Skipping

showcrop · Oct 27, 2010

Ford 172 with less than 100 hours on all new motor started skipping under load. Checked compression, 152,152,144,150. Checked spark, got the spark that "Old" always describes. Fat, blue, jumps 16 inches. All plugs spark. Pulled the plug wires off one at a time and identified the missing cylinder. All but one plug has a lot of soot on it. What is my problem?

Dean · Oct 27, 2010

Are the spark plugs new? Are they the correct heat range? Have you inspected for cracked insulators? Have you sandblasted the plugs and/or replaced the plug in the identified cylinder?

Did you replace the spark plug wires and distributor cap?

Remove the plug wires from the cap tower to check for corrosion, missing or damaged terminal ends, etc. If nothing suspicious is found, replace the spark plug wire(s) unless they are relatively new.

Dean

Allan in NE · Oct 27, 2010

A number of times, and always after a fresh rebuild, I've ran into this same problem.

Not saying that this is your problem, but if you had that head reworked at a machine shop and they got those guides too darned tight, the thing will start hangin' valves, droppin' vacuum and missing when it heats up.

When it's cold, it'll run just fine again.

Just an idea for ya to kick around,

Allan

JDseller · Oct 27, 2010

I will kind of second what Dean says. I would replace the spark plugs first. I have been getting bad ones this last year or so. They will fire fine cold but when they heat up they start missing. My pressure washer did this and I clean the carb twice before trying the plug. The plug was the problem.
As for the soot on them. If you have a weak spark under load you will have to have the carb adjusted rich to run. It will foul the plugs fairly quick. If the new plugs don"t fix it then try the coil and condenser. They could be breaking down under load. I have had coils act like the carb was adjusted wrong for load: stumbling as the load was applied.
Also did you replace the ignition switch lately? I have ran into several of these "new" China and India switches that will act funny when they get hot. Cost a good Ford friend of mine a lot of money to find out his "new" switch was bad. He thought he had a bad valve guide hanging like Allen suggested. He had the head rebuilt and the problem was the same. (The valves and seats where worn but not the problem)

JOCCO · Oct 27, 2010

Although the others are correct don't over look valve springs!!!

Wayne from Wi · Oct 27, 2010

Are you using pump regular gas of 87 octane, when the motor was designed for higher? Under load you may be getting pre-ignition. Wayne

SparepartsTN · Oct 27, 2010

You"re keeping secrets. Which cylinder is missing and which cylinder has the clean plug? The more details we have the better chance to figure it out.
Warren

old · Oct 27, 2010

Soot says it is running way to rich and that could well be your problem. Air cleaner in good shape?? Carb set as it should be. Is the skipping cylinder the plug that is not sooty?? Is that one also the one that has the miss and lower compression?? Valves set correctly?? I would try a set of auto lite 437s in it and see what happens

Skipping

showcrop

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Dean

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Allan in NE

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JDseller

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JOCCO

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Wayne from Wi

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SparepartsTN

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old

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