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Only two plugs are having the problem and why would the compression be affected?

Puzzling for sure.


thanks
Bill

Good Evening...Go back up and read #3 one more time. Ross has the answer. Decks and heads have been milled over the years and the clearances are not as great as they originally were. Ross suggests and addional brass washer under the plugs...works...99 times our off 100 or so. Ernie in Arizona.

Weak valve springs are not usually a problem but sticky valve guides from bad gas, sitting, whatever are. Just leave the valve cover off and watch it as it runs at an idle. You will be able to see then the valves begin to stay up. I assume you are not revving this thing into the 4000 rpm range to look for spring flutter. Spray the the #s 2 and 3 valve stems with some PB Blaster while its running.

If you wish to double check the clearance over the valves, take the plugs out and turn the engine over till the valve in question is up. Those are exhaust valves under the plugs BTW. Measure from the plug seat to the valve with a dial caliper and then compare it with a plug.

It sounds to me like you have two exhaust valves that have too little tappet clearance or no clearance at all. When the engine warms up, those two exhaust valves cannot seat, causing the rough running. (The exhaust valves get hot very quickly). When the engine cools down, the valves can seat once more. (The valves will cool down very quickly when the engine is stopped, so the problem will go away soon after the engine is stopped) Running the engine with tight valves will soon burn the valves and could bend them. // I would immediately check all valve tappet clearances and carefully adjust them. // Removing the right hand inner fender panel and right front wheel is not very difficult, and will give better access to the valve covers and valves. I would begin by checking the clearance at the two exhaust valves that are hitting the plugs. I believe it should be in the .010 to .012 range.
You can get by adjusting the clearances cold by adding .002" to the spec. Intake and exhaust clearance specs are different.
Adjusting the valve clearance and replacing the two damaged plugs will most probably correct the problem. It's also important that the cooling system be in good condition and that the water distribution tube in the block is in place and in good condition.

Thank you for the suggestions. Will be following your advice next week when we delve a bit deeper into it.



Thanks for the correction on exhaust valve too.



Bill