running rough, has equal cold compression of 95 PSI , when running and grounding out plugs one at a time. all show a miss, but one shows less miss ,
that plug was exchanged to adjacent, no change, vacuum gauge shows a small flutter when idling but has 18# of vacuum , where should I look? thanks
What plugs are you using?? If it where me I'd try a set of NGK3112 plugs and see if the problem goes away with a tad bit hotter plug like those NGK
Start with the basics. Do you have a strong spark at each cylinder? If not plug wire, cap, spark plug? Any leaks in the intake system?
If you have good compression, then look at the spark side, could be an issue with rotor, cap, or plug wires.
I recently picked up a '51 8N. It ran ok but puffed a more than a bit through the exhaust. Which was barely there when cold and got worse when warm. I serviced the carb, distributor, and wiring, repaired the voltage regulator, and checked compression, which was at 105-110 across the board. I decided to check the valve clearances and found all of them too tight. Like .005 or less. I adjusted the clearances to the max allowed (.012" intake and .016 exhaust) it runs way better now, I'd say a solid 95% improvement. It still has the occasional puff, but nothing like it was. Mine has the free rotating exhaust valves, which you can distinguish from non rotators by the stem at the lifter adjustment bolt. The exhaust valves have the rotator caps and are a larger diameter than the bare intake valves. The hardest part of the adjustmemt was holding the lifters from turning. But with the help of this forum I was able to make up some of the 3/16" rod type of holding tools. Now it runs with more tapping noise but if it's anything like an old air-cooled japanese motorcycle, you want to hear the tap tappity tap of the valve train. If it's quiet, they probably need an adjustment. Good luck with your project.
Your absolutely correct. Valves are happier being a bit loose as it allows longer to dissipate heat when the
valve is closed.
However, on the basic street driven automotive engine, loose valves diminish air flow into the cylinders
lowering performance. I strongly suspect that low revving tractor engine is not really going to notice.
I have found the adjustable exhaust lifter for the cylinder that was not firing properly to be cracked across the base and up both sides to the first cavity, it appears that when engine warmed up the lifter expanded enough to prevent it from lowering with the cam, hence valve didnt close quick enough ????? My theory anyway

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