LM3 Cyl. Compression TEST?

[GuzziFuria]

Ciao Tutti,

Did a compression test on a 83 LM3 with 60K miles, cold engine, carbs, headers, plugs removed and valve clearance checked...

Right Cyl. 170 PSI
Left   Cyl. 150 PSI

What suggestion would you go with, to make up for the 20 PSI difference?  replace cyl. rings, valve seating,..., or simply go with a new piston/cyl. kit L & R??

Molto Grazie
Carlito

[yogidozer]

Instead of guessing, do a leak down test. Watch this
https://www.youtube.com/watch?v=crzl7J6OSAc

[lucian]

Try putting a teaspoon of motor oil in each cylinder plug hole , crank motor over once and then repeat compression test . If numbers improve significantly I is leakage past piston rings. My guess is it's more likely to pittiing or carbon build up on exhaust valves. Maybe a leaking head gasket. A leak down test will be very useful in pinpointing the source of leakage. 60,000 isn't a lot of miles on that motor if it's been treated, maintained properly.

[moto-uno]

  If you're really concerned , I too would suggest a leak down test . It's a no brainer , and a treat ( in a strange way )
to put your ear near the exhaust pipe and listen for the sound of rushing air to verify what's amiss . Or your ear to the
carbs or to the dipstick hole :) . Peter

[John A]

I once pulled a cylinder off my Guzzi only to find the defect was the Schrader valve on the compression tester intermittently sticking. Just saying…

[GuzziFuria]

Instead of guessing, do a leak down test. Watch this
https://www.youtube.com/watch?v=crzl7J6OSAc

Good stuff...  Thanks

[GuzziFuria]

I once pulled a cylinder off my Guzzi only to find the defect was the Schrader valve on the compression tester intermittently sticking. Just saying…

Good point...  Had the same once, when oil in cylinders contaminated the Schrader valve, rendering completely useless..

[GuzziFuria]

  If you're really concerned , I too would suggest a leak down test . It's a no brainer , and a treat ( in a strange way )
to put your ear near the exhaust pipe and listen for the sound of rushing air to verify what's amiss . Or your ear to the
carbs or to the dipstick hole :) . Peter

Absolutely will go with it

[GuzziFuria]

Try putting a teaspoon of motor oil in each cylinder plug hole , crank motor over once and then repeat compression test . If numbers improve significantly I is leakage past piston rings. My guess is it's more likely to pittiing or carbon build up on exhaust valves. Maybe a leaking head gasket. A leak down test will be very useful in pinpointing the source of leakage. 60,000 isn't a lot of miles on that motor if it's been treated, maintained properly.

Kool tip, will also give it a try... Thanks

[guzzisteve]

I would clean the carbs & ride it, it's close enough not to worry, mine had 180 on warm motor at 50K. Do a leak down if really concerned, it may change if you ride it a while may not.

Most likely valve cut will bring it back, it is a square motor w/Nickasil. UNLESS it's been running intake stacks w/no filters.

[GuzziFuria]

Try putting a teaspoon of motor oil in each cylinder plug hole , crank motor over once and then repeat compression test . If numbers improve significantly I is leakage past piston rings. My guess is it's more likely to pittiing or carbon build up on exhaust valves. Maybe a leaking head gasket. A leak down test will be very useful in pinpointing the source of leakage. 60,000 isn't a lot of miles on that motor if it's been treated, maintained properly.

Could see through the plug hole, lots of carbon build up on pistons, as you mentioned very good chance it's the cause...
She had 3 previous owners, always well maintained, service schedules strictly followed, always garaged and best of all no one tinkered with her, untouched.  Just the rubbers are all cracked, will b replaced and the U-joint had too much play on one end, will also replace...
Overall I think no one did major work on her, simply followed the service schedules closely and she was never abused or driven hard on the road...  All stock, even with the OEM air box, and seems to hardly have ridden in the rain...
A true Gem

[Chuck in Indiana]

Could see through the plug hole, lots of carbon build up on pistons,

Ok, here's an old aircraft mechanic's trick. While you have the cylinder on TDC and the leak down tester turned on, rap the rockers with a copper hammer a few times. You just may see black smoke out the exhaust, and compression come up when the carbon on the valves is dislodged.

[TOMB]

Just pointing out,

The Guzzi engine has two diffrent manifold pressures,

With the engine warm and running at idle pull off one spark plug listen to how the sound of the pulse sounds repeat that on the opposite side you'll notice that the sound has a different pulse that's the  diffrent manifold pressures attach  manometers on a carburetors at idle they would not be even if you run  them up half way they start to balance themselves out just pointing out you may be chasing a rabbit down the hole that doesn't exist

TOMB

[SED]

Is there any other issue besides compression difference? Dark plugs? A dog off the line?

At 40,000 miles my LMIII had a little better compression than yours, but the intake valve guides were absolutely shagged.  I could push on the valve spring retainer and move the intake valve stem side to side - especially on the left (which also had lower compression). There was no detectable play in the exhaust valves - only the intakes.  Replacing the guides and grinding the seats made a big difference in the way it runs - especially the idle to throttle slide transition.

Your bike sounds like a good one - congrats.

[GuzziFuria]

Ok, here's an old aircraft mechanic's trick. While you have the cylinder on TDC and the leak down tester turned on, rap the rockers with a copper hammer a few times. You just may see black smoke out the exhaust, and compression come up when the carbon on the valves is dislodged.

Good stuff, thanks!! 

[GuzziFuria]

Just pointing out,

The Guzzi engine has two diffrent manifold pressures,

With the engine warm and running at idle pull off one spark plug listen to how the sound of the pulse sounds repeat that on the opposite side you'll notice that the sound has a different pulse that's the  diffrent manifold pressures attach  manometers on a carburetors at idle they would not be even if you run  them up half way they start to balance themselves out just pointing out you may be chasing a rabbit down the hole that doesn't exist

TOMB

Good point, will keep it in mind...  Grazie!

[GuzziFuria]

Is there any other issue besides compression difference? Dark plugs? A dog off the line?

At 40,000 miles my LMIII had a little better compression than yours, but the intake valve guides were absolutely shagged.  I could push on the valve spring retainer and move the intake valve stem side to side - especially on the left (which also had lower compression). There was no detectable play in the exhaust valves - only the intakes.  Replacing the guides and grinding the seats made a big difference in the way it runs - especially the idle to throttle slide transition.

Your bike sounds like a good one - congrats.

Very interesting will check it out...  Probably will go for a cyl. head job...

Thanks...  Yeah I always try to look for "untouched" Guzzis round 20-30K miles, hard to find...  Have seen it all what people do to them and all their mistakes...  It is my passion to bring back the Guzzis to correctness without cutting corners...  The LM3, am totally disassembling for deep cleaning and replacing what needs replacing...  Then will concentrate on the mechanicals...

Grazie

[Rough Edge racing]

Ciao Tutti,

Did a compression test on a 83 LM3 with 60K miles, cold engine, carbs, headers, plugs removed and valve clearance checked...

Right Cyl. 170 PSI
Left   Cyl. 150 PSI

What suggestion would you go with, to make up for the 20 PSI difference?  replace cyl. rings, valve seating,..., or simply go with a new piston/cyl. kit L & R??

Molto Grazie
Carlito

Its better to do a compression test n a warm engine soon after shut down...If you think the engine runs well don't worry about it..

[GuzziFuria]

Try putting a teaspoon of motor oil in each cylinder plug hole , crank motor over once and then repeat compression test . If numbers improve significantly I is leakage past piston rings. My guess is it's more likely to pittiing or carbon build up on exhaust valves. Maybe a leaking head gasket. A leak down test will be very useful in pinpointing the source of leakage. 60,000 isn't a lot of miles on that motor if it's been treated, maintained properly.

Tried it, no improvement in compression, Left cylinder still reads 150 PSI, this means its piston rings??

Will also do a leak down test...

[n3303j]

Tried it, no improvement in compression, Left cylinder still reads 150 PSI, this means its piston rings??

Will also do a leak down test...

That means it's NOT piston rings.

[pete mcgee]

Tried it, no improvement in compression, Left cylinder still reads 150 PSI, this means its piston rings??

Will also do a leak down test...

From my experience 35 yrs ownership of a mk 3, it will probably  be the valves.
A leakdown test will confirm this.
Cheers

[mtiberio]

I have been using a compression tester to check the condition of my Land Speed Racer. I have (had) a leak down tester, but the mice liked its hoses better then me. My LSR bike will bend an exhaust valve it I miss a shift, and the easiest way for me to find that out is through a compression test. The motor is at about 10.5:1 by my calculation, and reads 190 #'s each side when healthy. A recent Lemans 4 motor that I picked up as a core for my next motor build shows 150 #'s each side.

[mtiberio]

Ok, here's an old aircraft mechanic's trick. While you have the cylinder on TDC and the leak down tester turned on, rap the rockers with a copper hammer a few times. You just may see black smoke out the exhaust, and compression come up when the carbon on the valves is dislodged.

To remove carbon without disassembling your motor, spray water into the intake of the carb while you hold the throttle open. Yes, you spray water into your motor. Not so much it stalls, but as close to that as you can get. The amount of crap that will come out the exhaust is amazing. The water turns to steam and will decarbon your pistons and combustion chamber including your valves.

[lucian]

LOL! I remember back in the 70's when water injector rigs were being sold as the magical cure for everything afflicting you're gas guzzler. Any benefit could purely be attributed to knocking the carbon off the valves, or preventing it's formation in the first place.   

[Pescatore]

Seafoam sprayed into the carb sync hole on the
 manifolds (if equippeed) will dissolve carbon as well.
I fixed an EGR code on an old Honda Odyssey by letting it suck the Seafoam
through a vacuum hose, in small quantities.