quote:A hotter plug is one that permits a higher combustion temperature. A colder plug colder combustion temps.
Regarding Plugs:
I think there is a general misunderstanding here, I have always learned that a HOT plug means that its CORE is running hotter than a COLD Plug. It is only a matter of the Plugs ability to disperse of heat and therefore its ability to stay clean. A Plugs Temperature rating has NO control over the Combustion Temperature. Some Engines needs a colder and some a hotter, only the Lord knows which. The rest of us has to test them individually to find the correct one for a particular setup. The trouble is if you choose a Plug that is too hot it will build up heat on its Core under high load and eventually glow permanently and therefore induce uncontrolled ignition (detonation) It will blow up your Engine and ruin your Day! Therefore always start your experimenting with a colder and work your way to a hotter until it remains clean without any or only little deposit! You have to drive several hundreds of miles to get a deposit if you are close to the correct temperature Plug and normally deposit can form at prolonged idle but should easily be burned off at 3000 rpm for a short while.
Be aware some Plug manufacturers uses reversed numbers for Heat Range!
Regarding Holley 4150:
Idle Screws controls Fuel! Here the trouble is that with a performance Cam the Carb does not deliver enough Air UNLESS you turn up the Curb Idle Screw and THAT induces a problem. By turning up the Curb Idle Screw you disables the Idle Circuitry because the Throttle Blades are past the Slots from the Idle Circuitry and then turning the Idle Screws has now no or little effect on the mixture. Some Carbs has an independent adjustable idle AIR supply, the 4150´s that I have seen has NOT. There are several possible ways to cure it. One is (initially) to turn up the Curb Idle on the SECONDARY Throttle Blades. It is a bit difficult to get to but it can be done with a Slot Bits and a 1/4" Wrench. Then counter turn the Curb Idle Screw on the PRIMARY ensuring that Primary and secondary Throttle Blades are EQUALLY opened. This might be "just" enough Air and hopefully both Sets are clearing the Idle Circuitry Slots in the Bores. IF NOT (and its easily checked by turning the Idle Screws to see if they have an effect now) you will have to Drill Air bypass Holes in your Throttle Blades (Primary and Secondary also if you have the 4 Idle Screw version) to allow Air into your Idle Mixture. I think I Started with 2mm Holes and it worked on first attempt. You might have to experiment or even go higher.
The importance is that both Sets of Throttle Blades are only just opening enough to secure they are not binding when they are in "closed" position. The only Air supplied for Idle is the amount of Air that can enter true that little Gap. It is normally not enough if you are using a 4150/4160 an a Performance Cam. (I only know about Holleys)
Adjusting your Idle Mixture by the Main Jets is not a great idea as it affects the whole RPM range which is not what you wanted. The above mentioned is the cure to have a controllable Idle Mixture. I don't know if you have access to a CO Gauge or an A/F Ratio Gauge but either is a valuable tool to find the correct Main Jets and to check your Power Valve(s) under load/acceleration.
Btw Doug your Spark Plug looks just fine to me, is it a fairly new Plug or has it run for a while?
