I have a ‘49 8CM with Edelbrock heads. The plugs I pulled out were Autolite 216 “Shorties” approximately .500” from washer to top of strap. The depth of the cylinder from where the plug seals to the head at TDC is approximately .650”. My question is, Edelbrock says that with their heads use a .750” long plug but when I attempted to install these, they hit the top of the piston with a little under .250” before the plug seats in the head. I can’t find a number on the heads and I haven’t pulled them to see if they have been shaved down to increase compression. Has anyone experienced this? She’s running rough, badly, severely fouling plugs, and backfiring when accelerating. After reading Edelbrock stating to use .750” long plugs, I thought possibly the shorty plugs were a. contributor. I’m going to reset the timing which should stop the backfiring and loss of power and Im going to put a fuel pressure regulator and drop the fuel pressure from 7 to 4psi. One more question. I’m running 3 original Strombergs on an Edelbrock manifold. Should I still run only 3-4 psi or run a little more due to my fuel demand? The engine is my avatar and more pictures in my photos. Any help is greatly appreciated, unless you’re being a smart****. Keep that to yourself, this is a brotherhood, have some decorum and respect. Thanks in advance. Be Well! “Big” ED.
First look down the spark plug hole and see if you see carbon on the bottom threads. Then remember that there are "standard reach" plugs and extended reach plugs and that alone may be the difference as "extended Reach" does extend out a bit further. I'd have to go out in the garage that has no lights right now and dig in the tool box for a gauge that you stick in a hole to measure how thick the item the hole is in is to accurately tell you that you need to measure that part to see how long the threads on the plug should be.
Weird that the plug is hitting the piston when the plugs are normally positioned over the valves. Your fuel pressure is way too high being at 7 psi; 3 psi is pushing it, and 4 psi is still definitely too much, 2.5 psi would be best. Most flatheads with 3x2 setups are over carbureted anyway unless they have serious reworkings going on inside to have a need for that much air and fuel. That is where your problem is and not the plug reach, the fuel psi is overcoming the needle and seat in the carbs causing it to be too rich. If it came down to it and the plug reach is still bothering you after getting the fuel pressure taken care of, then the most sure-fire way is to yank the heads and install your plugs to see where they end up once tightened down in relation to the combustion chamber ceiling and swap them out accordingly. Take all the measurements needed to confirm clearances while you're in there.
The first step is knowing what type of Edelbrock heads you are running? They modified the castings in recent years for .750 reach plugs. The primary reason is the availability of a TON of different plugs in that length - by far the most of any standard length. The original/older Edelbrock heads used the standard H-10 style Champion plugs - which had about a 7/16" reach (after the gasket was installed). If you have the older heads, then you need to be running a 7/16" reach plug. The ones that a lot of us run are NGK B6L Also, Edelbrock heads (all of them) have the spark plug over the valves - never over the piston. It is possible that if you installed the wrong length plug (like a .750 in a .4375 hole), that the plug will hit the valves . . . obviously this is not what you want. Plug Reading: You can't just pull a spark plug out of the engine and look at it and know anything as far as rich/lean conditions. Once you drive the car around, come to a stop, drive it into your garage, idle it, etc. -- the plug color will mean nothing. You can only test on the highway with a properly conducted load test. Even then, it is hard to read the plug unless you cut it apart on a lathe and know what you're looking at. My suggestion is a properly installed O2 meter if you want to know your real AFR. On fuel pressure, I like to run about 2 - 2.5 lbs MAX with Stromberg type carbs. Any more than that and you risk flooding the engine as the float/inlet cannot handle the higher pressure.
Personally I think you would be better off to fit an appropriate low pressure fuel pump and regulator. Not relying entirely on the regulator to pull down 7psi to 2.5psi . The needle is incapable of seating and stopping fuel at any more than 3psi. I had the same plug electrode problem years ago ,the heads had been milled too much,
These days, most used heads are at least 50 years old. Anything can happen in those ensuing years, especially when most of them were in the hands of "innovative" hot rodders. Any time I encounter a set of used heads, I measure everything to see where I'm starting at. quench distance, valve clearance, plug reach, etc. Some heads qualify as :wall-hangers" from the get-go. Only after they pass the initial tests will I consider using them and even then, most require additional work. Even new heads may need work. There was a long discussion on the Ford Barn" about the proper plugs for new Offenhauser heads. The final conclusion was that the current ones right from Offenhauser needed a special spacer to get any plugs to fit properly. And anyone installing a set of heads on a flathead without first optimizing quench is leaving some free performance increases on the table. Installing a set of aftermarket heads on a flathead Ford is more than just simply bolting them on.
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