Misfire and backfire through the carb

[RyKR]

I have an issue with the 302 in my 83 Capri. It will misfire and spit back through the carburetor when i blip the throttle. It will start and idle fairly smooth and I can very slowly bring the RPMs up to about 3500 without issue, but hitting the gas rapidly causes the backfire through the carb. This engine ran very well when I pulled it from another car. I put it in my car where it has sat for over 10 years and I am just now getting around to messing with it again.

Initial timing is at 16 degrees with approx. 38 total
New plugs, cap, rotor, and wires
New ICM
carburetor was rebuilt (idles better than it did before but didn't help the spitting through the carb)
tried another carburetor with same results
Vacuum is steady around 13"'

Since the vacuum is steady and I can slowly rev it up, I'm thinking that there aren't any serious mechanical issues. I've tried two carburetors and they both have the same symptoms. Should I be looking into an electrical issue? If so, where do I begin? I have 12 volts to the coil. Am I supposed to have power to the distributor?

[Scott Peters]

This might seem stupid… but I had a “similar “
problem with a 79 5.0.
I solved it by cleaning the contacts of the engine ground strap.
It’s the strap that mounts to the back of the head on the passenger side, to the firewall.
When you installed that engine, did you reinstall that very important ground strap?

Scott

[RyKR]

Before it was installed, the engine bay was painted. I'll check the connection and make sure that it is making a good ground.

[gr79]

13" idle vacuum is too low if a stock engine, no radical cam.

Recheck for vac leak(s), vac connection routings, condition of carb gasket.
Recheck firing order of ignition wires at cap and plugs. Check plugs.
Old gas, not enough gas (lean).
They say usually is ignition timing related, or a leak.

[qikgts]

Adding to what folks have offered above, I'd verify that the balancer hasn't spun and isn't skewing your timing readings. You may wanna try a known good Duraspark box too.

[richpet]

It shouldn't cause a misfire under those conditions, but 16* initial might be a little much. Most run 12/13 IIRC.

I'd recheck all the wires. Revving slowly is minimal cylinder pressure so they spark a bit easier. Getting on it harder increases that pressure and the spark may be going elsewhere. I've had that issue and it was a bad boot at the plug end - when load increased it was sparking to the block instead of the plug gap.

Also agree with the vacuum leak potential. I have a moderate custom cam ground for my 347 build that should provide 14" vacuum. 13 is low.

[Bentley]

Do you have the firing order correct for this new to you engine? HO or non HO firing order??

[RyKR]

Do you have the firing order correct for this new to you engine? HO or non HO firing order??

I'm using the HO firing order. I would think that it would run very poorly if I was using the wrong firing order.

[RyKR]

I'm trying to respond to everyone so I may have missed something:

So the engine isn't entirely stock. It has a Comp Cams 268XE (I think), AFR 165 heads, and a Performer RPM intake.

I installed a new blue grommet duraspark box. I'm not sure how to test the distributor. The coil is also new.

I will add fresh gas, the gas has been there since around June. I replaced the fuel tank last fall and blew the lines out.

I've checked repeatedly for vacuum leaks. I even pulled the intake and reinstalled it because I was certain that was the issue.


Where I'm at now: I have purchased an HEI distributor. If that takes care of the issue, I will at least know that it is ignition related and I can go from there. If I can't figure it out soon I'm going to take it to a shop and likely pay as much as the car is worth in diag fees. It will be worth it though, I need to get this car going again.

[Bentley]

I'm using the HO firing order. I would think that it would run very poorly if I was using the wrong firing order.

Sounds like its running POORLY to me?

[85GTGuy]

What year is the motor? Is it a HO motor?

[RyKR]

Sounds like its running POORLY to me?

Would it idle smoothly and rev if it were the wrong firing order? I would think that it would spit and sputter, even at idle if it were the wrong firing order. It currently backfires when I hit the accelerator quickly. Not all of the time.

This isn't sarcasm. I'm genuinely curious if it would run worse with the wrong firing order.

[RyKR]

Sooooooo..... much to my embarrassment I think the problem was the firing order. I was led to believe that the engine was an 83 HO so that is the firing order I went with. It idles the same but it doesn't backfire when I stab the throttle.

I do appreciate all of the help.

[richpet]

Glad it was an easy fix! It's always nice when it doesn't cost any $.
And we have ALL had these moments...

[85GTGuy]

Good deal. Congrats on getting it solved.

Curious.... what kinda vacuum you pulling now?

And, like @richpet pointed out, 16° initial timing seems a bit too much. I think the factory spec for an '83 5.0 Stang/Capri is 12°. Might could go a little more, but 4° additional may be too much. Just make sure you avoid any (potentially damaging) pre-ignition.

[83GTTops]

Good find. Glad you are better

[Bentley]

Always do the simple things first....

[benab61114]

Good to see that

Sent from my SM-G965U using Tapatalk

[RyKR]

Good deal. Congrats on getting it solved.

Curious.... what kinda vacuum you pulling now?

And, like @richpet pointed out, 16° initial timing seems a bit too much. I think the factory spec for an '83 5.0 Stang/Capri is 12°. Might could go a little more, but 4° additional may be too much. Just make sure you avoid any (potentially damaging) pre-ignition.

The vacuum is nearly 15 now.