battery got very poor numbers, 60s or so. I had a battery hooked up like it was designed to and put a charger set on 50amps attached to it. Then I took the exhaust off, for a couple of reasons (had water inside from sitting for ~2 years and I want to get an aftermarket one anyway). So my buddy comes up with a better setup, we hook the battery up to the charger and then put some big fat jumper cables on it and run these to the ignition and a ground on the bike. All of a sudden I'm getting compression numbers 120+, up to 180 in one cylinder. Is it possible that the exhaust being off gives a false reading? or is it just because I had a better setup for turning the starter? I just don't want to get my hopes up.

 

[saaz]

Exhaust off should not make any difference. You may have some oil still in the cylinders, giving false hig numbers. Try running the bike to clear out the oil then doing another compression test.

The lower figures for the middle cylinders is not unusual as they run hotter, wear more.

 

[skip]

Was this bike sitting long bfore your first compression test? could be the mystery oil freed up the rings and now you are getting better compression. Make sure all the oil is out of the cylinder then check again

 

[Anonymous]

I'm not too sure about the compression but I'm happy for you. As for the battery charger, you are lucky that you didn't ruin your ignition system. Right before I bought my 400 the owner charged it with the battery in the bike and he ruined the regulator/rectifier. Luckily for me he said his bike was a runner when he sold it to me so he replaced the RR without any arguments.

Steve

 

[Dink]

The other thing that your results point out is that you have some wiring issues to deal with, you said that you ran jumper cables direct to the starter etc. and this made a difference? I f so you need to look closely at the condition of your wiring and connections.
Dink

 

[Anonymous]

As mentioned previously, run the engine first. If it has been sitting for a while, it should get run for a good long time at higher RPM's. Compression test should be done with engine warm. Most likely the higher numbers are a result of better cranking speed due to increased available cranking voltage.

 

[Anonymous]

checking your compression -

checking your compression -

Those published "OK Compression" numbers are based on:

1. A warm engine.
2. All spark plugs removed
3. All throttles and chokes wide open.

If removing the exhaust made a difference in the compression readings - it is at least partially clogged and calls for investigation for - mice nests, bees nests, last week's lunch, etc. etc.

stephen

 

[saaz]

Any oil still in the cylinders will give false (high) readings. It needs to ther run and warm, then try again. Watch out hooking up high output power while cranking over.

Hopefully the oil will have given you better compression.