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Old 02-02-2006, 11:32   #1 (permalink)
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Exclamation 1973 F250 - 390CI - runs poorly

I never thought I'd appologize for having a long post, but.......

I got this old Camper Special. It has a new 3" dual exhaust, and a Carter AFB (750CFM) instead of the original Holley. It ran so so.

I put in a new set of points, cap, rotor and plugs. Timing was OK. It seemed to run a little smoother than before.

Still it stumbled at low RPMS, it would really bog down coming off a stop unless I gave it quite a bit of gas. I have to rev it pretty hard before shifting or it stumbles again in the next gear.

Thus begins my quest for better performance. I had suspected all along the carburator was an issue. I don't think it's jetted right. So I took it to a mechanic. After discussing my symptoms and the problems I had resetting the timing, he suggested a I might need a new distributor. I purchased a remanufactured one from Vapex. Certain I had installed it correctly, I rolled it over, got a loud bang!, then started second guessing myself had I put it in correctly. I called the mechanic over, he verified that indeed it looked like the points were set right, the firing order was OK, and the distributor wasn't a tooth off. We tried it again. Same thing. Crank, bang! then nothing.

After doing a compression test, the mechanic determined it may have jumped time. From everything I read, it seemed plausible. He hauled it in and replaced the timing chain and gears. Afterwards, he claimed it ran very rough with the new distributor. So he took it off and put the old one back on. It ran better than it had before for the first few hundred miles. But now I'm experiencing the same issues I had before I started, even worse. It starts hard (I replaced the battery after all that cranking) it backfires when I try to start it, it backfires sometimes coming off of idle. It still stumbles like heck and I've lost confidence in my mechanic.

So...should I go after the ignition? (plug wires, update the distributor to electronic etc.) Or try to rejet the carb or go back to the Holley. Or something else (ring job, valve job etc.) Could I be overlooking something?
Thanks in advance for the advice. I look forward to hearing from other big block owners with similar issues and proven answers.

MTG
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Old 02-02-2006, 19:23   #2 (permalink)
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Re: 1973 F250 - 390CI - runs poorly

750 is too much for a stock 390, biggest they came with from the factory were 600's i believe, i would start there, any carb rebuilders near you? i just had my autolite redone and for $110 its like new. if the timing is ok, firing order is ok, and points are set right there isnt much left to cause backfiring/bogging from the ignition.could be a valve problem but i'd start with the carb.
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Old 02-03-2006, 05:22   #3 (permalink)
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Re: 1973 F250 - 390CI - runs poorly

Thanks for your reply. Don't stop now. Anybody else got something to add?

I'll verify the model number of the carb again (in the event it's really a 600 I'll try to rebuild it) Or would I be better off trying to find an original replacement? I don't have the original carb. The previous owner I think was talked into the Carter rather than a rebuild because the cost was about the same, the rebuild was tricky, and the Carter was better anway. Any feelings on that?
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Old 02-05-2006, 15:17   #4 (permalink)
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Re: 1973 F250 - 390CI - runs poorly

i havent run the autolite yet but my carb guy says its an excellent carb design, he says carters/edelbrocks are a reliable simple design, q-jets are good when tuned properly(but terrible to work on) and holley is his last choice, dont know why. its like anything else, people have their favorites and they'll blast anything else on the market. personally i havent had much luck with holleys, mainly my experience is with q-jets since i've always owned gm's until the country sedan. if that holley is a 600 or 650 i'd say get it rebuilt and make sure they know that its going on that heavy truck. you can post the holleys # on here, i'm sure someone can decipher it, i could but i'm moving and the books packed up.
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Old 02-06-2006, 09:57   #5 (permalink)
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Re: 1973 F250 - 390CI - runs poorly

What are the piugs gapped at? And what is the points set at? And what is the timing set at? I agree, 750cfm is to large for a stock 390, are you sure that it is a 390. Most of the trucks came with a 360ci engine. When wwas the last time the fuel filter was changed? I change fuel filters every spring in all my cars and trucks. I did not see in your post if you changed the plug wires, that is a very good thing to look into.
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Old 02-06-2006, 23:10   #6 (permalink)
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Re: 1973 F250 - 390CI - runs poorly

There might be a filter screen where the fuel line goes into to Carburetor, if that clogs up your motor will sputter and bog down losing power, so check that. I had a 76 3/4 ton years ago with 390 and had that problem. Also I am with the other two guys, your carb is too big. Being the truck is that old, you could have rust and dirt particles comming from the gas tank and clogging up the fuel system causing it to run too lean.
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Old 02-07-2006, 10:38   #7 (permalink)
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Re: 1973 F250 - 390CI - runs poorly

I've determined the carb is 600 CFM. It's a 9605, which is listed as having a GM linkage. The 9607 (with Ford linkage) is discontinued.

My plugs are gapped at .035 and the points were .017. The distributor was tight and didn't appear to have moved. Timing is 6B and idle is 650 RPM.

It's got 7mm premium Silicon plug wires, but I noticed the clown had twisted one of the wires so that it was pressing up against the exhaust manifold. It was as limp as a noddle when I caught it. Will it survive?

The fuel filter is too cheap and easy to overlook. I'll do that first. Then check the plugs for fouling, inspect the distributor cap for cracks and give it a try. If it doesn't want to fire easy, I think I've located a rebuild kit for the Carter AFB.

Can anyone think of a reason why the new distributor I had bought failed to work besides maybe the condenser? I'd like to try it again. I swear that old distributor shaft is wollered out. I also believed it would solve any advance problems I might have. Can I damage the engine installing or wiring a distributor incorrectly?

Thanks for your suggestions. I'll let you know how it turns out.

mtg
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Old 02-07-2006, 18:40   #8 (permalink)
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Re: 1973 F250 - 390CI - runs poorly

For sure replace that limp sparkplug wire, never did like those carbon resistor wires. I remember when I bought my 1976 F250 it was brand new and about a couple weeks later I installed solid copper core wire and it never once misfired. Just the tiny fuel filter screen where the fuel line goes into the carburetor gave me trouble a few times, had to keep cleaning it out, but finally I drained the gas tank and did get some crap out of it, after that it was ok. From your last post, looks as though you have done everything right.
Good luck and let us guy know how you make out with it.
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Old 02-08-2006, 09:11   #9 (permalink)
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Re: 1973 F250 - 390CI - runs poorly

To check the wires you can use a inductive timing light, just clamp on each wire and watch the way the light flashs. If you have a misfire in the wire you will get a intermitant flash or no flash. On the new dist., was it a rebuild or new? It may have been a bad tune-up kit in it. I have had rebuilt dist. that would not work and returned them for differant ones more than once.
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Old 02-22-2006, 19:12   #10 (permalink)
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Re: 1973 F250 - 390CI - runs poorly

I've got a Ford 390, and as far as it bogging down, mine originally did when I got the vehicle, due to a major rust build up in the old metal gas tank, it would bog and sputter horribly, it also made it hard to time b/c of the lack of gas flow, after I cleaned out my gas tank and cleaned out my fuel lines, took apart my carb cleaned it and put it back together, there was a major difference in performance, it was very quick at take off and idled and ran much smoother. might want to check for any rust build ups if its a old vehicle. I'm using the stock distributor, with an Edelbrock 650 and an Edelbrock 850 cfm carb, I alternate the two from time to time for different performances, but both the 650 and the 850 run perfectly fine for mine, if its old, I'd definetly check for clogs, and rust build ups or deposits in your fuel lines.
Josh
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