750 Ambassador head mystery

[neetones]

Hi all. I have a '69 750 Ambo that's fairly new to me. I've fixed up a few things but it always ran quite weakly.

I finally did a compression test and found a disappointing ~80PSI on each cylinder. Just pulled the heads for the first time and found Gilardoni cylinders (great!) with excellent crosshatching.

I noticed, though, that the diameter of the cylinder bore doesn't appear to match that of the combustion chamber. Cylinder measures 83mm and combustion chamber measures 72mm. I suppose that's normal.

Additionally, I'm seeing this odd arc of wear across the piston.
Looks like my Gilardonis don't have oval pushrod holes so I can use a regular head gasket?

Any insights? I haven't pulled the valves yet but they appear to be seating well.

[turboguzzi]

The mismatch is normal, designed into other models too, actually beneficial because it creates a squish area that helps combustion. the "wear" arc you see, is the squish area actually doing its job.

still, 80 is not a good compression, have you tried to do valves leak test by pouring fluid into the ports? valve / seat sealing would be my first suspect.

[pehayes]

I finally did a compression test and found a disappointing ~80PSI on each cylinder.

I must ask.  What is your compression check procedure?  Did you arrange to have both spark plugs out and have someone or something to hold  your throttle wide-full-open throughout the test?  Pressure should be around 160psi with no more than 10% variation side to side.  Your numbers are a clear indication that your throttles were closed during the test.  Common error. Reassemble and  you might find that everything is OK.

Patrick Hayes
Fremont CA

[michaell32]

I wouldn't be surprised if the mark on the piston is simply fuel coming around the intake valve and cleaning the carbon off that spot. I doubt it's wear.

[neetones]

Well this is very helpful. I did not have the throttles open. I had both plugs out, though. Tried with one plug in and got the same results.

I may just check the valves and bolt everything back up.

I must ask.  What is your compression check procedure?  Did you arrange to have both spark plugs out and have someone or something to hold  your throttle wide-full-open throughout the test?  Pressure should be around 160psi with no more than 10% variation side to side.  Your numbers are a clear indication that your throttles were closed during the test.  Common error. Reassemble and  you might find that everything is OK.

Patrick Hayes
Fremont CA

[neetones]

I continued taking the head apart and found that the valves are very loose in their guides.

Exhaust is looser than intake but both can be freely wiggled around. Looks like I have bronze guides and the exhaust valve is stainless (non-magnetic).

Having a difficult time finding a machine shop that can K-line them. Any suggestions? The bike doesn't get ridden much so I'm not sure how much I want to shell out, but I'd like it to run properly and reliably.

video of valve play: https://photos.app.goo.gl/jfE961oZUvhaHdoN6

[Antietam Classic Cycle]

I continued taking the head apart and found that the valves are very loose in their guides.

Exhaust is looser than intake but both can be freely wiggled around. Looks like I have bronze guides and the exhaust valve is stainless (non-magnetic).

Having a difficult time finding a machine shop that can K-line them. Any suggestions? The bike doesn't get ridden much so I'm not sure how much I want to shell out, but I'd like it to run properly and reliably.

video of valve play: https://photos.app.goo.gl/jfE961oZUvhaHdoN6

Do you have a micrometer or even a dial caliper to measure the valve stems in the area that slides through the guides? The valves might still be usable, but those guides are knackered. Guides cost $14 ea. and my local machine shop charges $280/pr. to clean and media blast the heads, replace the guides and recut the seats.