Motorcraft 4300 Questions & Answers

Question:

I was wondering if I could ask a question: the accel pumps I ordered from you are for rebuilding a Motorcraft 4300 with stamped code D2PF-SA …. this carb, which I found out was for the ’72 Tbirds with a 429, was what came installed on a ’67 Tbird I purchased that has a smogged 390 in it (a rarer California-only version).

I had a concern which was the jets in it are sized at .64 or something, rather than the .45 ( or maybe it was .44 ) jets which are spec’d for that year and size and were what I found in a second 4300 I had and rebuilt ( but had to abandon as the bowl was warped and the supposedly fuel resistant gasket sealer I used to combat the warp broke down and contaminated everythin inside the carb! aarrggh) .

Would I be better off putting the smaller jets in ? I notice
the spark plugs were quite sooty black in this car … would that be
from it having oversized jets ?

Feedback on this and any other 4300 advice/feedback is much appreciated
! Thanks,

…Dave

Answer:

There are things other than the carburetor that could cause the plugs to be black, timing being one of them, but  I will only address the carburetor here.
Spark plugs should burn gray. Black usually indicates a too rich condition. That could simply be that the carburetor is supplying too much fuel because it is dirty. If the carburetor hasn’t been rebuilt, then I would start there. It could also mean that the jets are too big. If the carburetor has been running on the car and was OK at one time, then I would discount the jets unless you have moved to a high elevation of 5,000 ft. or more. Higher elevations of 5,000 ft. or more will usually require a jet 2 sizes smaller. If you don’t know the history of the carburetor, then it could be a jet problem. Someone may have put the wrong jets in the carburetor, or as already mentioned they have it jetted for 5,000 plus feet and you are below that. Due to the lousy gas we now have to deal with, the jet specifications written in the manuals is out the window. The specifications could be a starting point, but for the most part watching the color of the spark plugs is the best way to get it right and you do need to get it right, or you will be replacing your engine.When the spark plugs burn black reduce the jet size by 2, then try again. White plugs mean the jets are too lean. Increase 2 sizes and try again. Do this until you get it dialed in right.Motorcraft jets are stamped on the side, or on the top with either 2 or 3 numbers. Two numbers indicate the size (53=.053), Three numbers also indicate the size, but use only the last 2 numbers (253=053). Can’t find the numbers? Use drill bits, or reamers to determine the hole size.

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