Sputtering under load

Hi All - could use your help on this one... My 64B has developed a sputtering issue while "under load" that is to say that if I'm going down the road and the engine starts to sputter -& I put the clutch in the sputtering stops & the engine will rev ok - when I engage the clutch & the powerplant is under load - the sputtering will start again.

Things that I have tried : A. draining the fuel then running the motor til it used up all the gas in the lines. B. replaced the gas w/ new fuel w/ dry gas & green/marine grade stabil gas additives to try to remove the chance of a fuel issue being the problem
C. put in a clear fuel filter just before the carbs to be sure fuel was getting there.

The car starts up fine it just sputters once you've gone a few miles down the road.

Any ideas ???
Thanks
Tj

Timing, carb adjustment, tune up? i`m just throwing a few ideas out there! good luck Timothy.

Well as for timing & or tune up- it runs fine when the clutch is in so??

NOW CARB ADJUSTMENT - I'd buy that but where to start?

needle out of adjustment & or choke tube???

thanks 4 the input

Tj

TJ- You don't mention the type or condition of distributor or ignition that you are using but that would be the first place I would look. Check the condition of the points, rotor, distributor cap, and change the condenser. Also while you're at it check for a loose plug wire and check the gap/condition of the plugs.

Ditto on Jim.

Sounds to me like a failing ignition coil. Substitute a known good one and see if things improve.

Jack