I bought a 1975 slant 6 4spd od truck in Tucson AZ.
i did the normal tune up to it oil,plugs Cap rotor,wires. I then began my journey back to Kansas it drove flawlessly ,but it seemed like it was down on power and trouble keeping 75 mph.. surprisingly it did not burn any oil !! could this be a fuel pump problem or ??

To be honest, that sounds pretty normal. I bought my 86 W150 new and when it was new, flat out pedal to the floor on level ground top speed was 80 MPH. Any hill and it dropped to 70 and required a downshift.

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If it was easy, everybody would be doing it.

My 76 D100 will do 80-85 tops, but that is because it has 3.9 rear gears. Is the 4 speed an overdrive or not? You may just have a truck geared to drive like a truck- reasonable engine RPM top speed of 50-55. With the 3.9 gears in my truck I am spinning almost 4000 RPM to go 70-75. The engine turns about 2500 RPM just to do 50-55.

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Casually looking for a Clifford hyperpak intake for cheap.

The truck has OD anything I can do to make it run better on freeway speeds ??? Thanks guys

A good exhaust system, distributor recurve, 2 bbl and careful tuning can make for a nice performance increase. After that it's head work (pocket porting and bigger valves), compression increase and maybe a cam swap.

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Joshua

Depending on your gear ratio, rear gears could help top end performance at the expense of low end performance and towing

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If it was easy, everybody would be doing it.

Buy a few bottles of gear oil and break cleaner. Open your def cover. Clean it out well with the break cleaner. Then count the teeth on both the ring. And then the pinion. Divide the two numbers and you will get your rear gear ratio. Mine is 3.55. The 39 ring gear teeth divided by 11 pinion gear teeth equals the gear ratio. (39/11=3.55) Then bolt it back up. Refill it with oil. And you just killed two birds with one stone.

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~Nic

Ok cool it's just a shorted rat rod type truck no towing or anything..I will check out the diff when I get back to the truck .

Check the rear axle gear ratio. Then do a full tuneup and check for vacuum leaks (especially at the distributor vacuum advance pod), accurate base timing (the outer ring on the vibration damper can slip making the timing mark inaccurate), proper valve adjustment, and proper carburetor adjustment.

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Casually looking for a Clifford hyperpak intake for cheap.

ditto about everything said,, my 83 D150 with 3.23 rear axle ratio when stock would top out at about 75 mph on level highway, about any rise in the grade would require a down shift to 3rd to maintain speed. That was with a factory 2bl, and highway mileage was in the 16 to 17 mpg

with the engine improvements: raised compression, cam change, HEI, os valves and head porting, dual Dutra's,,,, it will run 90 + and if I keep it in the 65 to 70 MPH range I can touch 20 mpg

With stock HP and aerodynamics your experience seems pretty normal.

You would need to improve either aero or HP to see a difference. I doubt a gear change will make any difference.

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Yeah....Im the one who destroyed this rare, vintage automobile.....

My 300.000 mile 77 d200 crew cab would run 75-80 all day long if I wanted to. I made a 600 mile move and loaded a 4000# slide in camper in the bed, then loaded 2 layers of fruit boxes full of books inside, and finished off with a 16' tandem axle trailer loaded with 7000# of stuff behind. On that trip I averaged 70 mph and 11 mpg, I don't think that sounds like it was "low on power". Yours sounds like it might need a little attention.

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Dave

1977 d-200 crew cab ex-army pickup wants it's /6 back
1962 Valiant 2 door, 170, three on the tree
1972 d-100 parts truck
80 volare wagon now a parts car

What did you set your timing to?

I'd look at how the timing curve comes in. It's pretty easy to do with any modern dial back gun. Map it, and it may tell some tales.

With stock everything, the OEM timing is 0 degrees base for cars. Everything in mechanical will give you something like 26 degrees, then whatever your vacuum can gives you. (Disconnected while checking the curve)

A happy slant of that era will like a short governor. Either a weld up or a 9R. 12 degrees initial will give you 32ish degrees mechanical, and your vacuum advance can do whatever it wants, as you won't have much signal at speed. An earlier ramp to full governor will help with acceleration, and you should try to get all your mechanical advance in by cruise rpm.

The trucks may have a different base timing. I Don't know. The engine, on the other hand, is the same.

I'd set it up to 10 degrees and see if that helps. Use caution and listen for detonation. If it doesn't ping, take it up to 12, otherwise back it off until it stops.

CJ

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Part of Tyrde-Browne Racing

ok cool thanks guys I will play with it little and see what I come up with I wont be around the truck until may I will check timing as well everything else seemed to be in order. I was smelling some fuel and the only part I could not be sure of was the Fuel pump.. and it did act like it was dying in traffic on me I had to feather the gas a bit...once the engine caught back up it ran ok

ok I upon looking a little a closer it is a 1bbl carb ,straight piped, no muffler very small pipe and I had a vacuum leak at the "tree " on the intake manifold so I plugged all ports that where not used on carb and intake...I did not check the rear end ratio yet I will when I go back home... think a better intake would and exhaust should wake it up ? any thoughts ?