Compression test- What do ya think?

[Jaybird]

Hello all! I just about got my 80 XS11 going. I never saw the bike run, previous owner said it was "strong" Compression test with rubber-tipped 'hold-in' type guage, throttle open:
135-140-145-155
Bike has 76K mles. Should I be worried? To me, it looks nice and high, but the spread is troublesome. I'm gonna run it and see. Any thoughts?

[sixtysix]

According to the manual the compression test sould be done on a warmed up engine, throttle open, spark plugs removed. If the test was done that way then 145-155 is 'good compression'. I am not looking at the manual right now so this is from memory but I beleive that the allowable spead is 15% from highest to lowest. 15% of 155 is 23psi so I would say that you were OK. BUT I have known list members that have run their bikes at well over this 15% figure (me included) with no ill effects. I beleive that running an engine with a large compression spread gives it an unbalanced feeling and puts uneven stresses on certain engine bearings, so longevity would be compromised.

Bad compression is going to be in one or both of these areas, piston, rings or bore wear and valves One way to check is add a couple of squirts of oil (1-2 tablespoons) to the cylinder with low compression. Let it sit for a minute to allow the oil to coat the sides of the cylinder and piston. If there are any gaps in the rings, the oil will temporaraly seal them.

Check compression again by cranking the engine, remembering to open the throttle. If the reading improved significantly (more than 30 psi), your rings are probably worn. If there is little or no improvement, you could perform a leak-down test to determine where the compression is escaping (intake, exhaust, crankcase, gasket, etc.)

If it is valves, it could be as easy as adjusting the valves per the tune up specs.

Anyway like I said yours is fine!

[Jaybird]

Thanks Gary! You confirmed what I had thought about the engine being OK. remember 10% being the figure from high school auto shop 20+ years ago! That's probably for a big V8 though. Engine was cold, hasn't run in over a year, maybe 2, and I bet numbers would be a little better with a screw-in type guage. She's gonna run fine!...soon as I get the sparks going. Thanks again.

[TopCatGr58]

Originally posted by Jaybird
Thanks Gary! You confirmed what I had thought about the engine being OK. remember 10% being the figure from high school auto shop 20+ years ago! That's probably for a big V8 though. Engine was cold, hasn't run in over a year, maybe 2, and I bet numbers would be a little better with a screw-in type guage. She's gonna run fine!...soon as I get the sparks going. Thanks again.

We must have attended the same shop class....that and the manual also states less than 10% variance across cylinders. It also states that COLD, it should be 142 psi at sea level, plus or minus 14. After I put a big bore kit, it must have increased my compression, because after breakin, I was running 175-185 psi!!

BTW, 9.2 to 1 comp, at 14 psi for sea level...multiplied to each other is: 128.8....apparently a little difference, but close!

[Mike Hart]

Atmospheric pressure is more like 14.7 psi, so 180/14.7 = 12.25:1 YIKES. You running race fuel, TC?

and 14.7 x 9.2 = 135psi

[TopCatGr58]

Originally posted by Mike Hart
Atmospheric pressure is more like 14.7 psi, so 180/14.7 = 12.25:1 YIKES. You running race fuel, TC?

and 14.7 x 9.2 = 135psi

I thought the 14.0 didn't sound right, but it was late, and didn't bother checking my Encyclopedia!!!! Not running Race, but definitely Premium....93 octane.

Tomorrow looks to be a nice but HOTday, I think I will fire Godzilla up, and then recheck those comp levels now that it's got a couple k-miles extra on it from the XSEast run, to see where it's at !

[TopCatGr58]

Still Pumping Strong

Hey there Mike,
Well, got lazy and just did a cold compression check, although Gary says that it should be done warmed up, which I believe it to help it seal better....pistons/rings expanded??

Anyways...still running 180, 180, 170, 180,(Sea Level) so looks like I'm actually averaging at least 12:1 comp ratio with this big bore kit. Daaaaayaaaaaam!! I had no idea that it would boost it that much, since I'm still running stock cams, heads, didn't even shave the head.

So this brings me to my next qaundry, which Ken Krieger has also been wondering. I went to Indy filters, have 4-1 pipes, and stock main jets of 110. Have no fuel starvation affects at all. Has the increased Comp ratio allowed me to get more bang out of what would/could/should have been a more leaner running system with these breathing mods??? The jetting guidelines state that I should have had to go up 3 sizes. Hey, I'm happy it's running great, just pondering.

[Mike Hart]

T.C., you say your plugs are tan, so that's a good sign that you don't have to change anything.

Maybe the higher compression is the key ingredient here? Does anyone have experience with milling a head for higher compression and how the fuel mixture behaved as a result?

I'm guessing that a smaller combustion chamber would require relatively less air/fuel mix, so all other things being equal, you could keep the same mains and provide more air, which is kinda what you're saying, right?

I'm about to go to individual filters on my XJ, which according to Denny's jet tip, says 3 sizes bigger. Then reduce by one (formula), then by another for mismatch (in this case, stock exhaust, leaving me with a possible increase from 112.5's to 115's. I'll observe the plug color, see if there's any change.

My bike has always run well at altitude (Mt Rainier, Mt Baker, Mt Hood), suggesting it may be on the lean side, but the plugs are a medium tan.