Spark plug "reading" question

[pazcarguy]

I have a 66 coupe with an 11.5 to 1 CR 289 using 351 W heads and all the internal goodies. I bought the car last January and the engine was built by a reputable engine builder (Ford guy with 40 years experience). I pulled the spark plugs to gauge the A/F ratio and all are running rich except cylinder # 3 (nice light tan color). I am using a new Mighty Demon 650 mounted on a torker manifold that has 4 corner idle adjustment. I was wondering why cylinder #4 wasn't light tan as this is on the circuit as # 3? I have not performed a leakdown test or static compression test on the engine yet. The engine has been properly broken in and wanted some suggestions from the forum. Thanks, Brian

 

[69Rcode_Mach1]

That is not a big deal. You need to remember that when running a plenum design intake manifold that the air is shared through each of the runners to each cylinder. Sometimes one will get a little more or less, they are never exactly the same, they are very close though. That is why a bigger cam will lope a lot, and why if you throw on a set of webbers or individual runner carb, all lope goes away. Just tweak the carb a bit but remember err to the rich, do not go lean. The best way to read the plugs is to run the car up to high rpms where peak power is made, and shutting off the car immediately before the revs come down, then pulling the plug. This will get you the most accurate idea with the engine under load.

 

[iskwezm]

mine does the same,its mostly due to manifold design and how the air is unevenly distributed between cylinders