Compression variation?

[Cam Lay]

So, one side seemed a little squishy as I was setting the valves, and so I bought a compression tester. I have 150 pounds on the right, and 120 on the left.

 

This is more variation than one would want, correct?

 

Step two is to rig up a leakdown tester (hmmm... looks like they're cheap enough to just buy one, now) and see if it's rings or valves, right?

 

And step three, God forbid, is... is... new rings, new valves, a second mortgage...

 

I petition the assembled wisdom for advice.

 

Thanks!

C

[jrt]

I'm sorry, Cam. It sounds pretty low. That's a 20% difference in compression.

Based on past discussions here and my admittedly flaky memory, I think the first things to look at are valves and guides. I recall several reports of soft valve stems and/or guides. Are you blowing oil?

The rings and cylinders are good ol' nikosil and man, that stuff is tuff.

It's not at all expensive to replace valves and guides. Any competent shop should be able to do it. You're in N.C. or thereabouts? Call up Zydeco Racing or Charlie Mullendore- they can probably hook you up proper.

 

Edit: MGCycle lists valves at @ $50 per. You might also consider having the guides 'K lined'. I haven't done it, but apparently it's all blingy and works well.

[docc]

This is a Nero Corsa of 2004 vintage? How many miles? What's the oil type and change history?

 

Were the valves out of spec more one side (the low side) than the other? Any history of engine work?

 

All of these questions on the theory that it is better to have nurse than Doctor . . .

[Cam Lay]

Yes, 2004. If you remember the one on Ebay in NC a while back with the long full-of-personality write up about "Uncle Mark," this is that bike. Mileage was "7800," but the previous owner included an extra speedo cable with the deal, and went on at some length about how easy it was to replace... Mileage is unknown, in other words, but greater than 7800. Still, it was only 50 miles from me, looked to be in pretty good shape, and the mechanic at the local Guzzi place said he had done routine maintenance on it a couple of times and was not aware of any problems. PO talked a good game, but I've since found that some of what he said he had done was not done, so am going over it stem to stern...

 

Valves were about what you would expect for not being checked in a while. A little off, more on the low side than on the other but still not more than "normal drift."

 

This morning I put some oil in the left cylinder and checked the compression again. With oil sealing the cylinder I get 125-130 pounds. Not a big improvement. That suggests that I have leaky valves, correct? (It has been a long time since I last did this kind of thing.)

 

What are my options at this point? Several older posts indicate that K-line valve guides are a good choice. Does anyone know of a machine shop near Clemson, SC that could do this, or would I be better served to ship the heads off to a Guzzi-specific outfit? (Suggestions?)

 

Thanks,

C

 

 

This is a Nero Corsa of 2004 vintage? How many miles? What's the oil type and change history?

 

Were the valves out of spec more one side (the low side) than the other? Any history of engine work?

 

All of these questions on the theory that it is better to have nurse than Doctor . . .

[Tom M]

Are you testing with the throttle wide open? If not that's the problem.

[Cam Lay]

No. Meaning no disrespect, but why would it make a difference?

 

 

Are you testing with the throttle wide open? If not that's the problem.

[Tom M]

No. Meaning no disrespect, but why would it make a difference?

 

 

Are you testing with the throttle wide open? If not that's the problem.

 

The throttle plates greatly impede airflow. When I first used my compression tester on my old Honda I got similar readings to you. Then I read the directions and tried it with the throttle WFO and saw 175-180 psi on all four cylinders. I checked my LeMans a few years ago and the numbers were lower but consistent between cylinders. Give it a try and let us know what you get.

 

Edit: Here's a good article that mentions compression testing but advocates doing a leakdown test and tells you how to make a leakdown tester. I have a leakdown tester and did the test on my Guzzi a few months ago. Once I figured out how to keep the 100psi form turning the engine over I saw 4% leakage in both cylinders.

 

http://www.xs11.com/forum/showthread.php?t=190

 

The author Mike Nixon is a pro. http://home.earthlink.net/~wrenchtwister/credentials.html

[Kiwi_Roy]

 

The throttle plates greatly impede airflow. When I first used my compression tester on my old Honda I got similar readings to you. Then I read the directions and tried it with the throttle WFO and saw 175-180 psi on all four cylinders. I checked my LeMans a few years ago and the numbers were lower but consistent between cylinders. Give it a try and let us know what you get

 

Yes, 150/120 sounds low, I got 165/170 on my V11 a few days ago.

With the throttle closed it would only need the throttle bodies slightly out of sync to give a variation like that.

[Cam Lay]

OK, so compression is bad on one cylinder with the throttle wide open. Marginally better with a bit of oil in the cylinder, but not much. So it's valves...

 

Questions:

 

1) I need to have the valve guides replaced. K-line seems to be the preferred option. Should I replace the actual valves, too? Or is that something the shop will do as a normal part of the job?

 

2) Is there anything else that I should get done while I have the heads off?

 

3) Who's good for this kind of work near the upstate of SC? Any good rebuilder? Guzzi specialist?

 

Thanks,

C

[gstallons]

Before you go throwing $$$$ out the window, why don't you verify your quazi compression problem?

If you KNOW the valves are properly adjusted proceed to this exercise below.

# 1 Remove the throttle bodies and the exhaust pipe(s).

#2 Take your compression tool ( I assume it has a coupling), remove the Schrader valve, install it back into the spark plug hole.Then apply shop air into the cylinder.

#3 With the rear wheel off the ground & the trans. in 6th gear, turn the rear wheel as if the bike were going forward. You will turn the crankshaft until the cylinder comes "up" on the compression stroke. When this happens you will feel it "kick back".

#4 While someone else holds the rear wheel in this position listen at the intake and exhaust ports for escaping air. There should be no air coming from any ports.

# 5 If this checks out o.k. do the other side just to verify if you are doing this correctly and the other head is o.k.

#6 If this checks out, your problem is in the sealing of the piston rings.

[Tom M]

I agree with Gene in that you need to do more testing to see what's leaking in the low cylinder. It might just be a head gasket or a chunk of carbon propping a valve open.

 

I had my valves and guides replaced last year by a local machine shop. I bought the replacement valves from Ferracci off of ebay for $20 each. I think they're still available. My top end rebuild thread is here in case you're interested: http://www.v11lemans.com/forums/index.php?showtopic=15401&st=0&p=166464&hl=ferracci&fromsearch=1entry166464

[Dan M]

I agree with Gene in that you need to do more testing to see what's leaking in the low cylinder. It might just be a head gasket or a chunk of carbon propping a valve open.

 

I had my valves and guides replaced last year by a local machine shop. I bought the replacement valves from Ferracci off of ebay for $20 each. I think they're still available. My top end rebuild thread is here in case you're interested: http://www.v11lemans.com/forums/index.php?showtopic=15401&st=0&p=166464&hl=ferracci&fromsearch=1entry166464

 

 

Yes, Gene's method will pin it down for you. The fact that oil didn't help does point to valves. Yes, guides have reportedly been the issue with this engine.

Talk to your local mechanic for advice on a good machine shop in your area. Valves & guides are pretty basic stuff. You don't need a Guzzi specialist.

[stormsedge]

I had a similar mismatch when I first checked mine...I backed off all four rockers to ensure valves were closed for a recheck and numbers fell in place. I'd try that before going very much further...sorry if I missed that in the thread. Keep smiling. k

[IBA 270]

I had a similar mismatch when I first checked mine...I backed off all four rockers to ensure valves were closed for a recheck and numbers fell in place. I'd try that before going very much further...sorry if I missed that in the thread. Keep smiling. k

 

 

As usual...I'm in on this thread late.

 

You likely have an update by now, and if you do, please share!

 

Ok, some thoughts; with most modern engines, a compression issue commonly occurs with valves. Nikasil lined barrels go huge miles without eating rings or the lining. I've seen bottom ends (not Guzzi) go 200K with the cross hatch still very visable. 7,800 is nothing...78,000 is nothing unless the engine was run dry...but it likely would have seized before damaging the barrels.

 

Ok...

 

 

I'd certainly look to valve adjustment and valve seats in that order. Guides don't impact compression, but if you're using oil, then guides may be an issue.

 

Top end stuff on motors like these are easy fixes. Valves are cheap and a rebuild shouldn't be much if you don't want to tackle it yourself.

 

My late .02 cents, but I just wanted to see where this project was..

 

Oh, and last point...is rideability impacted? If not...then what's the worry?

[Chuck]

OK, so compression is bad on one cylinder with the throttle wide open. Marginally better with a bit of oil in the cylinder, but not much. So it's valves...

 

Questions:

 

1) I need to have the valve guides replaced. K-line seems to be the preferred option. Should I replace the actual valves, too? Or is that something the shop will do as a normal part of the job?

 

2) Is there anything else that I should get done while I have the heads off?

 

3) Who's good for this kind of work near the upstate of SC? Any good rebuilder? Guzzi specialist?

 

Thanks,

C

Cam, before you spend a lot of money.. Did you buy that leak down tester yet? If so, here's what you can do. Bring it up to exactly TDC to fire the suspect cylinder. Turn on the air to the leak down tester, and set it at 80psi. What's it read? Below 70 on the second gauge? Leaking out of a valve? Take a copper or brass hammer (or a claw hammer and piece of wood or plastic and hit the rocker arm enough to open the valve. Naturally, the spring will close it. Do this several times. Watch the second gauge and see what the pressure is. Many times, it is nothing more than carbon (or corrosion) lightly holding a valve off the seat, and causing a leak. This is called "staking the valves" in aircraft maintenance, and is commonly done. I've seen many cylinders come back to normal (mid 70s) with this technique..

Edit: forgot to tell you why this works.. keeping air pressure in the cylinder blows the carbon and/or corrosion out that is dislodged by the valve hitting the seat.