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Old 12-22-2011, 05:54 PM   #1
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gas mileage question

I have a ford dumptruck that has been getting 5 mpg hauling firewood on one ofe my jobs. 90 mile round trip. leaving for work temps outside have been 17 degrees in the morning, 30 degrees in the evening . I am hauling the same weight load, driving the same speed , same route. I fill up at the same pump every evening. 18 gallons, 5 mpg. today, on the way home, I inadvertantly left my manuel choke on , didn't notice it til about 5 miles from home. truck seemed to run fine, no loss of power, but when I filled up, it only took 16 gallons! 5.6 mgp. do I need to richen up my carb?
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Old 12-23-2011, 05:51 AM   #2
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Re: gas mileage question

If, without the choke on, it is running lean and misfiring, then richening it just enough to eliminate misfires should help your mileage.

Before adjusting your carb, you could experiment with the choke knob for a few tanks and verify that choking it indeed slightly helps you mileage.
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Old 12-23-2011, 06:29 AM   #3
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Re: gas mileage question

Leaner is better for fuel economy, unless the engine begins to misfire. Erik's right.
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Old 12-23-2011, 07:04 AM   #4
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Re: gas mileage question

Might want to look into the vacuum-operated emissions gadgets that may be under the hood (what year is it?)...make sure they're all operating and adjusted. Also ignition timing...if it's too far advanced and you can't hear the knock then it might behave as described.
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Old 12-23-2011, 07:25 AM   #5
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Re: gas mileage question

If its old enough to have a manual choke, it likely has very few, if any emissions gadgets under the hood.
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Old 12-23-2011, 08:05 AM   #6
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Re: gas mileage question

it is a 74 with a 477 superduty. no smog. when I installed the carb, I had to turn the idle mixture screws out 3.5 turns to eliminate hesitation off idle. the truck seems to run fine. on a rig like this more power means better mpg. I am doing mountain driving, and the longer I am on a hill, the more gas is used. if slightly richening would give me more power, I would expect better mpg. it is a very low compression engion. timing doesn't seem to cause pinging. right now I have it advanced enough that it backfires going down hill
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Old 12-25-2011, 04:20 AM   #7
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Re: gas mileage question

3.5 turns doesn't sound like much, so you might be running lean. I would think you'd run better with 6 turns or so, but you really need a vacuum gauge to figure this out.

-Bob C.

(of course that's only at idle, and that doesn't have a huge effect when you're running at 1/2 or 2/3 throttle!)
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Old 12-25-2011, 02:54 PM   #8
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Re: gas mileage question

I want to be clear on how many turns. when I say 3.5 turns, I am talking about complete, 360 degree turns. are you talking about full turns, or 180 degree turns?
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Old 12-26-2011, 07:33 AM   #9
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Re: gas mileage question

IMHO- You should stop turning the screw if it begins to miss at idle as you are unscrewing the idle mixture screws.

Like Bob said, the idle mix adjustment doesn't have much effect when you get up to 1/2 throttle.

Rejetting, or raising the float level (assuming it is a little low) would definitely richen it under load.
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Old 12-26-2011, 09:00 AM   #10
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Re: gas mileage question

this carburetor came off a smogged bronco. I don't know what year. I assume it was set up lean for emissions. going up 1 or 2 jet sizes might be worth a try. just using the dumptruck gives me 5 mpg, but when I am pulling my backhoe mileage drops to 3 mpg. the compression on these engines is 7.5:1 so a turbo would be nice, though a blower would be easier. like I said before, more power on the hills means less time on the hill, which equates to better mpg.
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