Engine has a cough

[Freddo]

I recently gave my 67 a tuneup. Its the first tuneup I have given it in the 6 years I’ve owned it. (I put 1,000 miles +/- each year on it.)
It has a 283 with a Edelbrock 750 cfm carb. (Yes, ridiculously oversized, but it is what it came with & it’s on my list to change...it just seems like something more urgent always comes along.)
All in all, it has previously run well and idled evenly. Right after my tuneup, it developed an intermittent ‘cough’. It doesn’t cough when it is sitting still and I rev up the engine, it does it while I am moving. It seems to happen after it has warmed up and mostly when I am accelerating between 2,000 & 3,000 rpm. When I am accelerating & it coughs, I let of the gas a little bit and it seems to get better, but as I accelerate, it will cough again. When it coughs, the whole truck bucks. It has an auto transmission & a couple of times when it started to cough, I downshifted to 2nd(from drive) & once it was at about 3500 rpm, it seemed to do better, but it has also coughed above 3,000 rpm, so I’m not sure if that means anything.
My tuneup consisted of a compression test,(ranging between 125-145 lbs), a fuel filter, new points, condensor, rotor, cap, plugs & wires. I know there is a wide variation of quality in parts & I have read about problems with crummy parts on the board. My parts were mainly Delcos from Rock Auto. I gapped the plugs at .035 and did the best I could to get the points installed. (The last points I installed were 40 years ago on a Plymouth...this is my first Chev smallblock) I had to screw in the points a ways to get it to a 30 degree dwell, but it is steady at 30.

I have to think that something I did in my tuneup is what caused this because my truck ran just fine before the tuneup, but I am not sure what it is. Sorry about the lengthy post but I wanted to give you as much information as I could in the hopes that you might have some suggestions.
Any ideas would be appreciated.
Thanks!

[special-K]

How is the air filter? Did you time it?

[mr.precision]

Try putting your old plug wires back on..... sure sounds like plug wires are goofy.

[Sheepdip]

Here's a cut and paste that might help.
Why does my engine spit or backfire?


First off, you need to determine whether it is backfiring out the exhaust or back up through the carb. We usually refer to an exhaust backfire as a "backfire", and backfiring through the carb as "spitting" or "coughing".

"Backfiring" is usually caused by a spark plug "sparking" when it isn't its turn and the exhaust valve is open. If your air/fuel mixture is too rich and you have unburned fuel in the exhaust system, cross firing from one spark plug wire to another can occur if they are touching each other, and when this happens while the exhaust valve is open, it will ignite the rich / unburned mixture in the exhaust manifold and tail pipes and result in a big bang. This is why they make spark plug wire separators, to help keep cross firing at a minimum.

Having timing that is too late (retarded) can cause this same thing sometimes if your engine is running too rich. When the exhaust valve opens, the mixture isn't done burning so it ignites the unburned fuel in the exhaust manifold (or header collector) and causes a loud bang.

Back firing can also be cause by a cracked distributor cap, or one that has carbon tracking inside which causes cross firing between the terminals inside, which in turn, sends spark to a spark plug that isn't ready for it yet.

Backfiring through the carb (spitting or coughing) usually occurs in the morning when a carb's air/fuel mixture is a bit too lean. This usually goes away once the engine warms-up. It is also commonly caused by the accelerator pump in the carb not squirting enough fuel before the main jets start working. If you spit the instant you blip the throttle it's probably the accelerator pump in the carb not working, plugged up or out of adjustment.

As with backfiring, coughing or spitting can also be caused by a bad ignition system, such as cross firing, which sends a spark to a cylinder that has the intake valve open. When that plug sparks out of turn, it lights the fuel in the cylinder and the pressure has to go somewhere... so if the intake valve is open, it goes right back up through the intake manifold and out the carb with a "spit" and sometimes even a flame.

When, and how, it backfires or spits will give you an indication for where to start looking.

[Freddo]

Originally Posted by special-K

How is the air filter? Did you time it?

Thanks for your reply! The air filter is an aftermarket that is several years old and looked ok, but that is an excellent suggestion. I will replace it as it is probably time anyway.
I checked the timing and it is about 10 degrees before top dead center. I didn't take the distributor out when I replaced the points and didn't move the distributor at all. When I timed it, I idled it down as far as it would go and plugged the vacuum advance hose. (At least that's what I remember we used to do.) I figured at 10 degrees, it was probably ok?
Thanks again. I will try timing it again and report back.

[Freddo]

Originally Posted by mr.precision

Try putting your old plug wires back on..... sure sounds like plug wires are goofy.

I will give that a try. I saved the old wires.
Thanks for your help!

[Freddo]

Originally Posted by Sheepdip

Here's a cut and paste that might help.
Why does my engine spit or backfire?


First off, you need to determine whether it is backfiring out the exhaust or back up through the carb. We usually refer to an exhaust backfire as a "backfire", and backfiring through the carb as "spitting" or "coughing".

"Backfiring" is usually caused by a spark plug "sparking" when it isn't its turn and the exhaust valve is open. If your air/fuel mixture is too rich and you have unburned fuel in the exhaust system, cross firing from one spark plug wire to another can occur if they are touching each other, and when this happens while the exhaust valve is open, it will ignite the rich / unburned mixture in the exhaust manifold and tail pipes and result in a big bang. This is why they make spark plug wire separators, to help keep cross firing at a minimum.

Having timing that is too late (retarded) can cause this same thing sometimes if your engine is running too rich. When the exhaust valve opens, the mixture isn't done burning so it ignites the unburned fuel in the exhaust manifold (or header collector) and causes a loud bang.

Back firing can also be cause by a cracked distributor cap, or one that has carbon tracking inside which causes cross firing between the terminals inside, which in turn, sends spark to a spark plug that isn't ready for it yet.

Backfiring through the carb (spitting or coughing) usually occurs in the morning when a carb's air/fuel mixture is a bit too lean. This usually goes away once the engine warms-up. It is also commonly caused by the accelerator pump in the carb not squirting enough fuel before the main jets start working. If you spit the instant you blip the throttle it's probably the accelerator pump in the carb not working, plugged up or out of adjustment.

As with backfiring, coughing or spitting can also be caused by a bad ignition system, such as cross firing, which sends a spark to a cylinder that has the intake valve open. When that plug sparks out of turn, it lights the fuel in the cylinder and the pressure has to go somewhere... so if the intake valve is open, it goes right back up through the intake manifold and out the carb with a "spit" and sometimes even a flame.

When, and how, it backfires or spits will give you an indication for where to start looking.

Thanks for the information. I'm not sure if it is backfiring through the carburetor or out the exhaust. I will do some experiments and see if I can tell which.
Thanks again for the info. This gives me some additional areas to check!

[garyd1961]

Have you changed the fuel filter? I would also double check the firing order and the points.

[Freddo]

Originally Posted by garyd1961

Have you changed the fuel filter? I would also double check the firing order and the points.

Hi, I did replace the fuel filter. I will check the firing order and I am also a little suspicious about the points.
Thank you for the suggestions. I hope to put all of your suggestions to work this weekend and get this cleared up.
Thank you again to everyone for your help.

[garyd1961]

Check for vacuum leaks. It's not hard to knock a vacuum line loose while doing a tune up.

[geezer#99]

Close your plug gap down to .032.
Is your points and condenser the adjustable type accessed thru a lift up window in the distributor using an Allen wrench?