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General Category => General Discussion => Topic started by: neetones on December 24, 2025, 04:32:18 PM
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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.
(https://i.ibb.co/FkRvnNsz/PXL-20251224-221522974.jpg) (https://ibb.co/FkRvnNsz)
(https://i.ibb.co/MxcBGgps/PXL-20251224-221527845.jpg) (https://ibb.co/MxcBGgps)
(https://i.ibb.co/1fCt3rq3/PXL-20251224-222427499.jpg) (https://ibb.co/1fCt3rq3)
(https://i.ibb.co/zW9KR6t5/PXL-20251224-222402052.jpg) (https://ibb.co/zW9KR6t5)
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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.
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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
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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.
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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
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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
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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.
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Thanks Charlie,
Do you have some contact info for that machine shop? Sounds like you trust their work.
Nitin
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Thanks Charlie,
Do you have some contact info for that machine shop? Sounds like you trust their work.
Nitin
Sent you a PM.
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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
thats a HUGE play, cant remember ever seeing something like that in 40 years....
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I had an old Chevy PU with a worn out 350. I burned an exhaust valve, I planned to just throw a new valve in it just to get a couple more months out of it. After removing the head the valve even had more wiggle than this one. I installed the new valve and drove it a couple more years. The funny part I ordered the new valve by application, '71 Chevy 3/4 ton Camper Special. When I went to pick up the new valve they wanted $30 for it. Holy Cow, turned out it was sodium filled. I would have never thought that GM would go this far, after all they were known to build stuff just well enough to last a reasonable amount of time/miles. I got a $6 valve instead. :laugh:
kk