What could it be!

[Fordboy300]

Hey guys, how's it going?

So this weekend I decided to do the yearly tune up and oil on my truck.  My truck has always like to foul the plugs it has been 5 or 6 years since I've rebuilt the engine and recently adjusted and changed the setup spring in the carburetor (edelbrock 500 cfm).  Today I decided to check out the cylinder 1 plug just for fun.  Yep the plug was sooty it alsohad the oil residue on the threads and the ground electrode had started to build up carbon deposits.

My first thoughts are that valve seals could be shot.  But it's only 1,3,5, and 6 that the plug has heavy or light  carbon build up maybe piston rings  but I highly doubt because it burn any oil.

The engine is a Ford 300 six.  

[1986F150Six]

I do not know the answer, but I find it odd that the listed prone-to-foul plugs are in cylinders 1, 3, 5 & 6. What came to my mind is that the firing order is 1-5-3-6-2-4. Is it possible that the distributor cap has arcing issues?

[Fordboy300]

Funny I just realized that it is the most of the firing order.  Well I'm not sure because it has that fancy D.U.I. distributor.

20180530_110247.jpg 

[Gary Lewis]

What is the "setup spring" in the carb?

[Fordboy300]

The setup springs are the spring that are with the metering rods underneath the cover that are top of the carburetor.

[Gary Lewis]

Oh, I understand.  The springs that step the metering rods up under low vacuum.  So, did you install lighter springs?

[Fordboy300]

Well the carb came with orange spring which is rated at 5 inches of vaccum and I installed the pink spring which is rated at 7 inches of vaccum.  So I believe I went to a stiffer spring and I adjusted the carburetor because at idle it would just smell like fuel to the point if I let it run in the garage I would have a foggy garage and your eyes would burn. It's also had a little stumble when I would get on it from cruise.  I'm sure i have to change jets and metering rods to get it running like a champ.

[PetesPonies]

At idle metering rods and other main jet adjustments have no operation value,. They are not part of the idle circuit.

[Gary Lewis]

Yes, you installed a stiffer spring.  Now the mixture will go rich at 7" of vacuum instead of 5" of vacuum.  In other words, it will take less throttle to make it go rich.

But that's when underway.  It doesn't change anything at idle.  Nor do the jets or rods.  Just the mixture screws.  So the foggy garage and burning eyes don't have anything to do with jets, rods, or springs.

As for the stumble when "getting on it from cruise", that depends on how quickly you move the throttle.  If you move it quickly and significantly then that is likely to be not enough accelerator pump.  If you are moving it slowly then that could be the springs, rods, or jets.

[Gary Lewis]

You beat me to it.