Misfiring issues

[HYDRO]

Gentlemen I have a question concerning what seems like misfiring on my 73 challenger 340 four bbl. A little history, I totally rebuilt back stock but had to bore .030 over other wise totally stock. On initial start up she seemed to run best at 10 degrees before with vacuum advance plugged and around 900 rpm then hooked vacuum back getting around 34-37 degrees. She would start good, quick throttle response with no preignition shutting off. Due to having to do a lot of other things was not able to go very far just down the street and back and seemed to do well but unable to do a good run till last week took a 40 mile run. Turns out it wouldn’t go into 3rd so keep rpm around 25-2800 rpm and after about 10 minutes starting misfiring off and on , made it back home and removed fuel filter hoping it was clogging but was clean. I realize it could be many things mainly electronic. Has new standard coil, wires, plugs, blue ICM , regulator, rebuilt thermo-quad, and engine wire harness. Hoping someone can lead me closer to that needle in a haystack possible problem. Thanks in advance for any imput.

 

[Challenger RTA]

I totally rebuilt back stock but had to bore .030 over other wise totally stock.

How long and what engine speed was the break in? Does it go back to good character? Rocker arms loose after heating up. I would put a vacuum gauge on it. Check the cap and rotor for corrosion. Been there done that new cap the summer before, go figure.

 

[HYDRO]

Not broken in yet still needs 400 miles before I get on it the cap looks great. Thanks for the vacuum chart will recheck timing and hook up my vacuum gauge and see what happens

 

[Xcudame]

I'm assuming it's an automatic. Not going into third (direct drive) is certainly an issue that needs to be addressed.

 

[Ricks72Chlgr440]

Gentlemen I have a question concerning what seems like misfiring on my 73 challenger 340 four bbl. A little history, I totally rebuilt back stock but had to bore .030 over other wise totally stock. On initial start up she seemed to run best at 10 degrees before with vacuum advance plugged and around 900 rpm then hooked vacuum back getting around 34-37 degrees. She would start good, quick throttle response with no preignition shutting off. Due to having to do a lot of other things was not able to go very far just down the street and back and seemed to do well but unable to do a good run till last week took a 40 mile run. Turns out it wouldn’t go into 3rd so keep rpm around 25-2800 rpm and after about 10 minutes starting misfiring off and on , made it back home and removed fuel filter hoping it was clogging but was clean. I realize it could be many things mainly electronic. Has new standard coil, wires, plugs, blue ICM , regulator, rebuilt thermo-quad, and engine wire harness. Hoping someone can lead me closer to that needle in a haystack possible problem. Thanks in advance for any imput.

Did you initially run the rebuilt engine at ~2000 rpm for at least 20 minutes?

 

[HYDRO]

Only when I went on the short trip at 2800 max mainly between 2000- 2500rpm

 

[Ricks72Chlgr440]

Only when I went on the short trip at 2800 max mainly between 2000- 2500rpm

If you are using a flat tappet cam & lifters then you may have wiped your cam out.

 

[Xcudame]

Yes @Ricks72Chlgr440 might be correct, flat tappet cams need to be broken in on initial startup for 15-20 minutes at 2500 to 3500 rpms. And it's helps to have breaking lube installed on the camshaft lobes prior to installing the lifters. And priming the engine using an oil with lots of zinc phosphate prior to startup.

Years ago in the engine rebuilding shop I worked, all the reground camshafts were parkerized after grinding which also helps during the break in period. I believe Howard Cams are still parkerized. Not sure of other brands these days except a Mopar 440 camshaft kit I bought from Summit earlier this year was parkerized.

 

[Ricks72Chlgr440]

Yes @Ricks72Chlgr440 might be correct, flat tappet cams need to be broken in on initial startup for 15-20 minutes at 2500 to 3500 rpms. And it's helps to have breaking lube installed on the camshaft lobes prior to installing the lifters. And priming the engine using an oil with lots of zinc phosphate prior to startup.

Years ago in the engine rebuilding shop I worked, all the reground camshafts were parkerized after grinding which also helps during the break in period. I believe Howard Cams are still parkerized. Not sure of other brands these days except a Mopar 440 camshaft kit I bought from Summit earlier this year was parkerized.

Not sure what manufacturer the cam came from, but there have been quite a few problems with some. He will need to pull the intake and look at the lobes. If questionable he will need to pull the rocker shafts & pushrods to have a look at the lifters. That will tell the story.