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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

We removed the inner fender and front right wheel to give us better access to the engine.
We sprayed the valve guide area with sea foam and followed up with marvel mystery oil soak. After letting everything set for the Thanksgiving weekend, well get back to it next week and check tappet clearance and adjust.

Going to order a new gasket set and reinstall everything to see how it runs if all is good.
Adding spark plug washers too.


thanks for all the guidance and good vibes.


Bill

Update: She runs great !!!
After installing new gaskets and an extra spark plug gasket, I'm happy yo report she runs great. It idled and accelerated smoothly from initial startup.
Thanks for the pointers and tech info. Now we just have to get the brakes to work like they should and clean out the fuel tank. YUCK!
That's a project for after the New Year.

thanks again for all who listened and to those who gave us info to make her run again.

Bill
NPM, Warren, Ohio

Good news. I'd replace any spark plugs that were damaged to avoid any chance of the electrodes breaking off later on and causing trouble. Unlikely, but why risk it?