How Much Compression?

[MPittma100]

I checked my compression and had 100 PSI in all four cylinders. Yamaha service manual says: Standard is 142, Minimum is 128, and Maximum is 156.
seems to run fine at 100, but am I missing something?

I have had the cylinder head off to do a valve job. Cylinders looked good - still had the hash marks on them.

Cylinder leakage test is 10%-12% which is normal.

Is anything wrong here?

MP

[jwhughes3]

If its running fine maybe gauge is off?

If it was me, I would not worry unless I got wildly dissimilar numbers.


The Sun is slowly dying, but it burns OK for now so lets worry about burnout later


John

[Rasputin]

I guess I will be the first to ask...was the engine warm or cold when you did the test? Did you have the throttle wide open or closed when you did the test? Were the sparkplugs on the other cylinders in or out when you did the test? Is the battery fully charged or was it when you did the test? Finally how long since this engine has seen a good number of road miles?
Just stuff that we like to know so we can answer in a more informed way.

[DGXSER]

I know that mine is also in that 100 PSI range, on a warmed up engine, plugs out, WOT, well charged battery, and has about 5k miles in the last couple years.

I also know that it runs pretty well, and definitely has more power than I need. So, I do not plan to pull the head or give it new rings any time soon. JMHO

[ManagerMike]

If it was me, I would not worry unless I got wildly dissimilar numbers.

Agreed, the absolute numbers are less important than consistant numbers. The numbers you get can be different from the manual for a variety of reasons.

What kind of trouble are you having that led you do do a compressoin test?

[WMarshy]

Agreed, the absolute numbers are less important than consistant numbers. The numbers you get can be different from the manual for a variety of reasons.

What kind of trouble are you having that led you do do a compressoin test?

According to the manual, compression testing should be done on a regular basis to provide a baseline and possible trend in degrading performance which in return will drive rebuilding before catastrophic failure...

[MPittma100]

Compression Conditions

Engine was warm, throttle closed, all other plugs were out, and battery was fully charged. Bike has same compression whether it has been ridden or not.

The bike runs fine except that it is cold natured. What compression numbers have some of you gotten from your engines?

MP

[DGXSER]

The throttle being closed is your problem. You need the throttle wo be fully open when running compression check. I am willing to bet your numbers will go up to the 140 or so mark if you do that.

[crazy steve]

...throttle closed....

You'll get low numbers with the throttle closed, quite a bit low. You really have to hold the throttle open (or remove the carbs) to get accurate readings. As to the 'factory' numbers, take those with a large grain of salt. I've seen numbers all over the map, but from my experience a 'good' motor should be in the 150 psi + range, and don't be surprised if you see as much as 200 psi; typical readings seem to be in the 180 psi area. There's a lot of factors that can effect compression readings, but I've found that properly adjusted valves can make a huge difference. My most recent project checked at 130/90/60/130 when I first got it, after a valve adjustment the numbers came up to 180/180/160/180.

But if you're getting even readings (within 10% of each other or close) and the bike runs good without smoking, don't worry about it...

[natemoen]

My engine started out with 80-90psi. Now its around 140-150psi.

[airborneXS]

compression check: engine warm, plugs out, tci unplugged, ive put around 5k miles on it this year, the only thing i forgot to do was open the throttle, 149/150/155/155.....