Puzzling compression test

[11Rider]

Hi my pals. My '79 Special has 7,300 miles on the odometer and the rest of the bike looks like it's accurate. I did a compression test but I did it with the bike cold. I later read in the manual that it's supposed to be warm. My readings were 180 PSI on cylinder number one and 170 PSI on the other three. The manual states 142 +/- 18. Even at the high end of the spectrum I'm 10 to 20 PSI too high. Could doing the test cold account for this being that the cylinders would be a little smaller???? If not, what do you think?

[bikerphil]

Maybe the previous owner put in a big bore kit. It could also be carboned up too. Possible your guage is not accurate?

[natemoen]

I wouldnt worry about it really. As long as they are close at this point just get it running and after you start putting some miles on it then check again and see what it is.

[11Rider]

Maybe the previous owner put in a big bore kit. It could also be carboned up too. Possible your guage is not accurate?

Hadn't thoght of a big bore kit or high comp pistons, but it seems to have the same power as my first 11 and everything else is just about completely stock.

Guage is a nice one and brand new, but still can't rule that out - good thought.

I wouldn't think it would be too carboned up with 7k miles on it and apparently stock jetting (cold blooded at start up).

PS Bikerphil, I have my own version of your tag line "If it ain't broke, I can break it".

[natemoen]

PS Bikerphil, I have my own version of your tag line "If it ain't broke, I can break it".

Sometimes modifying turns into breaking! Or at least for me it does!

[skids]

You cant complain about that compression! If it knocks on regular, use higher octane. If not, just ride t!

[11Rider]

You cant complain about that compression! If it knocks on regular, use higher octane. If not, just ride t!

Thanks, I was thinking the same thing.

[b.walker5]

I wouldnt worry about it really. As long as they are close at this point just get it running and after you start putting some miles on it then check again and see what it is.

No offense Nate but Sorry, I believe this to be bad advice and completely disagree. 170 to 180 psi is too high for a stock 11 engine and unless you know it's been bored and has high comp pistons you need to find out why it's so high.

If the compression in a gasoline engine is too high, it can cause a problem known as pre-ignition or detonation. This can be very destructive, causing damage to internal parts of the engine.

Do the test again following the steps below (summerised from a mechanics trade training manual)

1. Warm your engine up properly. a cold engine will NOT give you a correct test.
2. Disable the ignition circuit (leave the kill switch off)
3. Hold the throttle wide open to ensure the engine gets an adequate air intake.
4. Crank the engine continually for at least 5 to 10 full revolutions to obtain an accurate reading.
5. Record and compare all readings and compare with the manufacturers specifications. If readings vary more than 10% from spec, or each other, then more detailled inspection and testing may be required.

Also from the same manual...

When the compression is found to be too high in one or more cylinders, this would be an indication of excessive carbon build up in the engine. This can only be corrected by performing a chemical de-carbonizing process on the engine (specialized service), or by removing the cylinder head(s) and physically removing the carbon that is attached to the cylinder portion of the head(s) and the tops of the pistons.

[11Rider]

I definitely plan on doing the test again after the engine is warmed up. If it is carbon, does anyone know if the Yamaha combustion chamber cleaner works, or of any other method short of disassembly??

[skids]

Gages typically vary right out of the box (i,e, not well calibrated). Maybe you can borrow a gage and do a spot check...your reading against the other.

There was some talk about misting water into the carb inlets with them near wide-open throttle to burn off carbon. You can also run them on the higher rpm range to burn it while riding. There are gasoline additives you can use also and they will turn your oil black right away. I am thinking you would hear pinging or detonation. I always thought detonation was a timing issue rather than autoignition due to high compression.

I think that bigger bore would not create different compression unless the pistons are different than stock with crowns consuming more of the volume at TDC. Maybe the kits do that, I don't know. Otherwise compression would not change much.

[DiverRay]

A spray can of Berryman's B12 Chemtool will clean the carbon as well. Remove the lower part of the air cleaner once the bike has been ridden for at least 10 miles to warm it up, and use the tube to spray into the carb intakes one at a time while keeping the bike at about 2500 RPM. Once you have all four done, shut down the bike and let it set for about 20 minutes. Take it out for a ride after that, with the air cleaner back together, and run it up to 7 or 8 grand in third gear once or twice. That will clean most of the carbon out.

[natemoen]

No offense Nate but Sorry, I believe this to be bad advice and completely disagree. 170 to 180 psi is too high for a stock 11 engine and unless you know it's been bored and has high comp pistons you need to find out why it's so high.

If the compression in a gasoline engine is too high, it can cause a problem known as pre-ignition or detonation. This can be very destructive, causing damage to internal parts of the engine.

Do the test again following the steps below (summerised from a mechanics trade training manual)

1. Warm your engine up properly. a cold engine will NOT give you a correct test.
2. Disable the ignition circuit (leave the kill switch off)
3. Hold the throttle wide open to ensure the engine gets an adequate air intake.
4. Crank the engine continually for at least 5 to 10 full revolutions to obtain an accurate reading.
5. Record and compare all readings and compare with the manufacturers specifications. If readings vary more than 10% from spec, or each other, then more detailled inspection and testing may be required.

Also from the same manual...

When the compression is found to be too high in one or more cylinders, this would be an indication of excessive carbon build up in the engine. This can only be corrected by performing a chemical de-carbonizing process on the engine (specialized service), or by removing the cylinder head(s) and physically removing the carbon that is attached to the cylinder portion of the head(s) and the tops of the pistons.

I wont disagree with your method because that is the way to check compression and that is how I do it. But when I first got my XS I did have very high compression much the same as the OP. I did NOT have any pinging from the high compression and it was not causing a problem, and I did not have any carbon build up inside the engine (I have had the heads and the jugs off and I can verify that), and I also do not have a big bore. After sitting things get stuck and clogged. After being run a little they most always break free and become loose again, and that's why you recheck after you have run it some. I am not saying to leave it that way indefinitely I am just saying don't go to any extreme measures right off the bat with it.

So I will again say, Ride it. Do the test on the warm engine with wide open throttle see what you get and then RIDE IT. Recheck the compression after 50 miles, 100 miles. See what happens. I highly doubt you have much for carbon inside the cylinders with only 7300 miles on it (could be a different guage set though so there could be more miles on it that what is shown could even be 107300 ).

[11Rider]

Thank you everyone for taking the time to give your advise. I have plenty to work with now and will put this on my "to do" list for the 11.

[ManagerMike]

I must ask first, why are you performing a compression test, does it not run well? Does it have a dead miss that you cannot attribute to lack of fuel or spark?

Compression tests are used to provide information regarding the ability of the engine to ingest and compress air, nothing more. It is a diagnostic, not a health test. It is used to determine if a mechanical failure condition exists (bent valve, broken ring(s) etc..) after you have eliminated fuel or spark as being the cause of whatever performance problem you are trying to diagnose. (there is no need to test compression on an engine that runs well)

Over and over have I seen 'gearheads' get a 'new' (used) engine and perform a compression test on it, get all bent over the results, and chase their tails for months over a percieved problem that just does not exist!

Also, the numbers mean essentially nothing as absolute values, they are only usefull when compared to one another on a cylinder by cylinder basis.
i.e., if you have a dead miss, fuel and spark seem to be good and a compression test shows that cylinder to be at, say 80 psi and the others at 150, 160, and 180, you HAVE found a problem. Addind oil to that cylider and rechecking can provide more info: if the reading goes up significantly, the issue is with the rings, if it does not, the valves are suspect.

The readings you posted mean nothing unless you are trying to solve a specific problem, too many variables in the testing equipment and testing methods. Also, the exact minutae of how the test is performed is of little relevance, so long as you reproduce the testing method faithfully for each cylinder, again it is a compairative test and not an absolute test.
Remember the KISS principle. These bikes are pretty simple, keep it that way.

[b.walker5]

I must ask first, why are you performing a compression test, does it not run well? Does it have a dead miss that you cannot attribute to lack of fuel or spark?

Compression tests are used to provide information regarding the ability of the engine to ingest and compress air, nothing more. It is a diagnostic, not a health test. It is used to determine if a mechanical failure condition exists (bent valve, broken ring(s) etc..) after you have eliminated fuel or spark as being the cause of whatever performance problem you are trying to diagnose. (there is no need to test compression on an engine that runs well)

Over and over have I seen 'gearheads' get a 'new' (used) engine and perform a compression test on it, get all bent over the results, and chase their tails for months over a percieved problem that just does not exist!

Also, the numbers mean essentially nothing as absolute values, they are only usefull when compared to one another on a cylinder by cylinder basis.
i.e., if you have a dead miss, fuel and spark seem to be good and a compression test shows that cylinder to be at, say 80 psi and the others at 150, 160, and 180, you HAVE found a problem. Addind oil to that cylider and rechecking can provide more info: if the reading goes up significantly, the issue is with the rings, if it does not, the valves are suspect.

The readings you posted mean nothing unless you are trying to solve a specific problem, too many variables in the testing equipment and testing methods. Also, the exact minutae of how the test is performed is of little relevance, so long as you reproduce the testing method faithfully for each cylinder, again it is a compairative test and not an absolute test.
Remember the KISS principle. These bikes are pretty simple, keep it that way.

Whilst I agree with your reasoning on most of this, and I'm one of those 'gearheads' that have been inside many engines, mostly V8's, over the last 35 or so years, I dont agree that the readings posted mean nothing. Readings considerable higher than the manufacturers posted specs should not be ignored. Certainly do the test again with a different guage to rule out guage inconsistancies, but if still high then it should be investigated further.

I also dont completely agree that compression tests are a diagnostic only and not a health test, as worn rings can also give a low test, and you've covered that with your oil test method. Oil in the bore will only expose worn rings, not broken ones and, as you've indicated, burnt valves.

I do agree that sometimes testing without reason can be fraught with unnessacary heartache, however it can, and often does provide useful information helping to paint an overall 'health' picture of the engine.

It doesnt happen any more but there was a time when compression tests as part of periodic tune ups was the norm.