Effect of fuel on compression readings

[Strizzo]

Just wanted to give folks an idea of why you should shut off the fuel selector and run the carb out when doing a compression test. I did compression tests on both skis yesterday as an after-first ride check to make sure all was well and forgot to turn the fuel selector off when i did the xp. so i pulled both plugs, hooked up the tester, full throttle, and cranked away. tests went approx - 148, 150, 152, 152. then moved to the pto cylinder, and the first test was 152ish, then 155, 155. hmm? these are higher than the tests i ran a year ago on this motor. Did a couple more and the numbers climbed to right at 160 before i realized that i hadn't turned the fuel selector off. went back to the mag cylinder and the numbers were similar, 155-160.

So i thought i'd post this for people that don't think leaving the fuel on or not holding the throttle wide open will have any effect on your compression readings. if you want to have consistent, reliable, comparable year-on-year compression tests that you can use to track the overall health and wellness of your engine, remember! fuel off, grounded plug wires, both plugs out, wide open throttle, and crank.

I did remember to shut off the fuel valve on the gtx and got consistent 148-150psi readings, just like last year when I bought it.

 

[Sportster-2001-951C-Stock]

Sounds like you've got a scored cylinder on that first one, or an air leak at the crank seal. (just kidding, LOL!)

 

[Strizzo]

yer hilarious :boxing_smiley:

 

[Spimothy Leary]

just got off the phone with a guy claiming 175 on a 787 with a stock head ....
not possible is it ?
well, at least not possible w/o something being wrong right ?

 

[Strizzo]

just got off the phone with a guy claiming 175 on a 787 with a stock head ....
not possible is it ?
well, at least not possible w/o something being wrong right ?

i tested compression while trying to dry out the motor after being flooded, still was spitting a bit of water out, compression showed 170 in one hole on the same engine i'm talking about in my OP

 

[Spimothy Leary]

i tested compression while trying to dry out the motor after being flooded, still was spitting a bit of water out, compression showed 170 in one hole on the same engine i'm talking about in my OP

i didn't think a gasket would push it up that far, you could push it up maybe a few pounds right ?

my first thought was oil in the cyl's ... i'm not a tuner so I normally let others worry about that stuff

 

[Strizzo]

i didn't think a gasket would push it up that far, you could push it up maybe a few pounds right ?

my first thought was oil in the cyl's ... i'm not a tuner so I normally let others worry about that stuff

any non-compressible material in the cylinder will reduce the air space at TDC and increase compression reading. if an engine has been sitting for a long time, the center RV shaft seals could have leaked into the case and cause higher compression readings. in my case it was the seemingly small amount of fuel from cranking wide open in the OP case, and last year when there was still a bit of water in the motor that hadn't been run out that was bumping up the numbers. after i got it cleared out and started, ran the water out the numbers were back at 148-150 like they should have been.

 

[IDoSeaDoo]

I thought you were supposed to leave both plugs in while doing a comp. test...

 

[Dr Honda]

just got off the phone with a guy claiming 175 on a 787 with a stock head ....
not possible is it ?
well, at least not possible w/o something being wrong right ?

Sure.... a thin base gasket, and oil in the cyl. BUT... it won't last long since the squashband is out of spec, and it will detonate. If you have a mod'ed head, and the squashband is correct, but you have a smaller volume dome... it will run good on pump fuel up to 175 or so.

 

[Strizzo]

I thought you were supposed to leave both plugs in while doing a comp. test...

Well you have to pull at least one otherwise youd have nowhere to screw te tester in! I think the reason to pull both plugs is so you don't have as much load on the starter cranking over.

 

[Seadoobuddy]

Well you have to pull at least one otherwise youd have nowhere to screw te tester in! I think the reason to pull both plugs is so you don't have as much load on the starter cranking over.

I think IDOSEASDOO was kidding.....at least I hope so

 

[Seadoobuddy]

just got off the phone with a guy claiming 175 on a 787 with a stock head ....
not possible is it ?
well, at least not possible w/o something being wrong right ?

Maybe he had oil in it.

Just thought of something, when buying a used ski/boat and testing the comp. would it be wise to crank it over a few time with no plugs or tester screwed in, so you can tell if oil was added just before you got there. I assume you could see some spurt out.

 

[Sportster-2001-951C-Stock]

"I thought you were supposed to leave both plugs in while doing a comp. test..."

That's the right way, Strizzo's method is for high performance development engines only.

 

[IDoSeaDoo]

I think having the other plug in the cylinder makes the other half of the engine act like a spring, which is closer to how the engine naturally operates. The air gets compressed and then pushes down on the piston after TDC. This helps the piston you're testing go up a little faster and allows for a more even load on the starter. I think it makes a lot more sense to keep the other plug in.

 

[Sportster-2001-951C-Stock]

I've noticed leaving one plug in is unnecessary as long as the piston springs are in good condition.

 

[Spimothy Leary]

I've noticed leaving one plug in is unnecessary as long as the piston springs are in good condition.

tried that once but the muffler bearings were bad, which was a dam shame I wanted to take the ski snipe hunting that night.

 

[AdamXP800]

tried that once but the muffler bearings were bad, which was a dam shame I wanted to take the ski snipe hunting that night.

Hahaha. lol

 

[Seadoobuddy]

you guys are terrible ...snipe hunting on a ski....bad idea too noisy.....use an electric trolling motor instead

Poor strizzo, IDK if his questions was ever answered

 

[Strizzo]

you guys are terrible ...snipe hunting on a ski....bad idea too noisy.....use an electric trolling motor instead

Poor strizzo, IDK if his questions was ever answered

I didnt' have any questions, i was merely posting this as an example of seemingly small things that could throw off your compression readings, making you think that your squish band or base gasket is wrong, or something to keep in mind when you go look at that ski thats sat for 3 years and "the motor was full of oil, but we got it all out now, should be fine" is most likely giving you compression numbers that are higher than they really are.

a cautionary tale, if you will.

 

[Dr Honda]

I think having the other plug in the cylinder makes the other half of the engine act like a spring, which is closer to how the engine naturally operates. The air gets compressed and then pushes down on the piston after TDC. This helps the piston you're testing go up a little faster and allows for a more even load on the starter. I think it makes a lot more sense to keep the other plug in.

It depends on the engine. With MOST 2-strokes... we remove the plugs.



But... TO ALL... let's keep things on track. (please)

 

[Dr Honda]

I didnt' have any questions, i was merely posting this as an example of seemingly small things that could throw off your compression readings, making you think that your squish band or base gasket is wrong, or something to keep in mind when you go look at that ski thats sat for 3 years and "the motor was full of oil, but we got it all out now, should be fine" is most likely giving you compression numbers that are higher than they really are.

a cautionary tale, if you will.

When I work on an engine that's been sitting... the compression can be way off. High, because of oil... or low, because it's dry. Normally... I will run the engine for 20~30 seconds to help wet, and balance things. Then... check it once it's cool.

If after that... the numbers are out of range... then I look for a prob.

 

[Sportster-2001-951C-Stock]

Yes, the correct way is to start it up and run it for at least a few seconds, shut the fuel off as strizzo suggested. Remove all spark plugs, ground the wires or disable the ignition, open throttle completely, insert compression tester and crank till gauge pressure no longer increases.

Oil or fuel in cylinder will give higher readings, which should be considered false. It engine won't start, I'll crank with no plugs to clear as much oil/fluid as possible before continuing test. If it's dry, I'll put some oil in the cylinder so as not to cause damage. I don't consider a compression test on an engine that hasn't been run recently to be as accurate as one performed after having run. Either/or are better than guessing.

The other ring/cylinder test method is a cylinder leak down test.

Have you guys ever removed the bottom piston ring to see if there's any performance gains? We used to do that years ago on 2-stroke dirt bikes but I never thought much of the idea.

 

[Strizzo]

Have you guys ever removed the bottom piston ring to see if there's any performance gains? We used to do that years ago on 2-stroke dirt bikes but I never thought much of the idea.

what's the advantage in this, reduced friction?

 

[Sportster-2001-951C-Stock]

Yes, reduced friction was the idea. Some claimed it helped get the rev's up quicker. I'd just smile and say "sounds good", twist the throttle and away I went up the hill. Never did it to mine so can't say.

 

[Spimothy Leary]