Thread: Miss on number 3 plug

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I have a miss on number 3 plug. Changed Cap, Rotor, wires and plugs. Still there. Could this be a valve adjustment? Car runs great except for that one plug keeps loading up. The rest are look great. Using R45TS on a 350 SB with HEI set at 035 gap. Motor has about 1750 miles on it. Any hints would be greatly appreciated. thanks

Does it smoke at start up? Might need to run a compression test.

No smoke at all. That's what puzzles me! thanks

Yep, first step should be a compression check of all cylinders. You are looking for them to all be close to each other in PSI. I bet you find something is low in that one cylinder. I just went through the same thing and it was carboned up valves in one cylinder sticking open.

Don

Don, would a product like sea foam work to clean the carbon up if this is the case? I have heard of using water in a spray bottle also to clear some carbon. Engine is new but I guess that does not mean much a compression test will give you your answers.

I've seen Sea Foam, but never tried it. I actually pulled one head and removed the two valves in that one cylinder and cleaned them up on the wire wheel. They were really encrusted. I then ran a product we sell at work called Power Tune that you spray down the carb of a running engine until the can is empty and the engine dies. Let it sit for a while (mine sat for 3 days, but they say 15 minutes works) and when you restart the engine crud like you won't believe blows out the pipes. My compression in that cylinder went from zero to 195 or so, through the wire brushing and Power Tune.

After that I put a heavy dose of Mercury Quick Kleen in the tank and drove it. When I checked the plugs they are burning light tan now. I also cut the fuel pressure down to 3.5 lbs and put a hotter thermostat in. The car now runs at 170 down the road instead of 150.

I never knew carbon buildup could kill an engines compression like that, which is why I agreed with you that he should check his compression. Bet he has a similar problem.

Don