Low compression on one cylinder

[ancho]

Hey all, new Guzzi owner here. Just picked up a 1972 Eldorado recently. Having some issues that I'm hoping all you knowledgable Guzzisti can help me troubleshoot.

The bike hadn't been ridden in nearly 10 years, but it was stored indoors with fluids drained. Part of what made this particular example attractive was the fact that it already had re-bored and nikasil plated cylinders, rebuilt heads and a rebuilt transmission. Cylinders were supposedly sent out to Moto Guzzi Classics who sent them out in a batch for nikasil plating, and the heads were rebuilt by Moto International. When I got the bike home I changed all the fluids, re-set the static timing, checked and adjusted the valves, and set the points. It fired right up, but was only running on one cylinder. The left cylinder was not firing at all. So after some diagnostics, I replaced the spark plugs. This helped, the left cylinder would fire now but it seems intermittent. And the bike is puking some oil from the crank case breather hose underneath. This made me worry about blowby. I did a compression test and the left cylinder is showing low compression; between 70-80 PSI. I pulled the head and the cylinder off, and inspected the rings. They look good. Not seized. Cylinder walls look good. Heads seem good condition.

Things I haven't done but need to do:

* Cleaned the carbs
* Measured the ring gap
* Measured the piston and the cylinder with calipers to make sure everything fits correctly
* Done a leak down test

I will be buying a leak down tester. Anyone have any other ideas as to the cause, or seen this before? I was hoping since all this work was done very few miles ago (like 500) it would be in better working order. But it has been sitting for nearly a decade.

[pehayes]

You sound knowledgeable, but I have to ask some basic questions.
Compression test with both plugs out, and wide-open throttle throughout the test?
Engine rotated to separate compression TDC positions when setting valves?
What was the compression on the 'good' side?
Who drove it for the 500 miles after restore?
Was it performing just fine on both cylinders before it was parked?
Yes, 80psi is too low but we have to compare to the unknown side which seems to be functioning.

Patrick Hayes
Fremont CA

[ancho]

You sound knowledgeable, but I have to ask some basic questions.
Compression test with both plugs out, and wide-open throttle throughout the test?
Engine rotated to separate compression TDC positions when setting valves?
What was the compression on the 'good' side?
Who drove it for the 500 miles after restore?
Was it performing just fine on both cylinders before it was parked?
Yes, 80psi is too low but we have to compare to the unknown side which seems to be functioning.

Patrick Hayes
Fremont CA

Hey Patrick, thanks for the response. Compression test was done with both plugs out, but I wasn't holding WOT. I will try it that way as well.

Yes, valves were set at TDC for each individual cylinder on the compression stroke.

Compression on the right side was reading about 130 PSI with the same methodology (meaning I wasn't holding WOT).

Previous owner rode the 500 miles since rebuild, then he moved across the country and stuck the bike in storage. He said it was running well, but "thought the timing might be a bit off."

[Dukedesmo]

If the rings and bores are good, and maybe you already checked but what about the valves, are they sealing properly?


It's amazing how they will run with low compression, my LM2 had a minor piston to valve interface - minor as in it occurred on a ride and although the engine suddenly lost power it was still running and got me around 15 miles or so home.


When I did a compression test I got around 25psi on one side and not much more on the other. Both exhaust valves had been 'kissed' by the pistons and were very slightly bent.


Now rebuilt with new pistons, exhaust valves and all valves recut etc., I've got 200psi on both.

[blackcat]

Both exhaust valves had been 'kissed' by the pistons and were very slightly bent.

What do you think caused this problem?

[pehayes]

Hey Patrick, thanks for the response. Compression test was done with both plugs out, but I wasn't holding WOT. I will try it that way as well.

It does make a huge difference opening the throttle.  Wrap a bungee or clamp around the hand throttle to hold it WOT throughout.
The closed throttles hold vacuum and restrict air intake.  You're trying to measure compression of that air.  Restricting in means bad data out.
Good compression should probably be 150-165.  Maybe a little more.  More importantly is the differential between the two sides rather than the absolute numbers.
Now that you have removed the heads for inspection, the gaskets may seat differently.  You might find different numbers regardless of technique.
Do you have any other twin Guzzi around (or friend nearby).  Pull those plugs and do a compression test with and without WOT just to see the results.

Patrick Hayes
Fremont CA

[Dukedesmo]

Quote from: blackcat on Today at 12:35:37 PM

What do you think caused this problem?

___________________ _____________

Don't know, was a mystery.

I hadn't had the bike long and I had only done about 500 miles when it happened (bike was 36 years old with 12k on it). I had serviced it including checking valve clearances etc. all seemed good and it was running well until one day it didn't.

Maybe I over-revved it? It did have non-stock higher domed pistons and a 'sporty' cam in it which may have made things closer than normal?

The shop that k-lined the valve guides and re-cut the seats for me did suggest the possibility of unleaded fuel maybe causing the exhaust valves to get too hot and somehow stick in the guides? but neither of use were convinced of that theory.

Despite the bores being fine, I fitted a Gilardoni 950cc kit as I couldn't find any new 850cc pistons at the time and it has been perfect for the last 10 years.

[Chuck in Indiana]

Ancho.. once you get your leak down tester, when you have the cylinder under pressure and can (probably) hear the valves leaking.. take your copper hammer and rap the rocker arm right over the valve. Aircraft mechanics call this "staking" the valves. If it is only carbon and corrosion from sitting, you will see carbon coming out the exhaust..the most likely.. and the compression start to come up. I've seen engines that have been sitting a long time come back to normal compression doing this.

[ancho]

Ancho.. once you get your leak down tester, when you have the cylinder under pressure and can (probably) hear the valves leaking.. take your copper hammer and rap the rocker arm right over the valve. Aircraft mechanics call this "staking" the valves. If it is only carbon and corrosion from sitting, you will see carbon coming out the exhaust..the most likely.. and the compression start to come up. I've seen engines that have been sitting a long time come back to normal compression doing this.

Thanks for the tip! I'm leaning towards an issue with the valves, hopefully nothing requiring major work. I'll know more when I head down to Harbor Freight later and pick up a leak down tester.

[guzzisteve]

Ancho.. once you get your leak down tester, when you have the cylinder under pressure and can (probably) hear the valves leaking.. take your copper hammer and rap the rocker arm right over the valve. Aircraft mechanics call this "staking" the valves. If it is only carbon and corrosion from sitting, you will see carbon coming out the exhaust..the most likely.. and the compression start to come up. I've seen engines that have been sitting a long time come back to normal compression doing this.

The only MCY mechanic I seen do this is me.  Look out sounds like a 45 Colt going off. That is how I set my keepers after installing valves & springs.
It works better if at BDC with pushrods out cause motor can't turn

[SIR REAL ED]

Ancho.. once you get your leak down tester, when you have the cylinder under pressure and can (probably) hear the valves leaking.. take your copper hammer and rap the rocker arm right over the valve. Aircraft mechanics call this "staking" the valves. If it is only carbon and corrosion from sitting, you will see carbon coming out the exhaust..the most likely.. and the compression start to come up. I've seen engines that have been sitting a long time come back to normal compression doing this.

Cool tip!  Thanks!

[SIR REAL ED]

The only MCY mechanic I seen do this is me.  Look out sounds like a 45 Colt going off. That is how I set my keepers after installing valves & springs.
It works better if at BDC with pushrods out cause motor can't turn

What is an "MCY mechanic?"

[guzzisteve]

motorcycle

[SIR REAL ED]

motorcycle

thanks!

[ancho]

I think I've narrowed it down to a leaky intake valve. On the compression stroke, with my thumb over the spark plug hole, I can clearly hear air escaping from what sounds like the location of the intake valve when manually rotating the crank. I'll have to remove the head again to inspect more closely. Tomorrow I will do an actual leak down test before I remove the head.

[ancho]

Big thanks to everyone who helped me out with ideas here. I think I'm close to resolution.

Thanks Patrick for the tip on making sure the throttle is open so air into the cylinder isn't restricted during the compression test. I had no idea that it could make that big of a difference but it definitely did. The compression was quite a bit higher when I disconnected the intake manifold. I also "staked" the valves to knock loose any gunk and reset the valve lash afterwards, which helped even more. Thanks Chuck for that tip.

So now I know compression is looking good (sitting at 175 now), I believe my only lingering issue is carbs. Going to spend this afternoon cleaning the carbs, dialing them in, and synchronizing them.