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Sorting out F250 390

March 12 2010 at 11:38 AM
Cook  (Login Gr8Cook)

1975 F250 390 V8 Camper Special 2wd C6 Dana 60

My grandfather originally bought this truck. With the exception of the cam (specifics unknown), Ignition (Mallory HyFire VI-A multi-spark and Promaster coil), Accel spiral core plug wires, The air filter and housing (big old K&N with substack), and Carburetor (Carter AFB) it is stock.

I rebuilt the engine about ten years ago. I did it because leaded gas was going or had gone away and wanted heads that would be reliable with that fuel. I had the head rebuilt with hardened seats and new valves (I think). That is when I put in the different cam (at the machinist's suggestion, he was a pretty big Ford fan and was building some pretty high dollar motors). I cannot find the card anywhere. It was supposed to be a good cam for towing, which is what this truck does. The engine had good compression before and the cylinder walls looked good so the machinist honed the cylinders and I put new rings on the old pistons.

The reason I'm writing is that the truck never felt as strong after the rebuild as before and it seemed to be more sensitive to spark advance. It is getting about 6.3 MPG and I think I can do better. It should at least feel strong at 6.3 MPG!

I have a Zeitronix wide band O2 sensor and have tuned the carb for WOT, Part Throttle, Off Idle, and Idle. The problem is that vacuum drops significantly under much of a load. I put a fuel pressure regulator in front of the carb to make sure it was getting fed consistently.

My current plan is to:
- Replace the stock manifolds and single exhaust with headers (http://www.sandersonheaders.com/product_info.php?products_id=88 )and dual exhaust with 2.5" Aero Chambers.

- Get with a cam manufacturer and figure out what cam is in there (degree wheel and dial indicator). I'm wondering if the new timing chain I put on there messed with the cam timing. I read on ford-truck forum that there were timing sets with retarded cam timing for emissions purposes. This truck has ZERO emissions equipment. I think its GVWR made it exempt. (Do you recommend a magnetic base or a bolt down base for a dial indicator on cam work?)

- Ask you guys here for guidance.

There are some leaks that need to be fixed so I may be pulling the engine. I was thinking of pulling the heads off and radiusing the combustion chambers a bit to prevent pre-ignition. I want to be able to maximize the spark timing.

How good are the heads and the stock intake manifold? I figure they are at least good enough to deliver the previous performance but I'd like to know if they are a big weak point. (where do I find the casting number that would tell you guys which heads these are?)

I don't have unlimited funds but as far as labor goes, I'm willing to go all the way.

I'm hoping to maximize mileage. I'm shooting for 12 MPG with an empty truck (more is always welcome) but if it is going to suck fuel it at least needs to do it with some POWER!

I've heard you guys here are real sharp engine folks who specialize in FE motors. I hope you can help me.


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(Login HugeBear)

It defintely should be better than that

March 12 2010, 1:17 PM 

In stock trim, my dad's 73 F100 would get 15 to 16 mpg. Loaded, unloaded, tailwind, no tailwind, it would get 15 to 16 mpg. We had taken the drivetrain (390, NP435, NP205) out of a 4x4 F250 and replaced the stock 360 with it.

Personally, I don't think you need all the bells and whistles you're mentioning, but this coming from a guys who's building a blower 390 for his pickup (I'm not too worried about fuel economy). Check the difference in cams for the model years, I think that an older cam grind will give you better performance and fuel economy.

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Joe D. Craine
(Login MsgtJoe)

Before you waste more money,

March 12 2010, 1:19 PM 

Check your vacuum advance for proper function. If it has a leak in the diaphram, or does not advance the timing properly, you will get poor performance, and terrible gas mileage. Joe-JDC.

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(Login Gr8Cook)

Re: Before you waste more money,

March 12 2010, 1:59 PM 

I've thought about replacing the vacuum advance just because it is 30+ years old. I've checked it and it does not leak and moves when vacuum is applied. I'm not sure how to tell if it is moving properly.

Here's a question. Should it be connected to ported vacuum or straight manifold vacuum and why?

Seeing as everything that I've done to it since the rebuild has accomplished, at least in part, what I wanted it to, I wouldn't call it a waste.

However, I do want to get the most bang for my buck.


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jim carlson
(Login jbcarlson)

Seems like too many changes...

March 12 2010, 2:30 PM 

The vacuum advance should be on ported vacuum, not manifold vacuum. Could be cause of detonation. Also, Ford vacuum advance cans are adjustable with an allen wrench. This can provide huge changes in mileage (and power). Also, why are you using that carb? I assume it's on an aftermarket intake. If so, what is it? Try a stock intake with a Holley 600 or old Autolite carb. Before, you do anything though, do a compression check and make sure the readings look right. This will tell you more than anything. Do it before you pull the engine and I'm sure something will be indicated.

Good luck.

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(Login Gr8Cook)

Stock intake

March 12 2010, 4:08 PM 

Good Question.

First off, this is my dad's truck. I write like it is mine because one, I'm doing all the work on it, and two it is easier to write "I" instead of "my dad"

It originally came with a dinky Holley 4 bbl. My dad was told by a fairly reliable source (I.e. he's not a moron for listening) that he should get a different carb and the AFB is the one recommended. I think the phenolic floats kept sinking and he couldn't find brass floats for it. He also got sick of the power valve blowing out every so often.

The intake is stock.

It has been quite a while but I'm pretty sure that the carb went on before the rebuild so it may not be ideal, I don't think it is the cause of the problem.

Great point on the compression check. I'll do that and see if that is revealing.


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(Login winr1)

Hey Cook....if it ran worse after the rebuild maybe the cam is holding the intake valve...

March 12 2010, 2:04 PM 

..open longer than the stock cam.

It seems to me, some of the torque cam specs dont look like they would work as torque cams.

It looks to me like they just took old grinds and called them torque cams.

Especially if you have low compression already.


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(Login Gr8Cook)

Re: Hey Cook....if it ran worse after the rebuild maybe the cam is holding the intake valve...

March 12 2010, 4:11 PM 

I've been wondering if the cam was the problem.

Do you have a preferred method for mounting a dial indicator? Magnetic base or something else?

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(Login hotrodfeguy2)

Dont know

March 12 2010, 2:56 PM 

How you get that bad. I get 10-12 with my 390 4X4 granted I have a stick but dam. I would check total advance also. I run mine @38 total with vac line pulled and plugged. I run 390 2V pistons also so I am at 9.5:1

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(Login lovesoldiron)

manifold vac

March 12 2010, 4:26 PM 

I would try the vacuum hooked directly to manifold vacuum. Never mind that the truck may have come with a ported vac setup. Many of the cars and trucks made during the emission era came with a goofy setup that had the vacuum disabled until the engine was begining to overheat. Even if the vacuum can on the dizzy is okay, are the hoses that lead to it all okay ? Is the thermal switch working ? Hooking up to straight manifold vac is a good way to test, and it is cheap and easy. On all of my rides that had dual vac on the dizzy, or had ported vacuum, I switched the setup to full manifold vacuum. I think it works much better. A nicer idle, and the tip in seems better to me. But it IS differant than trying to setup ported......The guys running ported vacuum need faster cetrifugal, IIRC. Cal/LSG

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S. Douglas
(Login ddbigengin)

I resemble that

March 12 2010, 9:03 PM 

I have 2 75 f-250 2wd trailer optioned trucks. ( They are the ones with the 390 4bbl. (big T cast in front of intake man.) The D5TE carb it came with was a 600 cfm 4160 holley-- not dinky, what it really needs! They were jetted with--assuming yours is the EGR delete heavy tk.-- 61 main front 41 plate rear a little lean front, even for no cam. the fact carb is plenty big enough---- unless you want to drive it over 140 MPH. ( I don't)


maybe a carb change and a 260-266 500 lift cam is in order.

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Rory McNeil
(Login RM428)

Re: I resemble that

March 13 2010, 10:00 AM 

I agree with DG, my 74 F350 ramp truck has a basically stock 75-6 390 4 barrel engine, and I`ve tried a few different carbs, including an Edelbrock 1406 (Carter copy), and none worked as well as the factory issue Holley 4 barrel. My truck has headers and dual exhaust, a 4 speed with 4.10 gears and 32" tall tires. It seldom goes out without a race car on the deck, and typically weighs around 10,500 lbs going down the road, on level ground at 55-60 MPH, it can occasionlly hit 10 MPG, usually more like 8-9, but I`d really have to be climbing some steep hills, of asking for a traffic ticket to get 6.3MPG!

428 powered Fairmont drag car, Best ET:10.03@132.11MPH, best 60 ft: 1.29
59 Meteor 2 dr. sedan 332, Ford O Matic
74 F350 ramp truck 390 4speed

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