1972 402 stumbles under load

[1972_C20]

Ok, I'm about to light this thing on fire. Truck idles ok and cruises ok. Once the 4 barrels even start to open up, it falls on its face and backfires out of the exhaust and the carb. It's different every time. Things I've done.

Cam ran flat, so the cam is new. Yes it's broken in correctly. Truck ran good after new cam was put in. Started acting like it was starving for fuel, so I installed an electric fuel pump. This fixed the truck for a week and it started sputtering under load again. Rebuilt carburetor and did not fix the issue.
Ended up replacing the carb due to it leaking fuel from the throttle linkage. I replaced it with a 750cfm summit brand carb.
Timed at 16 btdc
Adjusted valves with engine running.
New ignition coil
New spark plugs
Spark plug wires, cap, rotor, and ICM are no more than 6 months old. Cap is not cracked. Dielectric grease on all wires, plugs, and connectors.
Fuel filters are brand new.
Fuel pressure is 5.5
Vacuum advance is working
Mechanical advance has been cleaned of all rust and moves freely.
Replaced power supply wire going to my hei dizzy.

[geezer#99]

Is your carb a vac secondary copy of a holley?
Have you verified timing tab/balancer marks?
Did you flush out (hot tank) and redo the motor when you put the new cam in?
Have you tried more timing?

[1972_C20]

Yes, carb is vac secondary of the Holley. If I put too much timing to it, the truck doesn't run right. The motor was gone through yes.

[geezer#99]

I assume you've cut your mechanical back to 14 or so degrees.
Is the distributor centre shaft shimmed enough to keep the shaft from climbing upward changing your timing.

[Ironangel]

I bought a "Summit brand" Quadrajet four years ago and that thing was junk right out of the box! I immediately replaced it with a new Jet brand Quadrajet and she's ran like a top, very tunable. The Summit brand carb was sent to Cliff Ruggles who later told me the bushings were installed crooked and leaked. He went through that carb and jetted it for an L34 big block (SS396) motor and $365 later I wound up with an AWESOME Quadrajet that required no more than an occasional idle adjustment. The moral to this story is, you get what you pay for! You can have the worlds greatest motor and if you top it off with a questionable piece of crap, you still have a piece of crap, Slummit is junk! get a real Holly or Quadrajet from a reputable carb rebuilder or spend your money and time in wonderland..."Just sayin"

[1972_C20]

Geezer, I didn't mess with any shims on the distributor. Nor have I heard of that

[geezer#99]

Here's a video on the shaft end play.
https://www.youtube.com/watch?v=UTN5j9c5A9A
If the endplay is excessive it can alter your timing or give you spark scatter.

[1972_C20]

Sorry it's been a while. I found that the distributor had too much play in the mechanical advance. I replaced the dizzy and problem solved. Thank you for the help guys

[davepl]

Originally Posted by 1972_C20

Sorry it's been a while. I found that the distributor had too much play in the mechanical advance. I replaced the dizzy and problem solved. Thank you for the help guys

Wow! Thanks for coming back with the resolution... not one I would have guessed!

[Zone47]

Originally Posted by 1972_C20

Sorry it's been a while. I found that the distributor had too much play in the mechanical advance. I replaced the dizzy and problem solved. Thank you for the help guys

Well shoot! I was hoping for a video of the fire!

Anyway, glad you found the prob.