Compression Test Results

[HDKILA]

I did the compression test today and the results were a little strange...

Front Left and Right Cylinder were both 150-160

Rear Left and Right Cyclinder were both 180-190

I can't imagine this to be too good...

The rears seem MUCH stronger than the fronts?

Sorta opposite of an HD...

What do you guys think?

Thanks,
Abe

 

[hubeerjw]

You can try putting a little oil down the front cylinders and see if the pressure comes up... if it does, it is a pretty good indicator that your rings are shot, if there isn't much of a change, your valves may be leaking, or maybe even a head gasket...

Please correct me if I am wrong with this thought process.

Jeff

 

[lankeeyankee]

Was this test performed with the throttle wide open when testing each cylinder?

 

[HDKILA]

Was this test performed with the throttle wide open when testing each cylinder?

Yes. Throttle wide open as Jenna Jamisons mouth.

The only thing I can think of is that I had trouble tightening up the front two spark plug holes with the compression test gauge.

 

[lankeeyankee]

Yes. Throttle wide open as Jenna Jamisons mouth.

The only thing I can think of is that I had trouble tightening up the front two spark plug holes with the compression test gauge.

If you did everything correct
1) Remove all spark plugs
2) Tighten gauge hose into cylinder snug with no leaks.
3) Cranked engine while throttle wide open till gauge didnt increase any.

You have problems of a worn engine..

I wouldnt worry about it unless your blowing oil out the exhaust.

As I said my engine bumped between 155-165 on this 85 engine and it still ran like a raped ape in the start of the yr giving me my lowest 1/8 mile ET. It didnt make as much MPH but it pulled strong throughout the low and mid range.
Perform a leak down test...

Usually places like autozone advantage auto rent them out, all you need is to have access to compressed air. its simple to use... do a youtube search using keywords leak down test.,.. Ya can learn anything from youtube even be a doctor if ya wanted ..

 

[maleko89]

Make sure adapter is tight and battery is fully charged. A weaker battery will make for lower readings. Also, compression test should be done on a warm engine so run it for several minutes before testing.

 

[HDKILA]

Make sure adapter is tight and battery is fully charged. A weaker battery will make for lower readings. Also, compression test should be done on a warm engine so run it for several minutes before testing.

My engine was dead cold when I did this....I mean DEAD cold...sat in an unheated garage all night with temps in the low 20's.

I will try it with a warm apple pie motor.

BTW Mark,

I resent that check to you.

 

[maleko89]

Okay, no problem. Sorry it's been a pain!

 

[HDKILA]

Ok I retested with emphasis on everything being tight and I let the bike warm up for a few minutes...

Results were:

Front Left Cylinder 165-170

Front Right cylinder 170

Back left 190-195

Back right 185-190

So my front two cylinders are still way off the back...

Could it be the bike is overdue for a valve job? It's an 85 with 31K on the odo and I know the valves have never been touched.

Also I only let the bike warm up for a few minutes and it's dam cold out. Would riding it around for 15-20 minutes make even more of a difference?

 

[maleko89]

Abe, valve adjustment is definitely a good idea. Manual says to first check valves and make sure are in spec. Probably a good idea anyway since your covers are leaking if I remember correctly?

It's also possible your rears are higher due to carbon build-up. Two of mine were in the 190 range. Next oil change I used Rislone oil treament. 1/2 quart of this stuff replaces your engine oil. Also ran BG 44K and split that into 3 treatments. Compression came down 20 points.

Shouldn't make a difference as long as engine is up to normal operating temperature. 5-7 minutes should be plenty.

 

[HDKILA]

Abe, valve adjustment is definitely a good idea. Manual says to first check valves and make sure are in spec. Probably a good idea anyway since your covers are leaking if I remember correctly?

It's also possible your rears are higher due to carbon build-up. Two of mine were in the 190 range. Next oil change I used Rislone oil treament. 1/2 quart of this stuff replaces your engine oil. Also ran BG 44K and split that into 3 treatments. Compression came down 20 points.

Shouldn't make a difference as long as engine is up to normal operating temperature. 5-7 minutes should be plenty.

Mark,

I think your dead on about carbon build up. My rear two spark plugs were pretty carbon fouled and my front 2 were nice and brownish like they should be. What do you think?

 

[maleko89]

One more thought. Compression might be lower due to loose/leaky boots.

 

[2stangs69-91]

I forget why are you running the test again? all cyl should be within 10% of each other.

 

[Shuriken]

I've always wondered about compression tests.....

At my elevation (somewhere around 5500ft), I only got around 150-155 on all 4 with a warm engine on my '85. The VMax is a higher comp. engine than the XS1100, but I only saw 120-130 on one of my XS11s and as low as 90-110 on one that smoked. It always seems like the bikes I test have far lower PSI at my elevation. ??

I guess I'd freak out if I saw 185 on my max!