Hesitation, popping through carb under acceleration

[koonpup]

Hello everyone, I'm in need of some help. I have a 74 roadrunner with 318. The plug wires were a good 10 years old so I decided to replace them. I did and now my car will stall and die if you try to do a burnout. Also, when you accelerate quickly and hit 2500 rpm, it makes this god awful missing sound coming through the carb. So I thought easy fix, put the old wires back on. I'm still having the same problem. It's electronic ignition and the wires are ran so none of them touching each other. I checked 4 times and the wires are all in the correct order. I took every spark plug out and they look fine. I even tested them all with a voltmeter. I checked timing and it's still set where it was at 7 btdc. I put a different coil and ignition module on it. I rebuilt the carb two months ago and I thought maybe it was the accelerator pump. With the engine off, I pump the throttle and a steady stream comes out into the carb. I've taken carb clean and sprayed everything looking for vacuum leaks. This problem came from changing spark plug wires and now I can't figure it out. Any help would be greatly appreciated.

 

[Garys1969RR]

Remember the dist turns clock wise on a small block

 

[snakeoil24]

Sounds lean! Check your accelerator pump.

 

[Garys1969RR]

Only thing I can think of is firing order. 1-8-4-3-6-5-7-2

 

[Kern Dog]

You may have a camshaft going bad. Have you drained the oil recently to get a look at it? Any silver or shiny particles in the oil ?
Here is my reasoning: As the car runs down the road with very little throttle opening, cylinder pressure is pretty low. As you step on the gas, cylinder pressures go up. IF there is one or more exhaust valves that is not opening fully, (Due to worn lobes on the camshaft) The exhaust will try to exit through the intake valve and up through the carburetor. The only fix is by replacing the camshaft.
To determine if the cam is bad, you can remove the valve covers and feel for excessive free play in the rocker arm action. If any of the rocker arms move more than 1/8" up or down, it could be trouble. If any pushrod feels loose enough to rattle around, same thing: Trouble. No amount of tuning will fix a failing camshaft.
These cars are 40-50 years old and many have been Unearthed after sitting for long periods of time.
Sometimes a problem like this can happen out of the blue and be totally unrelated to anything else. It may be just dumb luck that it happened after changing plug wires. I hope for your sake it is something else than the cam.

 

[koonpup]

Sounds lean! Check your accelerator pump.

That's what I thought, but as I mentioned, I can see a steady stream of gas when I pump the throttle. Is there any other way to test it? Rebuilt 2 months ago. Thanks!

- - - Updated - - -

You may have a camshaft going bad. Have you drained the oil recently to get a look at it? Any silver or shiny particles in the oil ?
Here is my reasoning: As the car runs down the road with very little throttle opening, cylinder pressure is pretty low. As you step on the gas, cylinder pressures go up. IF there is one or more exhaust valves that is not opening fully, (Due to worn lobes on the camshaft) The exhaust will try to exit through the intake valve and up through the carburetor. The only fix is by replacing the camshaft.
To determine if the cam is bad, you can remove the valve covers and feel for excessive free play in the rocker arm action. If any of the rocker arms move more than 1/8" up or down, it could be trouble. If any pushrod feels loose enough to rattle around, same thing: Trouble. No amount of tuning will fix a failing camshaft.
These cars are 40-50 years old and many have been Unearthed after sitting for long periods of time.
Sometimes a problem like this can happen out of the blue and be totally unrelated to anything else. It may be just dumb luck that it happened after changing plug wires. I hope for your sake it is something else than the cam.

That is a scary thought! Lol thank you for the reply. I will most definitely pull the valve covers and check this because that seems to be the only thing I haven't checked. You might be right, it may just be a coincidence that it happened at the same time that I changed plug wires.

 

[Shorthorse]

If the car was running fine right up to the point when you changed the wires you are correct in concentrating on that first. It's not impossible for something unrelated to go bad at the exact time but the odds are highly against it. Check for cracks in the cap and make sure the rotor tang isn't bent. Make sure the cap is seated properly on the distributor housing. Look for obstructions in the plug wires or any linkage that might put pressure or pull on a wire. Check wherever your hands have been.
Will it break up during a free rev as well as under a load?

 

[Kern Dog]

A cam failure isn't the end of the world. Replacement does require some expertise though. My first cam failure was in my 440/493 in 2006. The replacement went bad too ! Since then I've read up on using better oil, proper camshaft break-in procedures, etc.

 

[MoparLeo]

You said a 1974 318. Is it the stock manifold and 2 bbl. carburetor ? Did you check the timing at idle, about 750 rpm with the vacuum disconnected ? Recheck the plug wires like said before 1-8-4-3-6-5-7-2 clockwise. Unless the wires are damaged or poor quality you should not have a cross fire problem. New Champion plugs gapped properly ? .035-.040 ? Float level checked ? Fuel level is not too low ? If it really ran good before with none of these current problems, something is not installed properly or got damaged in the wire change out . Be sure to use Dielectric grease in the cap and plug boots. Take distributor cap off and inspect for cracks and for carbon trails. Is rotor in good condition ? The carbon button in the cap that the rotor rides on. Damaged or ok ?

 

[khryslerkid]

If it ran ok before you replaced the wires then it's probably something you did. Retrace your steps. You might have pulled a vacuum hose off. Knocked off a vacuum cap on the carb. Looking at the wireing order wrong. Distributor cap not on exactly right. Cracked porcelain on a plug. Ect...

Think simple things first before rebuilding the whole car!

 

[MoparLeo]

Exactly. Problem diagnosing should always start at the beginning. 90% of the time, problems are basic and never assume any thing. Retrace your steps and recheck everything again. Like said before, it is never as complicated as we make things to be,.

 

[koonpup]

Update:
Spark plugs are 6 months old. Pulled them, checked for cracks, checked gaps, and all looked a good color.
Checked firing order. It's 1-8-4-3-6-5-7-2 clockwise.
Checked for vacuum leaks/unhooked hoses, none.
Checked distributor cap for cracks, etc. Everything still looks new. Seated properly.
Btw, its an edelbrock performer intake and edelbrock 1406 carb adjusted with vacuum gauge.

I pulled the valve covers. I could not find any excessive play in rocker arms or push rods. I decided to run it with the valve covers off and I noticed two things on the passenger side. One lifter doesn't move at all and another barely moves. So I take it that I do have a bad cam?

Thanks for everyones input.

 

[khryslerkid]

Or collapsed lifter. Either way, for the cost of a cam and lifters, you could replace them. If you're not on a budget, might even contemplate a complete rebuild.

So the engine was missing and popping before replacing the wires?

 

[koonpup]

Or collapsed lifter. Either way, for the cost of a cam and lifters, you could replace them. If you're not on a budget, might even contemplate a complete rebuild.

So the engine was missing and popping before replacing the wires?

It was not, which is why I find it odd that it happened right after I changed them. I hadn't driven it for a few months and I took it out for a spin right before I replaced them and everything was fine.

 

[khryslerkid]

That was an odd one. Unless when you got on it after changing the wires, the lifter or lifters gave out. Should have been quite a bit of rattling noise after that.

The intake will have to come off to at least inspect. If the cam lobes look wiped than you know its the cam. If it looks ok, you could get by with just new lifters. Kind of a cheaper way to do it.

If the cam lobe or lobes are wiped, then you have to think about how much metal from the lobe / lobes are in the oil, throughout the engine...

 

[dryheat]

I remember having the same issue years ago and I also checked the wires 4 times so I KNEW they were correct. 3 more examinations later I found 2 were swapped.

 

[Kern Dog]

Update:
Spark plugs are 6 months old. Pulled them, checked for cracks, checked gaps, and all looked a good color.
Checked firing order. It's 1-8-4-3-6-5-7-2 clockwise.
Checked for vacuum leaks/unhooked hoses, none.
Checked distributor cap for cracks, etc. Everything still looks new. Seated properly.
Btw, its an edelbrock performer intake and edelbrock 1406 carb adjusted with vacuum gauge.

I pulled the valve covers. I could not find any excessive play in rocker arms or push rods. I decided to run it with the valve covers off and I noticed two things on the passenger side. One lifter doesn't move at all and another barely moves. So I take it that I do have a bad cam?

Yeah, the cam and/or lifters are bad. Even a collapsed lifter will move a little if the cam lobe under it is still good. The 318 engines used a meager .373 valve lift so I wouldn't expect huge travel.
Sorry for the bad news. Cam failures can happen within a few hundred miles but if the engine has loud exhaust or a rough idle anyway, you may not immediately notice a bad cam. Now would be a good time to step it up slightly to a slightly bigger cam to gain a little bit of power. Even a stock 340 cam would be a nice step up while not making the engine lose any power at low speeds. If this car is not a daily driver, you have the time to think things through to make the right decision. New camshafts need all new lifters. Google search "Flat tappet camshaft swap" for tips on how to properly install the cam and lifters. After reassembly, the engine needs to start quickly and run at approx 2500-3000 rpms for 20-30 minutes to "Break-in" the cam and lifters. During this process, the lifters and the cam lobes develop a wear pattern to each other that is not visible except under a microscope. This mating pattern is absolutely essential for long camshaft life.
What caused the original cam to go bad? Several things are possible. First, it could simply be old age. Metal parts wear out over time. Secondly, the cam may have been starved for oil a few times if the engine was ran low on oil too many times. Also, the OIL that you used is surely a factor. ALL oils developed for todays engines are sorely lacking in a component that older style engines need: Zinc Phosphate. Granted, it is not as critical on a medium performance 318 as it is with a bigger engine with a bigger cam but the Zinc is still important. Valvoline VR1 has Zinc in it. Do not be fooled by synthetics! Being synthetic is good but they still lack the Zinc! Some Diesel engine oils have Zinc like Shell Rotella.
Feel free PM me with any questions. I don't know everything but I have been through this a few times.