I'm working on my in-law's 1978 D100 with a 318 engine. It runs fine at idle or revved up with no load. As soon as I start off in drive or reverse, it starts hesitating and backfires through the carb. If I put it in drive with the brake on and give it any gas, it does the same thing. New plugs, wires, coil, cap, rotor, vacuum advance. Timing is good and vacuum/centrifugal advance works as it should. No erratic spark at any engine speed. Timing mark doesn't "jump around" at any speed.

Compression 130-140 each cyl. Carb is only a year old, but I took it off and cleaned it just for grins. Good accelerator pump. Good jet of fuel from acc pump and good atomization when engine is revved up. Bypassed fuel pump and tank with an electric unit to no effect. Float level is good and I even ran it with the top off the carter 2bbl to prove the float level to myself with the carb mounted.

Valve timing dead on (new gears and chain). Engine runs GREAT - until you put even a slight load on it. It does seem to start bogging down a bit at 2500-3000 RPM or so, almost as if it's not breathing well, but I cannot prove it. It's more of a feeling I've got and I might be wrong.

Disconnected exhaust at cat flange (outlet of Y pipe), so clogged cat/muffler is not the issue. Heat riser is free.

No vacuum leaks. I sprayed carb cleaner everywhere looking for an induction leak. Vacuum is 15-17 on idle, which seems a bit low. Rapid fluctuation between 15-17 with each revolution. Drops to 5 when engine revved. If I let off the gas abruptly, vacuum jumps to 22" then settles right back down to about 15-17 again.

Plugs all looked good when I changed them. It does look like it's running a bit lean, but not excessively so. The plugs have always looked this way when I change them (slightly white instead of ash brown).

This started out as a hesitation when starting out from a stop or on acceleration It has gotten worse. The truck is a daily driver for my 80/90 year old in-laws, who bought it new. I'm afraid to have them drive it, so they're using the '65 Coronet until we get this figured out.

I'm stumped. I've tried all the usual tricks, but I'm not that familiar with Mopar engines, so I'm tossing this one out to the experts. Any ideas? Are there any special considerations or secrets that I should know about?

I would appreciate any ideas on this.

Thanks,
Bob