Spark Plugs

[Rob Matthes]

I just changed the AC-45 plugs in my 327/350 hp 1967 over to AC-43. After about 2,000 mile the 45's were black (maybe a rich mixture)What exactly is the difference in a hotter/colder plug? The service manual recommends either a 44 or 43!The 43's seem to be working fine ...

TIA, Rob

[Joe Ciaravino]

Re: Spark Plugs

Rob:

The 45's are actually 2 heat ranges hotter than the 43's you replaced them with. If you fouled the 45's, you will foul the 43's even faster.

The difference between a hotter plug and a colder plug is in the length of the insulator for the center electrode. The insulator length determines the rate at which heat is dissipated from the plug after it fires.

Colder plugs are recommended for severe duty, like towing a trailer, or racing, where a hot plug may cause detonation. Hotter plugs are recommended for stop and go driving, because light loads could cause fouling, and the extra heat retention assists in burning off excess fuel/oil deposits.

Have you checked your carb jets? Your float level? I would run the R45's, lower the float level slightly, and reduce the jet size (primary) by one size. One other thing to look at is the power enrichment valve. If that is ruptured, the car will run very rich.

Joe

[Duke W.]

Re: Spark Plugs

A "hotter" plug has more thermal resistance i.e. it does not transfer heat as readily as a "colder" plug. The AC 45 (or equivalent) should be suitable for normal driving and cruising. The 43 would be correct for hot lapping a road racing track, but would build up deposts quickly in "normal driving" and foul. For a given set of operating conditions the 45 tip temperature will be higher. The plug tip must maintain a temperature of about 750 degrees F to prevent rapid deposit buildup, but must remain below about 1650 degrees F to prevent preignition, which can lead to detonation and serious engine damage. If the 45s are getting black it's probably a carburetor problem, but I'll defer to one of the Holley experts on that issue. Check down the Board for the thread titled "Fouling Plugs" for more info.

Duke

[Bill D.]

Spark Plugs, Hot or Cold?

Rob,

The lower the number the colder the plug! If the 45 was fouling, the 43 will foul sooner. The length of the center electrode determines how fast the heat is transfered. Long is hot, short is cold. Check carb setting and jetting to run a little leaner and reduce carbon fouling. Also check timing and dwell. Have fun!

Wild Bill

[Dale Pearman]

Re: Spark Plugs

HAPPY HOLLEY Rob!

Varooom!


CLICK HERE

[Rob Matthes]

Re: Spark Plugs

Thanks for the replies. I have rebuilt the Holley and replaced the power valve and adjusted the float to the spec. I also just adjusted the timing to 12 degrees instead of 10. What does lowering the float accomplish?

TIA, Rob

[Joe Ciaravino]

Re: Spark Plugs

Rob:

I have found that lowering the float level slightly, maybe 1/16", can solve some rich fuel mixture problems.It will lessen the likelyhood of fuel spillover into the intake due to "percolation". This occurs after shutdown and is caused by a phenomenon known as "heat soak"- when the motor becomes excessively hot just after shutdown and the coolant is no longer circulating. This causes the fuel in the bowls to "percolate" , or boil over into the intake manifold. This is common to most carburetted engines.

When the carb settings are calculated at the factory, they are set up to provide enough fuel to supply the engine at WOT, with an extra safety factor. You can safely lower the level by a small amount without running into fuel starvation problems.

Joe

[grr]

Re: Spark Plugs

W this a gradual problem with the fouling plugs, or just started ? If it a just started and you changed out that power valve and adjusted the bowl level and did the ignition timing, I'd stay with the plugs you had before if that heat range worked fine prior to carb work. I take it your talking all eight plugs also. Thanks for the update.grr#33570

[Rob Matthes]

Re: Spark Plugs

Thanks to all responses. I just recently adjusted the timing and will adjust the floats slightly as well. The plugs which had the black soot were my old ones before changing anything which were about 1 year old with 3,000 miles. Thanks to all again!!

Rob