408 possible timing issue

[73 408dart]

Hey all I have a strange issue that I cant figure out. First off the car is a 73 dart with a 408, mp electronic ignition and a mild cam and 9.8 to 1 compression. Now for the problem, when in neutral I can rev to the moon, but when I start driving it will only rev to about 2400rpms then it starts to feel like its leaning out. I've bumped the jets up three sizes, changed out the dizzy 3 times, changed the ignition box twice, changed plugs and wires and have had the timing set from 5 btdc to 15btdc with full advance coming in at 2500 all the way up to 4000. The carb is a holley street avenger 750 vac adv, sitting on an edelbrock performer rpm air gap manifold. Any help would be greatly appreciated!

 

[3406pk]

You say it feels like leaning out. Feels like doesn't equal actual lean. What do plugs look like compared to plug dianosis chart. What is total timing without vacuum advance hooked? Usually total timing is 34 to 36, and initial set by best vacuum reading at idle. If initial is high, too much mech adv will get you lean. But should not shut down rpm. Maybe need to hook up to fuel/air meter to get jets right. I have 408 w 870 holley and jet at 76/78.

 

[73 408dart]

Spark plugs look perfect. My total advance is 34 without vac and right now it's all in at around 3500rpm. Side note my torque converter is a b&m holeshot 2400 stall non lockup. Another side note, it's not pinging. Right now I'm at either keep fiddling with the timing or mess with the carb, it's almost like the secondaries aren't opening up under vacuum

 

[3406pk]

I don't think timing will correct. Something is happening under load. Even on primaries only, should get up to at least 4500 rpm just slower. Are throttle blades open all way? Maybe run up to stall in gear and watch vacuum gauge, and make sure all cyl have spark under load. I'm bout out of ideas.

 

[73 408dart]

Thanks for the idea I'll give it a go