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  #1  
Old 01-09-2011, 08:08 PM
86sixbanger 86sixbanger is offline
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Gas milage? 1986 300 I6 with feedback system

I am tired of bad fuel economy from my 300. So I started by rebuilding the carbueretor. I seem to have almost no changes in mileage or anything, except that I forgot how the choke linkage goes together exactly so now the choke doesn't work. I get like 10 mpg. The air filter is clean, the mixture screw is adjusted nice and lean, and the truck runs smooth when it's warmed up. Maybe a new set of plugs would help? Any help on the problem is appreciated.
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  #2  
Old 01-10-2011, 09:53 AM
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What conditions are you getting 10mpg? Is this highway mileage? Cold starts and short trips around town? Etc.

As for your choke, one that isn't working can really contribute to poor mileage. If it isn't working, is it always closed or open?

Also, is your feedback system complete? If it isn't, or parts aren't working, it could be that your computer is putting the system into limp mode, which would explain your poor economy.
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1984 Ford Bronco. 300I6 Offenhauser C Intake Holley 600 4bbl, 31" BFG A/T, NP435, 3.55 rear 8.8" EFI Manifolds. 2.5" high flow cat/muffler.
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Old 01-10-2011, 02:30 PM
86sixbanger 86sixbanger is offline
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The choke is always open, and the truck seems to run good and strong once it warms up. I mostly make an 11 mile commute one way every day, so about a 22 mile round trip, usually with a few more added miles for random errands.
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Old 01-10-2011, 02:40 PM
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11 miles isn't too long for it to get fully warm, especially with the choke wide open (which it wants closed for cold driving).

I'd address the choke first thing though. That'll at least help some. If it doesn't, something might be not working correctly on the feedback setup.

My '84 that has a 600cfm 4bbl (ignition converted to the DSII), has the same gearing and transmission as yours, and I get around 12 - 15 with short trips (about 5 miles one way, barely enough time to warm up), but I've gotten as bad as 6mpg with enough short, cold trips.


Have you put a timing light on it to be sure the computer is properly advancing the timing?

Do you have a vacuum gauge? What is your vac at idle?
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1984 Ford Bronco. 300I6 Offenhauser C Intake Holley 600 4bbl, 31" BFG A/T, NP435, 3.55 rear 8.8" EFI Manifolds. 2.5" high flow cat/muffler.
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Old 01-10-2011, 03:37 PM
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Cold starts and short trips = MPG disaster. Period.
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Old 01-10-2011, 04:40 PM
86sixbanger 86sixbanger is offline
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even with the choke working before I got the same milage. how exactly do you make sure the ignition is advancing? I haven't checked the timing at idle for a very long time, but the last time I did it was 14* BTDC.
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Old 01-10-2011, 04:47 PM
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To check to be sure the timing is advancing, put a timing light on it and disconnect the SPOUT connector. See what it is at idle.

With the SPOUT disconnected, rev the engine and watch the timing as you go from 700 to 2500 RPMs. Should go from, say 10° to around 30°. If so, the mechanical advance is working fine.

Reconnect the SPOUT connector and repeat. Since the vehicle isn't moving, the timing at idle and the timing at 2500 RPMs should be a bit higher, like 25° at idle, 45° at 2500. If so, the computer is advancing it correctly.

It's a lot easier on a vacuum advance since all you do is put vacuum to the canister, but that should work well enough for a computer controlled timing. Someone may have a better suggestion.


If you haven't checked your timing for a long time, I'd start there. Have you ever timed with a vacuum gauge? Hook a vacuum gauge up to full manifold vacuum. With the SPOUT disconnected, advance the distributor until the vacuum won't climb any higher (it'll peak out). Then, back it down until it sits around 2hg below the highest. So if you could get it up to 20hg vacuum, back it down to 18hg. Your base timing should be perfect.

Also, use this to adjust your idle mixture screw. Adjust the idle mixture until you get the highest vacuum reading at idle.



If you haven't done them, a set of new wires, plugs, cap, and rotor is always a good tuneup.
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1984 Ford Bronco. 300I6 Offenhauser C Intake Holley 600 4bbl, 31" BFG A/T, NP435, 3.55 rear 8.8" EFI Manifolds. 2.5" high flow cat/muffler.
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Old 01-11-2011, 08:47 AM
86sixbanger 86sixbanger is offline
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Where/what is the SPOUT connector?
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Old 01-11-2011, 09:36 AM
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I'm not 100% sure, but if you have done your timing before, and didn't disconnect it then, then your timing is probably WAY off. It must be disconnected before setting your base timing.

As far as I know, it's on the distributor itself somewhere, and is a plug of some kind. It is the input from the computer to advance, or not to advance, the timing.
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1984 Ford Bronco. 300I6 Offenhauser C Intake Holley 600 4bbl, 31" BFG A/T, NP435, 3.55 rear 8.8" EFI Manifolds. 2.5" high flow cat/muffler.
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Old 01-11-2011, 09:39 AM
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Here's a picture I found online. It's for an '88 302, but I imagine very similar.

Click the image to open in full size.
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1984 Ford Bronco. 300I6 Offenhauser C Intake Holley 600 4bbl, 31" BFG A/T, NP435, 3.55 rear 8.8" EFI Manifolds. 2.5" high flow cat/muffler.
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Old 01-11-2011, 10:12 PM
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Quote:
Originally Posted by 86sixbanger View Post
Where/what is the SPOUT connector?
It's on the wire bundle that goes to the distributer, on a separate connector. It's usually a yellow wire.
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Old 01-12-2011, 10:55 PM
86sixbanger 86sixbanger is offline
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thank you. no, i did not set the timing, I just figured ahh, good enough, and left it alone.
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Old 01-13-2011, 09:48 AM
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Quote:
Originally Posted by 86sixbanger View Post
thank you. no, i did not set the timing, I just figured ahh, good enough, and left it alone.
I'll be curious what you find when you do do it.
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1984 Ford Bronco. 300I6 Offenhauser C Intake Holley 600 4bbl, 31" BFG A/T, NP435, 3.55 rear 8.8" EFI Manifolds. 2.5" high flow cat/muffler.
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Old 01-13-2011, 09:48 AM
 
 
 
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