Carburator too big?
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Carburator too big?
I just bought a 1976 f250 with a supposed 390fe. The previous owner installed an Edelbrock 1407 750 cfm carburator. Is this too muvh carburator for that engine? I'm getting roughly 7mpg and am wondering if this might be a possible cause.
Lead Driver
Joined: Oct 2018
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That's plenty of carb but it shouldn't be a big problem if it's in good working order. well as good as Carter works anyway.
Pull a couple plugs and see what they look like. there are many causes for poor fuel mileage.
What is the build on the engine? maybe 7 is as good as it gets. the mild built 428 CJ in my high boy is running perfect and I bet 8 is as good as I'm getting.
Pull a couple plugs and see what they look like. there are many causes for poor fuel mileage.
What is the build on the engine? maybe 7 is as good as it gets. the mild built 428 CJ in my high boy is running perfect and I bet 8 is as good as I'm getting.
Laughing Gas
Joined: Apr 2018
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Depends how it is built, but my 390 gets around 12-14. I do have a 600 cfm Holley. Did the old plugs look like a rich condition? is this 2v or 4v? Auto/manual? Rear gear ratio?
What engine does your vin say you have?
What engine does your vin say you have?
Post Fiend
Joined: Apr 2002
Posts: 19,311
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From: Waterloo, Iowa
Post a clear close up pic of a couple spark plugs. Not the new ones, the old ones.
Unless somebody knew what they were doing, and spent some time at it, it's almost certainly not getting anywhere near the highway mileage it's capable of. Not that it will be anything to write home about when tuned-up but there's no sense wasting fuel. 7 mpg to 14 mpg is a 100% improvement, not bad on a percentage basis anyway.
Unless somebody knew what they were doing, and spent some time at it, it's almost certainly not getting anywhere near the highway mileage it's capable of. Not that it will be anything to write home about when tuned-up but there's no sense wasting fuel. 7 mpg to 14 mpg is a 100% improvement, not bad on a percentage basis anyway.
Cargo Master
Joined: Sep 2013
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From: Pryor, OK
Unless you are driving a really hopped up 390 with a wild cam and high compression pistons, I'd say yes, you are running too big of a carb. A properly tuned 650cfm carb is going to be much better for a mildly built 390. But if you are ALL stock, a 600cfm. People over carb engines all the time. Everyone thinks they are improving their performance with a 750 carb and really they could be doing much better with a smaller carb. Just speaking from experience here.
Lead Driver
Joined: Oct 2018
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It is an excessive carb but if it's jetted right it really shouldn't effect fuel economy under normal driving conditions. a little over carburation effects throttle response more than anything else. but I agree a 650 would be about ideal if it's stock.
If something is going on the plugs should be black, or at least dark.
If the engine is basically stock it should get 10 or so. I don't think the 390's we had around here ever got much better than that from the day they were new. stock 360 2v was 10 or 11.
the modern fuels don't help anything either.
If something is going on the plugs should be black, or at least dark.
If the engine is basically stock it should get 10 or so. I don't think the 390's we had around here ever got much better than that from the day they were new. stock 360 2v was 10 or 11.
the modern fuels don't help anything either.
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Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
F250 is heavy, with aerodynamics of a brick. How much more does it weigh than F100? I seem to recall the official hwy mpg was around 14 or 15 back in the day. If you can target Ford's official mpg hwy rating that will always indicate you're getting close to a decent tune. The problem with a poor tune is it fouls plugs out and this just compounds the problem, and wastes gas without improving performance. These trucks will never get stellar fuel economy but there's no sense wasting half of it out the tailpipe and diluting engine oil, wearing piston rings and bores excessively.
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