Poor fuel mileage in my C cab truck

HI everyone, hope the New Year is great for everyone.

I have a 51 C cab that I have had for some 32 years. I drive it nearly daily, and it has around 650,000 miles on it.

When I got it there was a 232 V8 sitting in it that the original owners son said the dealer had installed.

It now has a 62' 289 in it with a 3spd/OD transmission and a 4:11 rear end (Ford 8"). It is running on a set of 16" truck radial tires.

The carb is a Holly 2 barrell that was a stock unit for either a Chevy

283 or Ford 289, can't remember which, not that it matters what it was supposed to fit. It has a manual choke.

I am running stock distributor and the system was converted to 12V. Engine compression is good, even in all cylinders, but the exhaust and plugs always look a little black.

When driving in a reasonable manner, trying to be sensitive to fuel economy, I can get about 10MPG city and 13 MPG on the highway traveling at speeds in the 65 MPH range.

If I am pulling my motorcycle trailer or have a couple of my bikes in the back (I collect early Japanese bikes, pre-1959 and slightly later) the MPG drops to about 10 on the highway and less in city or congested traffic.

This just does not seem right to me and after years of complaining about it, I thought the collective minds in the group might have some thoughts.

I have a 4 barrell manifold and several friends have told me to put an Edelbrock 500 CFM 4-barrell on it, claiming that my carb is probably junk, and that the smaller primary barrells in the Edelbrock will give me better city fuel consumption as long as I keep my foot out of the secondaries.

Others have recommended an electronic ignition conversion for the distributor and a hotter coil in addition to a new 2-barrel or the Edelbrock.

The exhaust is a single unit, that has some years on it but seems to be flowing OK. I found a sticking manifold heat valve 6 months ago, replaced it thinking that it could be contributing to poor overall performance, but it had little to no effect on the fuel economy. I had thought of converting to a dual exhaust, but have some routing problems due to the second stock fuel tank that I installed on the passenger side (during the fuel crisis in the 70s').

SO.....my question to the group has several parts: Should I expect better fuel economy? What should I check or do that I have not done? Thoughts on the 2-barrell vs. 4-barrell?

I have thought about a gear ratio change since the OD transmission allows me to accelerate from a stop easily even with a load and perhaps dropping the engine RPM at cruising speed would help, but so far, the only way I have found to get better mileage is shutting it off........

Thanks! Allen

Reply to
Allen Siekman
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Reply to
Pat Drnec

back to basics: an engine is an air pump that compresses a combustional mixture that is ignited to produce a accelerated piston down stroke. Any restriction on intake or exhaust will affect efficentcy , so labor free breathing is the target. the 4 bbl. primary vs. a 2 bbl. is a valid item, but gas savings vs. cost of swap may negate savings. rebuild the existing 2bbl., use a low restriction air filter and a low restriction exhaust and I do believe you will optimize power/gas milage/driveability. try removing the muffler and replace with pipe to have a complete length exhaust system and see if you have power and mpg increases. I run a sbc in a 1/2 ton pickup with a Y pipe into a single exhaust with no muffler or other inline parts (straight pipe), it is quieter than my neighbors truck with duals and fartmaster mufflers and works well with sufficent back pressure and free flow. also never lug engine my sbc runs best between 2000 rpm and

3200 rpm so I shift > HI everyone, hope the New Year is great for everyone.
Reply to
oldcarfart

I don't recall reading anything about the compression you have but assuming it is stock at around 8 to 1, what is the timing you are using? If it is stock (4 BTDC) try moving it up to about 12 degrees. Listen for any "pinging" so you don't damage pistons, etc. and check the total timing less the vacuum. Total should be up in the 36 degree range and all in by about 3000 rpm. These are ball park figures and not exact as you will need to experiment to find the best. Also check the vacuum advance to be sure it is working. If it is not, this can lower cruising speed mileage.

Just some other things to check that don't cost money.

Ted

Allen Siekman wrote:

Reply to
tedharbit

Excellent ideas from all. If can add anything,I have a question.Does this Holley 2 bbl. have a power valve? If it does and it is bad (blown ,cracked,seeping) it will pass more fuel into the motor than it needs darkening the plugs,discoloring the motor oil sooner (diluteing it also!) and causing lousy gas milage.I had plenty of 300 cubic inch Ford

6 cylinder motors with the 1 bbl Motorcraft carb that had a power valve that was very troublesome. Not that a 6 cylinder powered 3/4 ton pickup gave good gas mileage mind you,but when that valve screwed up you noticed it right quick. 4 BBL Holley carbs have power valves that used to rupture during backfires,but Holley has since changed to blow proof power valves like on the Street Avenger series of carbs. So if your carb (assumeably older in manufacture) has one,be sure to replace it during rebuild,or just as an experiment. Next it is also possible that old muffler's baffles have colapsed internally causing undue resriction. Go for the duals,if just for the sound of it. The bypass pipe Calvin suggested will show you if a restriction is present. Make sure your vacuum advance is working properly,and is hooked to a PORTED vacuum port,NOT a FULL TIME VACUUM port. Why? Your motor dosent NEED vacuum to the advance at idle.It needs vacuum to advance the timing when the motor's RPM is riseing,which a ported vacuum advance port provides because it is above the throttle plate,so the vacuum increases as the throtle is opened and the motor RPM increases. Full time vacuum has the distributor in full advance at idle,and when your foot opens the throttle,a momentary pressure drop occures below the throttle plate (where full vacuum comes from ) which backs off the advance the distributor sees right when it needs it the most to match the increasing RPM and load of the motor. Try adjusting the idle mixture screws( before and definately after rebuilding / replacing that 2 BBL carb) with a vacuum gauge hooked to a manifold (full time )vacuum source,which will be off the base plate of the carb. or the intake manifold. With the motor full warmed up and at an idle,hook up the vacuum gauge and see what the amount of vacuum is.Now first try turning the mixture screws IN about a 1/4 turn each and see if your vacuum reading increases. (Do one side,then the other) This will LEAN out the carb. If the needle drops and shows LESS vacuum that before,return the screws to where you started,then, turn the mixture screws OUT a 1/4 turn or so at a time. (Again,1 side then the other) This will RICHEN the mixture. What you are trying to do is get the highest vacuum reading you can at idle,which will ballance out the carb,and provide you with better mileage. Screws are backed out too far will cause you to use more gas from a RICH mix, BUT a too LEAN mix will cause you to be into the throttle more to get it to move,and increased throttle opening equates to less mileage. I will make a SWAG that the mixture screws have backed out over time and you will find the carb too rich. Now if your carb is wore out and sucking a bunch of vacuum around the throttle bores (or warped body) and it needs RICHENING to ballance the mixture out,your mileage will suffer accordingly and no rebuild kit will help unless the throttle shaft is rebushed to stop the vacuum leaks.Might as well invest in new at that point. Check the carb body carefully where the shaft comes thru. I am saying this because I cant see how old your carb is or its condition. And remembering Calvins air pump explanation,is this motor breathing thru a little snorkle air cleaner? An open element air cleaner with a good flowing element might free up a few MPGs and a few more horsepower. More horsepower equates to more output for a given throttle opening.(actually torque is what moves a vehicle,and more torque is allways a good thing). At the risk of over simplifcation,if you can get the vehicle to a certain speed with 1/2 throttle,rather than the 3/4 throttle it took before your improvements,you can see the savings. Its up to you to decide if the up front costs of improvements will be worth the savings returns in gasoline. Hope I have helped.
Reply to
Champ TruckingCompny

Reply to
Allen Siekman

ALWAYS start with the cheap and easy .

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Reply to
oldcarfart

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