interchange
Elder User
Joined: Sep 2003
Posts: 908
Likes: 4
Carbs do not have larger jets on larger engines. This is a fallacy. The jets are set to maintain a 14.7-1 fuel air ratio against the CFM flow of the venturi. The only reason you would change the jet is to compensate for altitude.
Now, there were different size venturis available. It could be that a smaller engine would have a smaller flowing carb, but the difference wasn't tremendous, and I would opt for the largest flowing 2V available on any engine. I think it was 350 CFM.
I once had to put a smaller flow carb on my 429 for awhile, and there was a noticable drop off in power.
Now, there were different size venturis available. It could be that a smaller engine would have a smaller flowing carb, but the difference wasn't tremendous, and I would opt for the largest flowing 2V available on any engine. I think it was 350 CFM.
I once had to put a smaller flow carb on my 429 for awhile, and there was a noticable drop off in power.
Junior User
Joined: Dec 2003
Posts: 58
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I reckon Ford didn't know that. In the interest of science, I went and took a look. One has 53F jets and the other has 54F jets. Not a big difference to be sure, but a difference nonetheless. One is part #D1OF and the other is #D1MF. Both have 1.21 venturi's and are identical in all other respects as far as I can see.
Elder User
Joined: Feb 2002
Posts: 987
Likes: 2
From: East Bend, Carolina, U.S
I have a 351C that I rebuilt to have better performance. It wouldn't run as good as before with the stock jets. I installed a larger set and the difference says it all. There is definately a need for larger jet sizes based on the engine a carburetor is on. In my opinion it falls into the same category of why your engine needs a larger cfm rated carburetor. I suppose if you had the ideal size carburetor for a certain engine then the jet sizes wouldn't need so much tweaking. It is way too easy to change the jets and not the entire carburetor to get the correct air to fuel ratio.
I wonder how much the power valve is affected by altitude since it is operated by vacuum? Obviously vacuum is affected by altitude changes.
Altitude I am sure adds to the need to change to larger jets, but it is not the most important nor the only reason. I do not know the scientific terms to explain it, although I recognize the need to change to larger jets. Maybe a scientist will spread a bit of knowledge here and explain why.
I wonder how much the power valve is affected by altitude since it is operated by vacuum? Obviously vacuum is affected by altitude changes.
Altitude I am sure adds to the need to change to larger jets, but it is not the most important nor the only reason. I do not know the scientific terms to explain it, although I recognize the need to change to larger jets. Maybe a scientist will spread a bit of knowledge here and explain why.
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MSEE
Joined: Apr 2004
Posts: 10,386
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From: Austin, TX
ok, so the jets change, but will i run into problems if i use a carb with the original 351c jets and put it on a 400? or will it just be a bit less powerful.
thanks
thanks
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Junior User
Joined: Dec 2003
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Only one way to find out. Bolt it on. It will likely be close if your engine is stock. There are some small differences in the choke settings for the two I have, but you can always adjust that if necessary. One jet size is very small and it would probably be hard to tell the difference. I suspect vehicle size and weight and gearing all effect jetting also.
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