1968 Dart 273 intermittetent engine issue

[1968 Dart 270]

Stock 1968 Dart 270 with factory original stock 273 v8 and Carter 2bbl carb. Car recently started having an issue where it would drive fine,engine run smoothly and accelerate smoothly,then after about 20-30 minutes the engine will run roughly like it's missing when I accelerate,then smooth out. Then it will start idling rough too. If I leave the car to sit for several hours or next morning,it will run fine again but then after a while run rough and idle rough again.
It is NOT a fuel delivery issue. It's got a brand new fuel pump,the clear fuel filter before the carb is full of fuel and the carb was just professionally rebuilt in hopes of solving this but the problem still persists. I am thinking it's got to be an ignition issue. Cap and rotor and points look newer. The plug wires are new. And if it were the cap and rotor or points it wouldn't just start working well again after it sits since those would have wear mechanically and that wouldn't change if it sat. I am thinking maybe the condenser? Or coil? If the distributor were bad it wouldnt just fix itself by sitting either,i wouldnt tbink.Any of the experts here have an ideas? The only other thing I could think is maybe a bad muffler? I had a car once where the baffles inside fell apart and restricted rhe exhaust,but there seems to be a good srong flow coming out the tail pipe still. Anyone here have this same issue?

 

[RustyRatRod]

Have you ever cleaned the gas tank out?

 

[1968 Dart 270]

Have you ever cleaned the gas tank out?

Yes,and I've installed a new fuel pump and new clear fuel filter and the filter is full when the engine is running of clear,clean fuel. Carb was also professionally rebuilt just the other day as it was due for it anyway. Runs the same.Idles smoothly and quietly. Drives fine for a while,then starts having a rough spot when you go to accelerate and gets worse until it.idles like crap and then barely will drive.

 

[RustyRatRod]

Then I would have to agree it sounds like something electrical breaking down. Do you have an extra ignition coil you can try?

 

[Oldiron440]

The condenser is inexpensive, try a new one clean the cap and look for carbon tracks, pull and verify the points clean and reinstall everything if they check out but it’s been my experience that you can’t just pop the cap off look under it and say it’s ok, verify everything. Check the voltage before and after the coil when it’s running, I’ve found repairs in the harness with odd wire that dropped voltage as it ran, so check it out. On this old junk you can’t assume that anything is correct.

I’ve had a condenser go bad on a new mallory distributor and it acted very similar. Cold it would run about five miles then run rough and finally quit, cool off start up and do it all over again. It was the only time I’ve ever had a problem with a condenser.

 

[1968 Dart 270]

Then I would have to agree it sounds like something electrical breaking down. Do you have an extra ignition coil you can try?

Yes,I have a new old stock in the box Standard Uc12 coil I bought years ago and never used. Do coils behave that way? Only issue i ever had with one it failed outright

 

[1968 Dart 270]

The condenser is inexpensive, try a new one clean the cap and look for carbon tracks, pull and verify the points clean and reinstall everything if they check out but it’s been my experience that you can’t just pop the cap off look under it and say it’s ok, verify everything. Check the voltage before and after the coil when it’s running, I’ve found repairs in the harness with odd wire that dropped voltage as it ran, so check it out. On this old junk you can’t assume that anything is correct.

Agreed. I just said that they looked like they were replaced recently,but i will buy a new cap,rotor and condesor anyway since it's routine tune up stuff anyway. I just wonder why it only happens when the car's been driven for a while. To me it seems like a heat related issue Like a bad condenser or coil?

 

[RustyRatRod]

Yes,I have a new old stock in the box Standard Uc12 coil I bought years ago and never used. Do coils behave that way? Only issue i ever had with one it failed outright

Yup. They sure can. I had one recently do just that. Fail when hot and run like crap or stall altogether and restart when cooled off. Replaced the coil and it hasn't done it again. Since you have another one, it costs nothing to swap it out and see. Right?

 

[RustyRatRod]

Agreed. I just said that they looked like they were replaced recently,but i will buy a new cap,rotor and condesor anyway since it's routine tune up stuff anyway. I just wonder why it only happens when the car's been driven for a while. To me it seems like a heat related issue Like a bad condenser or coil?

I would try one thing at the time. That way, you will know if you fix it.

 

[1968 Dart 270]

I would try one thing at the time. That way, you will know if you fix it.

I'll try it when I get home from work. There wasn't time this morning and it was 40.degress out. I'll probably buy the condensor too and replace that as well

 

[Dana67Dart]

Next time it's running poorly run a jumper from the battery plus to the coil plus.

If it smooths out look for a overheated / bad electrical connection from batt to ignition switch to ballast to coil.

It could also be a coil going bad.

Any chance there is full 12v going to the coil all the time?

Check that.

 

[AJ/FormS]

That thing got Solid lifters?

 

[Murray]

Coils can overheat and fail.

 

[str12-340]

you say that it isn't a fuel delivery issue, but what you actually know is that the system fills the fuel filter. How many gallons of gas since the carb was rebuilt? Are you doing anything to deal with ethanol in the gas?

 

[Bewy]

With the engine idling, measure the voltage at the coil [+] terminal. Take the meter with you & next time the engine goes rough, check the voltage.

 

[AJ/FormS]

That thing got Solid lifters?

cuz you know, if they are getting tight, increased coolant temp/normal increasing coolant temps, might be driving a few exhaust valves into insufficient seat time to cool off, and away goes your cylinder pressure.
Or if the intakes don't seal, and they leak pressure back into the intake, same thing, loss of pressure; but now, you also loss of engine vacuum, so she may idle like it has a "bit of a cam" in her .
Still runs great cold, cuz none of thatchit is going on. yet.

Jus saying.

 

[1968 Dart 270]

That thing got Solid lifters?

No. 1968 went to hydraulic lifters.

 

[1968 Dart 270]

Problem solved. It was a bad ignition coil

 

[1968 Dart 270]

Does anybody know what the correct Gap is for the breaker points. I'm going to replace them too.

 

[AJ/FormS]

I think around .019, but I'm sorta guessing.

 

[1968 Dart 270]

Anyone one whats correct? Ive searched and most say set the points at .016 others say .018

 

[Dana67Dart]

Anyone one whats correct? Ive searched and most say set the points at .016 others say .018

I know I should like a broken record....

What does the FSM say?

0.014 to 0.019 gap gives you a dwell of about 28-33

The dwell is really way you want to tune to.

Once you get the dwell set, then recheck the timing (closer to 33 is better than closer to 28) the greater the dwell the longer the coil can charge, but there is a limit the dist and coil can have.

 

[Vaanth]

Point contact gap for a 1968 273 with two-barrel is 0.014" - 0.019". Optimally, set it at 0.016" - 0.017".

Dwell angle is 28-33 degrees.

 

[1968 Dart 270]

cuz you know, if they are getting tight, increased coolant temp/normal increasing coolant temps, might be driving a few exhaust valves into insufficient seat time to cool off, and away goes your cylinder pressure.
Or if the intakes don't seal, and they leak pressure back into the intake, same thing, loss of pressure; but now, you also loss of engine vacuum, so she may idle like it has a "bit of a cam" in her .
Still runs great cold, cuz none of thatchit is going on. yet.

Jus saying.

Again,the car has hydraulic lifters. It was a bad coil.

 

[1968 Dart 270]

I know I should like a broken record....

What does the FSM say?

View attachment 1716356821

0.014 to 0.019 gap gives you a dwell of about 28-33

The dwell is really way you want to tune to.

Once you get the dwell set, then recheck the timing (closer to 33 is better than closer to 28) the greater the dwell the longer the coil can charge, but there is a limit the dist and coil can have.

Thanks. I'll do that. I need to adjust the timing too. It now pings under hard acceleration after changing the points,cap,rotor and condensor. I believe the previous owner ran it too advanced.