motorcraft 2100 jetting ford 390

O

Old2New

Member
I have a stock 390 that is running lean at cruising. not by much but the plugs look pretty lean after highway cruising. The carburetor I have is a motorcraft 2100, 1.23 and #54 jets. The engine is a stock 390, 1968. Would I benefit from running bigger jets? I bet about 19" vacuum at 12° btdc timing. Idle mixture screws 3 1/2 turns out. 650 rpm idle in gear. Im jjst trying to figure out how to make the engine as happy as possible and dont like the engine running lean. The plugs look to be in the ok range. would jumping up to #60 be too much?
 
We are blessed with at least 10% alcohol in our fuel. Alcohol has less heat energy than gasoline. Therefore the diluted gasoline will naturally tend to run lean. #60 is a pretty good jump, but what the heck, give it a try. It can't hurt, it just may decrease the gas mileage a touch. John
 
Hoosier Hurricane said:
We are blessed with at least 10% alcohol in our fuel. Alcohol has less heat energy than gasoline. Therefore the diluted gasoline will naturally tend to run lean. #60 is a pretty good jump, but what the heck, give it a try. IT CAN'T HURT, it just may decrease the gas mileage a touch. John
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Thats what I was wanting to hear. I'll give em a try. I would think better rich than lean.
 
You said it's running lean when cruising, but the plugs look to be in the OK range. Are there driveability symptoms like surging at a steady state, or any other issues? If it runs good and doesn't ping, I would leave as is, although 3 1/2 turns out on the mixture screws for best idle is about twice as far out as normal. Larger main jets do richen up the idle system a little bit too. Not much. If you change jets, I would think maybe 58's tops and see how the plugs look.

Sal
 
I meant that the plugs are barelh in the OK range. Not great, not even good, just barely ok. When cruising it does have a slight surging issue. I keep telling my buddy its because of my rotten old tires but he insist that its surging. I went ahead and ordered the 60s so I will slap them on when they come in and update everyone
 
I know a lot of people say you shouldn't hand drill your own main jets, but I do it and re-stamp the size sometimes. Never ran into a problem. Autolite/Motorcraft jets are stamped basicly the same size they are. For example, the #54 jets you have should be .054".

Sal
 
scicala said:
I know a lot of people say you shouldn't hand drill your own main jets, but I do it and re-stamp the size sometimes. Never ran into a problem. Autolite/Motorcraft jets are stamped basicly the same size they are. For example, the #54 jets you have should be .054".

Sal
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I also remember reading somewhere that tbey are straight cut so drilljng shoukdnt be a problem I would think. Am I correct in remembering that sone jets are tapered?
 
There may be jets that are tapered, but none I know of. Been working on all domestic carbs for a long time. Metering rods are tapered, but not jets that I know of. I could stand corrected.

Sal
 
do you need a drill press or could I just put it in a vice and hand drill it? how critical is it that the holes are perfectly vertical?
 
You need to do it as straight as possible. Your only opening up an existing hole slightly. Most ideal way is a small high speed drill press with a fixture to hold the jet. but can do it by hand too with a hand held pin vise to hold the drill bit.

Sal
 
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