Interpreting Compression Test Results

[Barnburner]

I have a couple of questions about the results of a recent compression test of my CL350 that I was hoping you guys might be able to help me with. Luckily, my wonderful wife also came along with a ASE certified Father-in-law, who very very graciously let me "borrow" this awesome Snap-On Compression Tester Kit. Though this is the first time I've had the pleasure to test a bike's compression, I'm sure it won't be the last!


Compression Tester Kit by RStutzman, on Flickr

Here's the tip I ended up using. It most closely matched the threading and length of the plugs and had a washer, which I figured was necessary since the B8ES has a crush washer already.


Compression Tester Fitting by RStutzman, on Flickr

Anyway, here's where my question comes. The following show the results of my test: the left cylinder had a PSI of about 175 and the right was at 195. This was on a warm engine after about 6-10 kicks with the throttle fully open and both plugs out.


Left Cylinder Compression by RStutzman, on Flickr


Right Cylinder Compression by RStutzman, on Flickr

After my initial relief that they were both above the minimum of 150 I remembered that the rated compression was supposed to be 170. I understand that the left cylinder being 5 PSI over is probably the result of carbon deposits, but what would make the right be +25 over? That seems like a really high number! For the Dr. House types out there, other symptoms on the right include running rich and slight overheating.

Thanks in advance for any thoughts/answers/advice.

 

[crazypj]

If it's running rich on one side only, you will have more carbon build up on that side.
Carb sync is probably out a bit
If valve adjustment isn't exactly the same on both sides I've seen a 20psi difference (bit wide on the 'high' side, valve closes early and traps more air at low rpm when your kicking it)
Set them all at 0.003" intake and 0.005" exhaust
Then use a 0.004"in and 0.006"ex to make sure you didn't get them too big and a 0.002"in. 0.004"ex to make sure they are not too small

 

[Barnburner]

Thanks PJ! I knew a valve adjustment was something I'd have to get around to eventually but I had no idea it could throw my compression off that much.

 

[crazypj]

Do a valve adjustment first, check cam chain tension (easier to check it adjusts with valve cover off)
Full service, get carb's synced and toss some fuel injector cleaner in tank (about 1/4~1/3 bottle)
It's a good idea to use 89~93 octane with injector cleaner
That should help clean some of the carbon out.
Air cleaner should be washable foam, use dish soap and squeeze don't wring it (it will break up) make sure there is nothing under seat blocking inlet (I've seen gloves, waterproof trousers, rags, etc restricting intakes)

 

[AlphaDogChoppers]

Just an FYI, the throttle isn't really "open" when you have CV carbs. The slides will still be closed unless you stick something in there to hold them open. It's probably why it took 10 kicks to get your max reading. If the intake is open, pressure will usually peak in four strokes.

 

[Barnburner]

Just an FYI, the throttle isn't really "open" when you have CV carbs. The slides will still be closed unless you stick something in there to hold them open. It's probably why it took 10 kicks to get your max reading. If the intake is open, pressure will usually peak in four strokes.

Thanks, ADC! Do you think this could have affected the readings? Also, I'm a little confused about what you mean by sticking something in there. Are you talking about the buttlerfly valves or the diaphragm/slide assembly? If it's the latter, how would i prop that up?

 

[AlphaDogChoppers]

I'm talking about the diaphragm slides. You can stick something like a popsickle stick in there to hold the slide open a bit. A piece of wood shim, a piece of thin plastic sheet. Just about anything that doesn't block the airway.

It's not that big of a deal, but you will see a BIG difference in the number of strokes it takes to get a reading. I wouldn't worry about it too much. It doesn't invalidate your results, I'm just being a know-it-all. <G>