Need help determine cause of missfire

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pjc360

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so I have a 300hp crate 360 magnum in my 91 half ton short bed 4x4 truck.
The engine has around 30k miles on it.
I’ve discovered a rough spot around 2000-3000 rpm that seems to get better once you punch the throttle but it comes back the second you ease off and stay at a steady throttle setting.
Doesn’t matter what gear I’m in, doesn’t matter if it’s parked or moving,
The rough spot is there.
I’ve got a brand new firecore 50 electronic vacuum advance distributor, timing is 20 initial and 34 total.
Vacuum at idle is 20hg.
Ignition is crane cams hi-6 cd ignition and a crane cams Ps92 coil.
Plugs are Ngk BKR5E’s gapped at .040
Wires are brand new Taylor thundervolt 8.2mms.
Carburetor is Quickfuel ss 680 vs carb.
Intake Mopar dual plane M1.
Tonight after work I hook the timing light up to cylinder 8s plug wire and bring the rpm up to 2500 rpm and I can see the break in the light.
Deffenitley miss firing on cylinder 8.
So I pull the plug and it looks semi fuel fouled like it’s having a hard time running and like something is letting to much fuel into that cylinder.
I do a warm compression test on cylinder 8 with carb wide open cranked on it for about 5-6 seconds.
I got 155 psi on cylinder 8.
So that’s a pretty good number and isn’t indicating anything wrong with cylinder 8.
So I pulled the valve cover and looked at the valve springs on cylinder 8 and I didn’t see anything that jumped out at me but it is dark outside and I was using a flash light.
Hoping to get some idea’s or some advice on what to do next to narrow this down cause it’s been driving me insane.
Number 8s plug has been doing this for quite awhile and I always thought I had my carb a little too rich but now I’m certain it isn’t a carb issue and that there is something going on in cylinder 8.
I’m leaning towards a bad valve spring because the missing goes away under load for the most part but is most prevalent when vacuum is high.
 
Well, if you are seeing the miss on the timing light, its either a bad wire,shorting plug,cross firing distributor cap,air gap in distributor off,bent distributor shaft or worn bushings vacuum plate loose or a bad ignition module.
 
Swap #8 wire with another and see if it follows. How are your grounds? Seems a lot of ignition setups like good clean grounds.
 
AJ/FormS said:
Is 8 the only one doing it?
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Yes as far as I can tell number 8 is the only cylinder that’s missing.
I’ve pulled all the plugs multiple times looking for an issue and number 8s plug is always semi fuel fouled and even smells like fuel a little bit.
The other 7 plugs looks great.
It’s not an issue in the distributor.
This is a brand new firecore 50 distributor with less then 100 miles of run time on it.
This is an isolated issue on cylinder 8.
I will hook my light up and check the other 7 cylinders at 2500 rpm.
But like I said every time I have removed all 8 plugs it’s always number 8 that is showing signs of too much fuel.
 
Have you been swapping different plugs into 8?
Do you have power brakes?
Are you loosing Brake fluid?
Where is your PCV hose plumbed to?

If the plug is wet for whatever reason, the spark will leak down the electrode and short the energy to ground. The light will see it as a missing strobe. If you have swapped a different plug into that hole, and it still does it, then take an old plug and break off the outer electrode, then stuff that plug into the wire and lay it on the engine. then start it up and observe the spark. That will prove the wire. If the wire is good then put it all together; spark is not your problem.
 
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dano said:
Swap #8 wire with another and see if it follows. How are your grounds? Seems a lot of ignition setups like good clean grounds.
Click to expand...

My crane cams hi-6 cd ignition box is grounded directly to the negative battery terminal on the battery.
I have a 4 gauge wire from block to negative on battery and a 4 gauge wire from body to battery negative and a 4 gauge from frame to body.
 
AJ/FormS said:
Have you been swapping different plugs into 8?
Do you have power brakes?
Are you loosing Brake fluid?
Where is your PCV hose plumbed to?
Click to expand...

Haven’t been losing brake fluid to my knowledge.
The brake boosters vacuum hose goes from brake booster to the back of the intake manifold on drivers side.
Close to cylinder 5
 
Plug wires are brand new by the way too they don’t even have 100 miles on them.
Same with coil.
I bought new plugs, new wires and a new coil and a new distributor thinking this was ignition trying to solve this and none of that made a difference.
 
Yes I’ve tried multiple plugs in cylinder 8 they all end up the same, semi fuel fouled and smelling like fuel a little bit.
 
Drop your float levels, see if it changes. If its over fueling,may be a place to start.
Buddy had a nasty over fueling problem,brand new had junk in the bowl. He wouldnt pull carb and start from scratch thinking it was a spark problem. Hauled it 100 miles to a speed shop,where they determined it was -you guessed it,over fueling. They went through carb,started with base settings and dialed it in from there.
I never got any oppurtunities to fiddle with holley 4bbl performance carbs or any reasonable facsimilies so aj i’m sure will shed some light.
 
AJ/FormS said:
Well if you don't do the wire test, I'm going home.
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I will do the wire test.
The only issue is I don’t have a shop at the moment and my truck is on the side of the road and the entire neighborhood would be enraged with me if I went there reving the engine up right now.
My neighbor already hates my truck says its too loud complains about it to me all the time.
So tomorrow night when I get off work when it’s not too late I’ll do what you suggest and report back.

So you reccoemnd breaking the strap off an old plug and putting the wire on it and laying it on valve cover and starting it and let it run without a plug in that cylinder? Is that going to be ok?
 
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Float level is exactly half way up the sight glasses which is where quickfuel says to have the float level.
 
Verify ignition system is good as AJ said. Around 2K is when the carb transitions from idle to primary circuit. Make sure idle and main air bleed circuits are clean. Every small block I've owned with dual plane has always needed split jetting to get all 8 plugs to burn the same. AFR gauges only give you an average of left and right bank. If you have a temp gun you can check each header tube temp.
 
I had a missing problem with my newly build 360 for my Barracuda. I tried everything but didn't test the wires because they were brand new MSD super conductors. Finally, I measure the resistance and found I had a bad wire. MSD replaced it. My advice is test everything even if brand new. Good Luck
 
I too have had problems with new parts being bad. Years ago I did a tune up including new plug wires on an old car I picked up. The car ran ok but would not start when it was real damp out. Put another set of wires on it and the starting problems went away.
 
67redcudavert said:
I too have had problems with new parts being bad. Years ago I did a tune up including new plug wires on an old car I picked up. The car ran ok but would not start when it was real damp out. Put another set of wires on it and the starting problems went away.
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Is it possible that my crane cd ignition box could be causing this?
I ohmed all my plug wires before installing them and they all measured at 50 ohms per foot.
The crane ignition box is the only thing I haven’t replaced in the entire ignition system.
 
One thing I forgot to add is ever since I got this crane cams hi-6 cd ignition box I have gone through a significant amount of pick up coils.
And one time the button under the coil wire inside the distributor cap melted away completely causing a terrible miss fire that I diagnosed very quickly as being the coil button inside the cap being burned away basically.
I bought a Mopar distributor from don at fbo systems and I had to replace the pick up coil 2-3 times with that distributor.
I’m wondering if it is the box that’s causing this issue.
Also a few months back in the summer time I held the throttle at 2500 where it starts missing and I could feel the ignition box getting fairly warm to the touch.
I have it mounted inside the cab behind the seat.
It almost feels like the rev limiter is kicking on and off at 2500 rpm and up like it’s just struggling to fire evenly.
I’m going to test the spark on cylinder 8 tonight by installing an old plug into number 8 and laying it on the valve cover and starting it and I’ll brong the rpm up to 2500 and see what the spark looks like.
I was getting a normal flash from the timing light on cylinder 8 at idle.
It wasn’t untill I brought the throttle up to around 2500 that I saw the break in the light.
 
So I was able to do a little more testing tonight and I have some information pertaining to my missfire issue.
I’ve determined that this is not a missfire that’s limited to just one cylinder... this is a missfire that’s affecting all 8 cylinders.
I hooked the timing light up to every cylinder and brought the rpm up to 2500 rpm where it starts missing and I could see the break in the light on every cylinder.
I put a plug into cylinder 8s wire and started it and I did have spark while idling.
I didn’t want to bring the rpms up to 2500 with the plug on the valve cover but at idle it is getting spark.
So after discovering this I called an old mechanic I know who is a Ford Guy but has some experience with Mopar’s and explained the problem and explained everything I’ve done to try and solve the problem and he said based on the fact that this issue was happening with your previous distributor and now you have the same problem with your new distributor and that you’ve tried a new coil to no avail and you’ve tried new plug wires that you have ohmed and your pick up coil is testing within spec he said I think it’s your crane ignition box.
Thoughts?
 
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