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Spark plugs and engine run on.

Posted By 312YBlock Last Year
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312YBlock
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I’m using Autolite #46 plugs gapped to .034 in my 312, it runs great but every once in awhile I’ll get a little run on. I’m familiar with the carbon, fuel grade etc causes but I’m curious if anyone has found a plug that is good at combating it?

1955 312 T-Bird Warwick, NY
Ted
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The spark plug heat range is not much of a player when having engine ‘run on’.  The engine not stopping cleanly when killing it is more related to the gasoline octane level being too low and/or too much throttle opening on the carburetor.  You can try setting the idle speed lower and see if that helps.


Lorena, Texas (South of Waco)


312YBlock
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I’m running Sunoco 91 octane, in addition to reducing idle speed is there any harm in trying the 93?

1955 312 T-Bird Warwick, NY
Ted
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Gut feel says you’ll not see a difference between 91 and 93 octane fuels.  That’s assuming those fuels are as advertised.  Idle speed being too high is still at the top of my list.  Have you looked at your ignition timing?  Could simply be too much ignition advance.


Lorena, Texas (South of Waco)


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Hmm, I’m at 12ish BTDC, no pinging or hesitation, great acceleration. The idle is comfortable at 500 on the factory tech that sits at 100 with the engine off but I’ll try shaving a few RPM and see what happens. Thanks much Ted for the feedback.

1955 312 T-Bird Warwick, NY
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I add an oz of Sea Foam to every gallon, I recently replaced both exhaust manifolds and put my finger into several exhaust ports finding only a tiny trace of carbon. The cylinders used to be choking on it.

1955 312 T-Bird Warwick, NY
Ted
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Are you using a conventional or ‘Dial Back’ timing light?  If a ‘Dial Back’ light, then I’ll suggest checking the timing with a conventional light just to ensure that that the ignition timing is the same with both timing lights.  ‘Dial Back’ lights are notorious for giving erroneous readings as they get older.

Lorena, Texas (South of Waco)


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I have a relatively new conventional conductive timing light.

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This brings us full circle and now back again to the quality of the fuel.  Have you tried fuel from another location?

I bring this up as I had a 454 BBC on the dyno that had a serious ‘run on’ issue after making the first dyno pull.  Once it was verified that the ignition timing and idle speed were not the problem, a quick check for the ethanol content in the gasoline being used showed it had 35%.  The fuel was purchased by the customer from his local station and was advertised as 93 octane Premium on the pump.  This was from pumps with stickers staying ‘May contain up to 10% ethanol’.  I had the customer go to the nearby local grocery store and purchase five gallons of 93 Premium and a quick check showed it had only 5% ethanol.  That fuel in the same engine without any other changes solved the ‘run on’ problem.

Lorena, Texas (South of Waco)


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Excellent point Ted, thanks.

1955 312 T-Bird Warwick, NY


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