76 d21 440 backfiring issue

[Rollingmenace]

Ok last summer I changed the plugs and wires . (Can't remember if I did cap and rotor) ran great last year and good for 2-1/2 trips this year.. On our way home yesterday I had an issue .. This happened about 4 times in a 40 mile or so trip.  I would either be off the accelerator on on it slightly maintaining speed. I'd push in accelerator to 3/4 or to floor to either pick up speed or lockdown trans.. As soon as I did this I would get some carburator back fires and loose power. I'd let off and try it again or feather it sometimes (I'm guessing there was still a bit of fire in carb) it would still do it sometimes not for a couple hundred feet.. Then all back to normal and run fine . This was sporadic and not every time I was hard on accelerator..

Kinda has me stumped the engine temp was good..

I have some suspect carb issues but I'm replacing that here as soon as the plate gets here..

It seems to be possibly ignition related would be my guess and a mechanic friend said the same thing... Seeing as I can't recall if I did cap and rotor last time it would be a place to start..

But I'm kinda funny that way if it were me instead of replacing a single component and hoping it works I'd just do it all .it makes me nervous about a potential break down outside of the city... Replacing the Cap rotor module and balast.. That being said what do you all think for about the cost I could get an entire hei setup and I'm way more comfortable with that as opposed to the mopar stuff

Thoughts or suggestions??

[Rickf1985]

Sounds like the accelerator pump is going bad in the carburetor which is creating an extremely lean condition when you hit the gas quickly. That or you could have a vacuum leak also creating lean condition.

[turbinebronze]

A bad spark plug or wire will cause a miss / dead hole / back fire under load.  It can idle nice and smooth with out a load. You might be able to duplicate the problem by lightly brake torqueing the vehicle (Vehicle in drive, foot on brake, and lightly pushing down on the gas at the same time, to load the engine).  I'd be careful if you do it with the air cleaner off.  You could get flames out of the carb.
  (my 2 cents, Craig)

[legomybago]

The Mopar ignition is good stuff....I wouldn't change over to HEI. Sounds like a fuel issue to me

[Rollingmenace]

Thanks guys...
The thing that stumps me is it is so random ..  Bottom of a hill hit the gas an she pulls just like normal.. Next hill backfire .. Run great next few hills  and go to grass is to go up another and same issue...

My first thought was oh crap vapor lock.. But usually when u get to that point it just goes all at once not comes and goes.  That's where I thought possibly ignition

Well I guess I should start with #1 since it was allready the plan ..
Install the electric fuel pump (as a backup precaution)
Remove as much vacuum and emissions stuff as humanly possible
And then install new edelbrock carb and go from there..

[Rickf1985]

You need to find the problem BEFORE removing the emissions stuff. If you start removing things you are just adding to the possibilities of problems. Solve the running issue as it sets before doing anything else and do it in a normal diagnostic routine or otherwise you will be chasing your tail for months. You have low vacuum, check the manuals for low vacuum related issues and address them first. I have offered advice but a lot of others are disagreeing with what I say so I will not say anymore on the subject. Do yourself a favor and read up on the vacuum issue.

[Wildman5977]

could be one of two things electrical, main starter relay or the balast resistor, I have a 440 that will not start when warm, I found dirty wiring connections for voltage to the ignition being dropped below a good voltage even though the starter was turning the motor good, and the balast resistor was starting to crumble from age