Today's Conundrum

[SaxMan]

All excellent advice above.

Check valve adjustment, look for arcing plug wires or coil (as Trevor suggests, look at engine running in a dark place to spot any arcing).

Also look for cracked rotor or cap. If in doubt, change them (not too expensive).


...If that's OK, then look more closely at carbs (see picture below):

Unscrew 3 screws in float chamber cover (the thing under #16) of each carb, remove fuel line and carefully lift up the float chamber cover. Do not damage float mechanism(#12, 14, 15). Look in float chamber for any sediment. If it's really dirty, remove the 2 screws of the piston damper cover (the thing behind #1 in the picture). Pull straight up. The piston and needle are inside it and may come with it. Be sure they don't drop out as you are removing it and don't bend needle (#6). Use a can of WD-40 with "straw" and blow into hole at bottom of float chamber. Clear spray should blow out of jet (#9). You can also blow WD-40 into jet and spray should come out at bottom of float chamber. Watch your eyes! If not...there may be dirt in there which can cause problems like your's. Sometimes these cars end up running on one carb (poorly, of course) because the other carb has a clogged jet.

It's always useful to change the fuel filter if things are dirty. Your fuel pump is probably OK (since the car runs better at higher speeds).

By the way, my '73 currently has only 19,500 miles on it and it was a mess when I got it. It would have been better if it had 100,000+ miles and had at least seen some use. Sitting around is no good for these cars.

I did check out the floats tonight. The looked okay. Both floats did have a bit of sediment at the bottom. Both float chambers still had fuel in them, and with it 10 degrees outside, I really had no place to put the gasoline without freezing up the entire garage. One thing I did notice was that the front carb probably needs more damping oil. I could lift the piston about 1/4 to 3/8" before you felt the hydraulic pressure of the dampers. The rear carb had only about 1/10" of play before you hit the dampers.

 

[58Custom]

You always need damping oil. The oil is slowly consumed over time, so checking the level every month or so is a good habit to get into.

 

[AN5Sprite]

How do the damper springs look?
They have to be:
1.the same and
2.not all stretched out.

 

[SaxMan]

I haven't pulled the covers to get a look at the springs, but will take a look and see.

 

[SaxMan]

I took advantage of the nice day to work on the carbs again, among other things. Sure enough, when I did the WD-40 trick, all kinds of brown stuff came spurting out the other side on the front carb. Rear carb seemed to be fine, but I cleared the sediment from the float chamber on that one, too. The springs are matched and the needles were nice and straight. The front one had some sediment on it that I cleaned up. When I put the carbs back on, I'm going to replace the fuel filter while I'm at it.