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Discussion Starter · #1 ·
Good Morning Ya'll!
I don't have a hot rod per say, but have been thinking about one of my vehicles fuel ratio and what better place to ask about carb adjusting/fuel ratio than a hot rod forum.

Here's what I have. I while back, I acquired a 1969 Ford N-750 Medium truck with the original 391 Industrial V-8 in her. To get her running, the previous owner stuck a Carter 9635SA on top of the intake, adjusted the idle mixture and called it good. She's been running good for 2 years now hauling houseboats but it's time for a tune up.

The Carter is a 625 cfm carb & I'm sure it's jetted for a 350/351. I cross referenced it with an Edelbrock and I came up with a Edelbrock 1400 as being the replacement. But anyway, I'm looking to get the most horsepower out of this old girl with the configuration I have.

I have an Edelbrock calibration kit (#1487) and was told to follow the chart for carb #1406. Looking at the jetting chart, I can do a 6% richer (both cruise & power) with the seats & needles I have or I can go richer (10% rich on cruise & 12% on power).

Doing a little math, the 391 has 11% more cubic inch than a 350 does so the question is, would it be correct to go 11% richer on the fuel ratio? She's not a hot rod by any means but she is geared low enough to pull everything but a gas station.

I'm looking for a proper ratio for what she does best. The low end torque and the pulling hp around 2500 rpm. Yeah, she doesn't spin very fast either. haha

What would be your recommendations for tuning this to get the right ratio?

Here's a picture of the old girl. Not many of these left on the road from the research I've done.

Thanks for any help on this that comes my way!



615471
 

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The Edelbrock 1406 is tuned for economy right out of the box, so they are seldom too rich. Considering the high load on your truck engine I would go as rich as you can on the power mode first, and then move back down the curve if needed. Stock Cruise mode setup might be okay, although a little richer might help smooth the idle circuit to cruise mode transition when the truck is loaded.

You may also need a step-up spring tuning kit, since you often need a higher rated spring to get rid of a cruise-to-power mode transition bog. The stock 1406 spring transitions from cruise to power when vacuum drops to 5”. If you switch to a 7” or 8” rated spring it will transition sooner as you accelerate and vacuum drops, and this can get rid of a bog. Of course you can also adjust the accelerator pump for a stronger shot, but by itself this will usually not resolve a bog.

Bruce
 

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Discussion Starter · #3 ·
My 1487 kit does have the springs in it so I should be good to go on that. I'll wait to see what the vacuum is on the motor and get a spring that corresponds with the vacuum I see. I'll start with half the in. Hg and work from there.

Please remember, this isn't a 1406 that we are dealing with. That chart is just one that I was told to use as a guideline. Unlike all the Edelbrocks in their books, this Carter is a 625 cfm (not 600 cfm), which will probably changes the numbers a little.

Never really had an issue with bog. More concerned about running too lean under a load.

One thing that brings this on is I lost an exhaust donut on the last tow. Got me wondering is I was running hot due to a lean condition or just old age of the lead donut that caused it to fail. Probably will never know but might as well richen her closer to what she needs and get rid of a possible dangerous lean condition.

Thanks for the reply!
 

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You might want to do a plug read before you start tuning. Take it out under a hard pull and just drop it in neutral and shut it off, then pull a plug or two and see their color.
 

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Discussion Starter · #5 ·
The tune up I am giving her is probably the first one she is getting in probably 20 years so it will be interesting seeing the condition of the plugs in the first place. But, she has pulled well since I have gotten her and the pulls are pretty hard with the hills and load she has been put under.

With the spark plugs that are in it now, wouldn't it be more informative to look at the condition of them from the repeated use she goes thru other than just a single pull test?

I can see how the plug test would give a rich condition indication but with a truck like this, wouldn't that be normal under a heavy load for the pulling power?

Just some info on what she is doing for me. Since I have gotten her, she has pulled 4 houseboats successfully. It has been thru rolling hills and up a long (about a mile long) 8% grade The boat in the picture above weighs about 15,000 pounds plus the trailer that it is sitting on. The total distance is about 60 miles.

Since this carb was new when I got the truck, I would think there will be a lean condition on the plugs from the general, overall use of the pulls she has done.

But it would be interesting to see how silted up they are after a quick little pull.

The question is, how much richer does this Industrial 391 have to run on than a personal 350/351?

I found a carb CFM calculator and just punched in a few numbers. I did a 350 c.i. compared to a 391 c.i. and both at a max RPM of 6500. The 350 gets a calculation of 559 cfm where the 391 gets a calculation of 625 cfm. That matches directly with the % increase of the c.i. of 11%. If I bring the rpm's down to the 391 specs of 3000 rpm, it remains at 11% increase.

This last pull, I lost an exhaust gasket and there is absolutely no "soot" around the leak. Which also indicates a lean condition under the normal use/heavy pulling.

What adjustments would you guys do to a 1406 straight out of the box on a 1969 Ford 390 for the initial test knowing the 1406 was set up for a 350/351? Keep in mind that I purposely pointed out an engine that is pre-emissions. ;)

Thanks for the replies!
 

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I think my next post will include a picture of the plugs and get a base conclusion as to how she is running overall.
See you guys in a day or 2. :)
I am not that familiar with the Carter carb, but I can tell you that the engine /trans/axle combo of that era got the job done. I drove an IH 1800 loadstar with a 392 Holley governed vac secondary, single axle tractor. Running a 5 speed w/2 speed axle. Pulling a 35 foot tandem axle flat bed. Hauling steel components for RV industry. This was in 72 and was mainly between local plants, it was no speed demon, but would pull real well. Later I worked for an IH dealer repairing trucks. Many farmers had the same truck as a grain/field truck. Always seemed to do a good job.
The cfm of the carb is not so important because you are looking more for torque at a certain rpm. That is why truck engines were governed to about 3500 rpm. Which is the peak given bore/stoke combination.
So 600-650 cfm would make little difference, more in the jetting to venturi booster size, and how the secondaries transition.
 

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Discussion Starter · #8 ·
I am not that familiar with the Carter carb, but I can tell you that the engine /trans/axle combo of that era got the job done. I drove an IH 1800 loadstar with a 392 Holley governed vac secondary, single axle tractor. Running a 5 speed w/2 speed axle. Pulling a 35 foot tandem axle flat bed. Hauling steel components for RV industry. This was in 72 and was mainly between local plants, it was no speed demon, but would pull real well. Later I worked for an IH dealer repairing trucks. Many farmers had the same truck as a grain/field truck. Always seemed to do a good job.
The cfm of the carb is not so important because you are looking more for torque at a certain rpm. That is why truck engines were governed to about 3500 rpm. Which is the peak given bore/stoke combination.
So 600-650 cfm would make little difference, more in the jetting to venturi booster size, and how the secondaries transition.
[/QUOTE

You're right about the cfm. The minimum cfm goes down greatly with the lower rpm's and I'm pretty sure that is what the calculator was showing. With the hills that this old girl climbs, it would be hard to notice the transition unless it was completely screwed up. I can see the importance in a hot rod or especially a daily driver, but of course a properly tuned carb is important for the motor in general.

And yup, these old girls definitely get the jobs done. Not very fast, but they do it. My collection?

1972 International Loadstar 1600, 345 ci with Holley 2 bbl. Only 40k original miles and runs like a champ.

1969 International Loadstar 1800, 392 ci. Is set up as a tanker truck for fighting forest fires. 4x4, sitting on 15x22.5 inch swamper tires, industrial drivetrain (much like a versa hauler) and diff locks on front and rear. Max speed is 40 mph. She'll climb over everything but a gas station. Still a project in the works.

1976 International 2010 dump truck. 548 V-8. Not pretty at all but runs great. Can't open the passenger door as the rust has made the door completely removable. Top speed is about 25 mph otherwise the death wobble will rattle your fillings loose. haha

Then there is the 1969 Ford N-750 that is the subject of this discussion.

I love these old beasts. haha



615482
 

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They look pretty normal for street driven plugs. You’ll see cylinder to cylinder differences this typical with a center plenum intake whether that plenum is mounting a TBI or a carb as the mixture varies cylinder to cylinder unless you apply port or in cylinder (direct) injection, it’s just the nature of these type beasts. In a carb or TBI you set the mixture so the leanest is correct which will make the naturally rich running cylinder a bit more than it needs, that’s just the way it is. If you lean out the rich cylinders it will fry the naturally running lean ones so gas being cheaper than internal replacement parts we just live with it. Plus cylinders run at different temps which results from the mixture deviations, compression pressure differences, and coolant circulation where it’s typical the fronts being at the pump inlet to the block run cooler than those in the rear of the block.

I don’t see oil on the plugs which gives confidence that the oil control rings are good and the valve stems are sealing.

Bogie
 
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