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Discussion Starter #1
I have a 71 Mustang with a 351C-4V, an EDL 2665 intake manifold and a Holley 1850 carb. I am only getting about 7 miles per gallon. The intake manifold pressure is only about 12 pounds which is the only problem I can find. I checked the compression on all eight cylinders and it is about 155 pounds with only a few pounds variation. I replaced the intake manifold gasket and checked all the vacuum connections and got no improvement in vacuum pressure. I also checked the timing and it is O.K. at 6 degrees BTC. The intake manifold and carb. were added by the former owner and the engine was rebuilt at the same time, about 1500 miles ago. I did notice that the ports on the EDL manifold are condiderable smaller than the intake ports on the head (about three/eights of an inch in each direction). This seemed strange but a speed shop guy said this was probably just the way the manifold worked. Any ideas on what my problem is?
Thanks, Dave
 
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Discussion Starter #3
I'm running 4v heads, the pressure is 12 at 600 rpm
 

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The 351C-4V is notorious for low manifold vacuum, but 12" is on the low end, as is 155# of compression. I would expect to see closer to 175-195# and 15-17" of vacuum. I would contact Edelbrock about whether this manifold is for the 2V heads and can, or even should, be port matched to the 4V heads. Also, do you know what cam is installed in this engine and is it correctly indexed. This could also be the reason for low vacuum and cylinder pressure. :)

'66 A-code Fastback (therapy)
'89 Town Car (SWMBO D'driver)
'87 Caprice (as req'd by Hagerty)
 
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Discussion Starter #5
sounds like the intake is intended for the 2v heads which can
affect intake air flow

SEAN
 
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Discussion Starter #6
I can't really help you with your milage problem but the #2665 intake is definately for 4v heads. The reason the runners are smaller has something to do with the rate the air flows through the ports. I think you should be looking some where other than the manifold. I'd be looking at the carb. Are you running an electric fuel pump or mechanical?
 
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Discussion Starter #7
Hi, thanks for the response, its a mechanical fuel pump.
 
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Well, If its a mechanical pump you can probably count that out as a problem. Are you running open or closed chamber heads? If they are closed and your using good fuel try advancing the timing to around 10 degrees. If they are open I would definately advance the timing right away to atleast 10 degrees and 38 total. You got me on the fuel consumption part it sounds like a thirsty car. Like mine!
Actually, do you have a vacuum advance on the car?<P ID="edit"><FONT SIZE=-1>Edited by 1970coupe on 04/01/01 06:35 PM (server time).</FONT></P>
 
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I don't know if they are open or closed, as far as I know they're stock 351 heads, there is no vacum advance, its a mallory electronic distributor.
 
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Are you hard on the throttle or just a daily driver. Any chance you might have miscalculated your milage? I would figure you should be getting close to double that (12 or so). Is the air filter in good shape?
 
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I don't have any new ideas or deep insight, but I do have a similar setup, and I'm getting 12 to 13 mpg (last time I checked). I had a really bad vacuum leak (caused by NEW vacuum caps that rotted within 3 months of purchase). With the vacuum leak, at 650 rpm the manifold vacuum was at about 12 to 13. With the leaks fixed the vacuum came up to 14. If I reved it to 700 rpm, it smooths out and the vacuum pops up to 15 to 16. I also used a bunch of spark plug wire separators to get the wires off each other, and with that the idle vacuum came up to 15 at 650 rpm.

Maybe you've got an old carb with a blown out power valve? Is it running ok? are the plugs comming up light tan/brown or black and sooty? Is the carb like an 800 cfm unit or something? The carb sounds mighty suspect. Is it a vacuum secondaries carb or a mechanical secondaries? If it was a mech secondary carb, and the secondaries were kicking in at, say, 2000 rpm, you'd be running all 4 bores all the time. Same with too soft of a spring in the vac. secondaries, and although I really don't know, I'd guess that would hose mileage.

I am running a vac advance stock distributor, so maybe that is the problem (but I can't imagine that vacuum advance would double the gas mileage).

By the way - I'm an amature at this and a lot of the ignition stuff I'm just in the process of learning, so take anything I say with a grain of salt.

Good luck!
 
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Discussion Starter #12
it's a new 600 cfm carb, secondaries are vacum operated. When I first got it, the old distrubtor was causing it to misfire through the carb and could that have damaged it. I adjusted the timing from 6 to 10 degrees, and the vacume pressure increased a bit and it idles a little smoother, but the pressure is still not up as high as it should be.
 
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Discussion Starter #13
Do you know what cam you have? If you know the manufacturer and cam id, the tech support lines (email and phone) are a good place to ask what vacuum it should be making at idle. Holley's tech support (via email) has helped me a lot recently. Also, I tried 72 jets in my carb (same as yours - 600 cfm vac. secondaries) and that did drop my mileage a bit - so it might be worth checking your jetting (especially if your plugs are comming up black or sooty).

Since misfiring at idle seemed to peg my idle vac. too low, I wonder if you still might have an ignition problem? Also, if your crank damper is old, the scale on it could be spun by a few degrees or more, in which case there is another timing procedure that has been deliniated here (and everyone who knows about it says it is better than tuning by the strobe light anyway).
 
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Discussion Starter #14
I emailed holley and acorrding to them, I don't have a blown power valve, I e-mailied isky about my cam. but I haven't heard back from them. I adjusted the timing and it helped with the pressure a bit, but i haven't had the time to fully adjust it. Thanks for the help eveyone. Dave
 
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