[nigelrharris03]

I was with a friend today having been asked about his engine.
It was producing some smoke only on the overrun, this could indicate oil being sucked in when the butterfly is closed.
The other observations that I made. There was good oil feed to the Exhaust cam and therefor the cam box.
There was no visually discernible wear on the cam lobes then even from my fingernail, so I am happy that there are no cam issues.

We decided that a compression test would be a good place to start to try and eliminate possible issues.
I will precis my 66.3 workshop manual following :-

Rated compression 12kg/cm^2 = 170psi
When Compression measures over 12kg/cm^2 150psi ? (not my typo, it's in the book), the head is probably carboned.
When the compression is below 10.5kg/cm^2, valve, piston ring, gasket, head and or cylinder gasket may be leaking.

The results on the starter motor were 165 Rt 150 Lt (Hot, WOT & no Choke).
We then tried the compression tester with the engine running on the opposite cylinder fully opened the throttle and keeping it open as the revs were building to app 6K cut the ignition. Nearly 180psi.

My question is this. What rpm should we run the compression test at? Is it specified at starter motor speed? Kicking over speed? Or indeed, is the value we obtained valid?

Before venturing into the engine there are other things we can look at. The valves need re gapping the exhaust were running at 0.08mm. Just over 0.003" which is 3 times the prescribed and 50% more that we normally run. I did not check the inlet valves as there really was not time and the light was going but I expect that they will be similarly loose.
I can't see this really reducing the compression although I am always willing to learn more.

I will check the Head nuts torque settings, although I can't hear, or see any sign of leak past the head/cylinder gaskets.

I would welcome learned comments

 

[-alan-]

Can't give you a definitive answer on anything Nigel, but one thing that strikes me is that at 6k rpm, the dynamics of the gas-flow in a cylinder will be quite different than at kick-start/electric cranking speeds. I would have imagined that at a mid-high range rpm like that, you would be well into the range where for example the reverse pressure pulses generated from careful tuning of the exhaust system will have a marked increase in the combustion chamber fills and pressure levels.

 

[longdistancerider]

Generally speaking oil smoke on decel is oil being sucked past the valve seals, could be bad seals, worn guides, worn valve stems or any combination thereof.
Oil smoke on accel is the rings and/or associated parts.
Compression tests are done with a dead engine, usually with all the sparkplugs removed so you have the highest possible cranking speed.
When adding oil to do a wet compression test use a measured amount into each cylinder just before testing said cylinder. Common error is adding to all cylinders at the same time and varying unmeasured amounts.

 

[nigelrharris03]

Thanks Jim

That is pretty much where I was coming from with respect to the burning oil. The engine has done app 1600 miles since a full, and allegedly complete, rebuild where no expense was spared. It certainly looks realy nice. That said, it stood for app 15 years before running although I understand fully oiled. I don't know of course, if the valve guides were replaced or not. Equally can't see that the current mileage would have damaged them. I'm possibly thinking that he should opt for a top end strip down to check over the guides and bores. It's a shame to have to suggest this, but having considered it overnight I'm thinking that ride until the winter then get the engine looked at.

Generally speaking oil smoke on decel is oil being sucked past the valve seals, could be bad seals, worn guides, worn valve stems or any combination thereof.
Oil smoke on accel is the rings and/or associated parts.
Compression tests are done with a dead engine, usually with all the sparkplugs removed so you have the highest possible cranking speed.
When adding oil to do a wet compression test use a measured amount into each cylinder just before testing said cylinder. Common error is adding to all cylinders at the same time and varying unmeasured amounts.

 

[Rob Axel]

A compression test is only one part to testing the "health" of a cylinder ...
Do yourself a favor and either buy or make a "leak down" guage.. this will let u know if there is a weak point, rings, valves/seats, head gasket, possible cracks..

 

[nigelrharris03]

Would the wet compression test be more valid cold or hot Jim?

Thanks Rob it's certainly worth considering the leak down test I'm not yet at that point. I'm just starting with the simplest tests first.