Little help here please

[macdaddy70]

My 70 GMC with 307 and 2bbl Rochester carb with divorced choke. New tank, sending unit and new plugs, wires, points (set with dwell meter), coil, etc. Recently rebuilt carb because of this issue.


Truck idles fine, but when I cruise 20-30 mph it wants to run rough (jump and surge but never go dead) Get on the interstate and can cruise 60 mph with no issues. If I let off the gas and get back into it, there is about a second hesitation before combustion and a noticeable THUMP, then it runs fine.
Any thoughts on what to check next? When the carb was rebuilt. I did use the paper gasket and not the 1/4" thick gasket that came with the kit. Should I have?
For what it is worth, it is hard to crank after sitting for a few days...not sure if that is relevant.
Any help would be appreciated.

[Steeveedee]

Does it do the lurching in high gear with a manual transmission?

Do you have a manual transmission, and what is the rear gear ratio?

The hesitation at speed sounds like the accelerator pump shot is not sufficient. I would recheck the float height. Some carb kits specify that the air horn gasket must be in place when setting the float height. This is important if the float hangs from the air horn. Didn't bother to look at the manual to see, since there is so much going on here, and you should have the instructions and the parts still, right?

Hard cranking, meaning it cranks a lot before it starts? That's just the lousy fuel we get these days evaporating from the float bowl, methinks.

[garyd1961]

Try spraying some carb cleaner around the base plate where the shaft for the butterfly comes through. I have seen these wear and sock air.

[macdaddy70]

Its a 4 speed manual with 373 ratio and the surge will occur in any gear. THought about the accelerator pump and will check float. THanks

[Steeveedee]

I was thinking that it might be lugging in high, but if it does it in all gears, that's not the case. Keep us posted on your progress!