i have an 87 slant six with a 78 distributer and carb, at present time it will barley run over 50 on the highway , but it is running smoth and idleing really well. other than the speed this truck i have been working on is doing very well. oh, i useing the 4 pin hei on this also.

can some one give me some idea of why my speed is so far off?

Is it an automatic? maybe the kickdown linkage keep you from getting full throttle?
I guess you have checked the throttle wire?

Just a place to start...

Well as the saying goes, pick the "low hanging fruit" first:

- Is the basic timing correct? Does the mechanical advance mechansim advance the timing as you rev the engine up? Does the vacuum advance work?

- Is the carburetor linkage adjusted to allow full travel? (especially since you say its a 78 carb)

- Got good compression?

If all those check out OK, then I might start to wonder if the cam has jumped time. How many miles on the engine? Is the timing mark stable when you shine your timing light on it? If it dances around a lot, the timing chain *might* be loose enough to have jumped a tooth.

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so far all i have done is take the 78 carb out of the box and put it on the motor. the 87 carb was leaking at the feul inlet and woulkd not idle, the solution finally found from my last few posts. before i took the leaking carb off, it would run about 75. but stutter a little on the way up, with slow but smoth accelleration, and was using about twice as much gas as the oridgion setup. now it's getting great gas mileage, idleing the best it ever has with smoth accelleration still a little slow going up to 50, that is top speed right now, pegged to the floor. before i did anything else i just wanted to ask if their is a vacuume fitting i may have to use in this setup or a special throttle adjustment i may need to make. presentley the only vacuume running to carb is pvc and vac advance.

Could you possibly have a plugged up exhaust system-catalytic convertor, muffler etc?
Just a thought.
Todd