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Kirkham Motorsports

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  • 2 Post By tgandy
  • 1 Post By CompClassics

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Old 10-06-2018, 06:06 PM
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Default 44IDF Issue

I've got a dead cylinder #6 on my SBF. I have compression, spark, maybe too much fuel. The spark plug is totally black and nasty but when I put it into a different cylinder, it worked just fine and the defective cylinder still didn't appear to fire. I used an infrared thermometer and the offending header tube remained more than 100 cooler than its neighbors or the cylinders on the other bank. When I was cranking the engine, without spark plugs, to do a compression test, I got a good mist out of the bad hole. Had to have been gasoline but it evaporated immediately since the engine was at operating temperature.

Whaddya all think?
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Old 10-06-2018, 06:47 PM
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I'm not a Weber expert, but cool exhaust and black plug both point to rich mixture.
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Old 10-07-2018, 12:53 AM
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Or an ignition issue. Three things you need for an engine to run, compression, fuel and spark.
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Old 10-07-2018, 03:42 AM
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It is missing only at idle?

What was your compression readings, all cylinders?

Have you measured your ignition cable resistances?

Have you checked for a loose idle jet on that carb for cylinder 6?

Gary
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Old 10-07-2018, 11:15 AM
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I have not yet measured plug wire resistance, but will. I used a spark plug tester that goes in line between the plug and the wire and the spark in the glass looks identical to the other plugs I tested. (not very scientific).

The mist puffing out of #6, when I crank the engine is pointing me towards a fuel issue. Also the blackened nasty plug screams there is an over rich mixture in that cylinder.

I'm also going to pull the main and idle jets and give everything a good shot of brake cleaner to see if I get any improvement. If not, I'll pop the top on the carb to see what I can see. ARGH.

It does miss at idle but sounds just fine at higher RPMs.

Maybe I should just leave it alone since it has a nice lumpy idle. Ha NOT
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Old 10-07-2018, 12:35 PM
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If your washing down that cylinder with a overly rich fuel mixture you will damage the cylinder walls and rings due to lack of lubrication. If your running that rich you can also be diluting your engines oil enough to cause bearing damage.
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Old 10-09-2018, 09:51 AM
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Karl,

I'd give you dollars to donuts it is an ignition issue as has been already suggested. Either the plug wire, the plug boot or the distributor cap are complicit in the problem.

Ed
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Old 10-11-2018, 06:42 PM
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It's true that it's a piece of cake to throw one spare spark plug wire on there and see if things change...
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