Yeah, It's Me Again

Still going through the teething pains. The stock 440 in my car still hesitates,back fires and stumbles under hard acceleration. Other than that annoying and aggravating trait, it starts right up, idles forever and runs smoothly under partial acceleration. It is a four gear car, btw. What I've done so far: Replaced the fuel pump, adjusted the BRAND NEW Holley's float bowl levels, tuned up the idle mixture screws, replaced the coil, replaced the condenser, spark plugs and ballast resistor. HELP ME. I have rusty razor blades near my wrists.

have you checked the distributor and vacuum advance?

Your distributor may be worn or maybe even worn out?? I agree with Stumpy,

Mike

During hard acceleration/loads: the Secondary (HV) portion of the ignition system sees the most 'stress'.
Concentrate there first.

Carb details, size etc... What is your timing at, full advance and idle? Where is your vacuum advance hooked up...

By HV do you mean High Voltage? Plugs and wires are brand new. Th carb is a 700 cfm double pumper, w/ #72 jets in the primaries and #80 in the secondaries.

Incorrect point gap/dwell

A cam with lobes going flat will pop through the carb under accel.

I had a girl friends car years ago act just like that and thought it was the carb and I took it by a friends shop and he laughed and said it ain't the carb the cams flat.

what kind of distributor?
what cams do you have on the accelerator pump on the holley?
had one that had some weird vacuum leak in the vacuum advance pot and would cause the car to pop and sputter under hard acceleration. unplugged the advance and it fixed it.

Burnt valve? I'd do a compression test and see what the cylinder pressures are like. Accelerator pumps? Checking this with the engine not running, is easy to do. An AFR gauge would be helpful in this situation as well.

Dang, Andrew treed me on the accelerator pumps....

The pink cams. And what the hell is this talk about burned valves and flat cam lobes. You guys are overthinking this. It's a baffling problem, but I'm sure it's a simple fix. I'm surprised no one suggested rebuilding the engine!

Have you rebuilt the engine since your first post on this? I still think it's a worn distributor if it is original to the car. (DAMHIK and why I think this)

Well if it was that simple you should of had it fixed by now. You have been trying to fix it for months. Time to get it in the hands of someone smart enough to diagnose and fix it.

Ouch! Well that ain't gonna help.

you still didn't say what distributor you are using.
also apparently there is a higher cam than pink now according to holley's site.

The distributor is a stock 1970 model w/ points and condenser.

Does it stumble then clean up at WOT? What's the initial timing? How much advance? Have you inspected the rocker arms for excessive play? Did any of the plugs look much worse than the others? Distributor cap cracked? Bad plug wire/wires?

Burn't or mis adjusted points (or something with them) would be my guess.

sticking advance weights ?

I'm in the camp of looking at the points. I had the identical symptoms to what you describe with my 440. It would idle and run under partial throttle just fine, and run terribly whenever you got on it. Turns out the points were worn so badly that I am amazed the car ran at all. Replaced the points and it runs great and performs like it should.

Question #1 Where is the Engine back firing? Carb or Exhaust? #2 What kind of distributor, points or electronic? #3 Where is your base timing set at idle? #4 which carb do you have?

so additional questions.
does it do it in park?
if not does it matter from what rpm you start to get on it from?
and the questions above are important.
have you inspected the distributor points?
is it backfiring from the carb or exhaust?

1 exhaust
2 points
3 factory setting
4 Holley 700 double pumper

Not in neutral (it's a four speed)
points are new
backfiring from the exhaust

Did you or can you verify the TDC on the balancer and the timing cover to make sure what actual ignition timing you have now?
If not get some help to do that and then let us know what you found.
Also verify the firing order on the spark plugs wires and distributor cap are correct, 1843
6572

Back firing in the exhaust most of the time is a fuel/carb problem. Can you post a picture of a couple of the spark plugs? Few things to check, fuel pressure should be 5 to 7 psi, what is your base timing set to? Is this a new carb? Verify float levels at the bottom of sight on the bowls. do you have a different carb to try out? I would try to put 2 to 4 degree's more advance to see if any thing changes. Check engine vacuum at idle and 2000 rpm and note readings. Check timing at 2000 rpm and note reading.

Cab. First off. Thanks. The ign timing is 2* ATDC. I'm changing wires this week. I like the distributor cap idea. What are they thru RockAuto, $10.? lol

I'll take pics this week. . Replaced the mechanical fuel pump! Engine vacuum is a steady 21 in. of merc. lol. Have not checked timing at 2 rpm. Thank you for some great feedback!

Have you tries advancing the timing. I would be at least 5 to 10 BTDC.

ATDC [censored]

Your issue is the retarded ignition timing
Set the initial to between 10 to 16 degree before top dead center and then rev the motor up with a good dial back timing light hooked up and working to find out where the max ignition timing is at, if it is less than 36 degrees BTDC go drive it, if it is more retard it until it is between 32 to 36 max BTDC
Beware, your going to wake up the Tiger so be careful on the first couple of WOT blast
Please let us know what you do and the results

Don't buy the cheapest ignition parts you can find , you will get what you for and they may not last very long

2 AFTER ?? That I believe is the wrong direction, should be more like 5 before. Without knowing how much mechanical advance your distributor currently has ( factory is seriously 30 to 33 degrees) what you want to do is bring the RPM up to 3000 and set your timing at 34-36 before TDC and see how it responds ... make note of any pinging. Once you have it up at that RPM bring it up to 5000 and make sure it's not advancing more.

x2. With everything stock it probably won't be able to take much more than 36* above 3000 rpm, especially with a 4 speed. Total is more of a concern right now than initial with the backfiring through the exhaust and sluggish performance.