jet size for 400 engine
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Senior User
Joined: Dec 2012
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From: Wildwood, Mo
jet size for 400 engine
My 351M is in the machine shop, when I get it back it will be a 400 with about 9.5 compression and a mild rv type cam. I am keeping the stock two barrel carb and intake manifold. I'm looking for information on the carb. Is it the same as what came on the 400 engine? And if so, will it need larger jets?
It is going back in our 79 Bronco. First time it ever been out. Thank you, Patrick
It is going back in our 79 Bronco. First time it ever been out. Thank you, Patrick
Postmaster
Joined: Nov 2011
Posts: 2,553
Likes: 10
The higher displacement and cam will increase air flow. Whatever you are doing on the exhaust, hopefully duals or duals plus headers will increase it some more.
The carb will very likely need bigger jets to add more fuel. Hopefully somebody will have a more specific suggestion where to start.
It also matters if it is an EGR carb to start with and if it has EGR now.
The carb will very likely need bigger jets to add more fuel. Hopefully somebody will have a more specific suggestion where to start.
It also matters if it is an EGR carb to start with and if it has EGR now.
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
Start with the stock size jets or maybe 2 sizes richer. It's going to take some experimenting and plug reading unless you have an O2/exhaust analyzer installed. Not a bad idea really they aren't all that expensive and work very well to really dial in the fuel mixture settings. It's a lot tougher to "read" plugs these days. Use the chart on the jet drill sizes, they deviate as they get larger from the stamped size. It's OK to drill out jets for test purposes but once the correct size is determined it's best to replace.
More Turbo
Joined: Apr 2007
Posts: 656
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From: Salisbury, PA
You really need to dial it all in after you get the engine in and running. Every engine is different. Even the air intake will make a difference. A cold air intake will take bigger jets vs an air filter simply on top of the engine.
With the "mild" cam you may need slightly larger jets.
Get it running and then take it for a short drive after break in. White plugs, too lean. Black plugs, too rich. Reddish/brown/pinkish plugs...good air/fuel mix.
With the "mild" cam you may need slightly larger jets.
Get it running and then take it for a short drive after break in. White plugs, too lean. Black plugs, too rich. Reddish/brown/pinkish plugs...good air/fuel mix.
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
I've been toying with the idea of an exhaust analyzer so in reading the reviews and such, one thing that keeps popping up often is the users who make a point to mention how they thought they had everything dialed in by plug appearance, till they actually started measuring air/fuel ratios and found they were still way off from optimum. Whether tuning for performance, or mileage, it's probably worth it. Today's fuel just doesn't "color" the way it used to.
Mountain Pass
Joined: Aug 2012
Posts: 114
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Everybody over jets carbs, bigger engine will probably use same or a size or two smaller jets. Bigger engine on same size carb makes a better vacuum signal to carb pulling more fuel through carb. Egr will make a difference though. Egr needs a leaner mixture. You do need to get it running, get timing set, etc, before jetting carb. Also might need to change power valve opening point with bigger engine also.
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