Periodic miss is eluding me. Accepting random suggestions.

Just a dead spot every couple 5 or 6 seconds on what sounds like one cylinder.

I found a spark plug with broken ceramic, and thought that had to be it, arcing to the header or head, but swapped it out and still have it.

Runs GREAT, but you can hear that miss, and I know it is robbing me of a little power.

Running Pertronix on big dizzy cap, 5.0 firing order.

Any other suggestions on where I might find this elusive little bugger?

 

Are the engine's rpms dropping (engine "stumbling"), or, do you "hear" something else (exhaust leak, arcing, other)?

 

take off the air cleaner. start the motor. stick your head over the carb and look down it. open the choke if closed. watch the venturies. do you see any drips of fuel falling off them? if so then you have a carb issue.
my other sugestion is plug wires. not sure how to test them but ive fixed alot of misses by changing out plug wires when i cant find any thing wrong.

 

At idle, it just drops a bit every couple seconds.

At cruise is when I hear it most.

If I could figure out how to post a video, I would show you...been messing with it for an hour.

How can I post an wma file? I swear I have done it before, but I am failing miserably this time.

 

What gap are you using at the plugs?
What wires?
If the cam specs that are listed in your signature are at .050" then I would suggest that uneven air/fuel mixture at an idle is causing your miss.
Your cam is for mid to upper end efficiency, the intake is designed for the mid to upper end, and the headers you are using aren't equal length which can make fine tuning impossibly difficult especially at an idle.
You may be able to hide the miss if you increase the idle fuel supply by enlarging the idle fuel feed restrictor but that will richen all the cylinders and not just the ones that are leaning out due to distribution of the mixture. Increasing compression might help fire the mixture even when it is a bit leaner than you want.
Basically you built an engine to run between 2800 and 6000 RPM. It may never idle well.

 

Here it is at idle. I will get a tripod and record at cruise speed in the next couple days when I get a chance.

http://scooper77515.fordmaverick.net/pichostpics/idle.wmv

You can hear it drop once early on in this video.

 

The problem isn't so bad at idle. Yes, it is a little rough, and when in gear and idling through a parking lot, it jerks every few seconds.

What I am concerned about is what I hear when cruising around 3000-4000 rpms.

It runs smooth, but you hear a miss every few seconds, and sometimes it will do it 2 or 3 times and then run smooth.

The only other time I heard anything like it was when my P heads had burned through my plug boots, and it was arcing over.

Up to today, I was running with the advance vacuum plugged, and it was maxing out at 30 degrees or so. I just plugged it up and will run it and see if it improves. It now maxes out way above 40, way off the balancer scribes.

 

The cylinder that had the cracked plug. Pull the wire off and have a good look inside the boot and make sure you dont have a carbon track inside the boot from the cracked plug. I change all kinds of wires that do this and fire down the outside of the porcelain. Just a thought.

 

hmmm...never heard of this.

I wish I knew which wire it was. I know it was on driver side, but not sure which.

Am I looking for a burned mark in the boot or what? Just some soot? Or actually a burned portion of rubber?

 

Change them all, if the wire is the problem.

I had a similar problem, one wire was arcing to the block. I moved it so it couldn't touch the block and start arcing. Four days later, all the wires were arcing and the power loss was very noticeable.

 

Well, these are second-hand wires. there is no reason why they shouldn't be faulty.

There is no harm is swapping them.

they look fine.

I guess I should fire the engine up at night and see if there is any arcing over the wires to the frame...

 

FWIW...My dads 51 Pontiac was doing the exact same thing.SBC 400 with a 2V rochester on it.Mechanicle pump.Good tank/fuel lines and filter.Fresh tune up etc...
Turned out to be a piece of crud in one of the mixture screw passages.Blew both out with compressed air,readjusted mixture...problem solved.Sometimes its the silly little things that drive ya nutz...good luck Scott!!!

 

I'm having this same kind of problem, but with a 250. I read the above post and was hoping maybe I could get a little more info on what I should see in the carb. I have looked down the the carb at idle, and see fuel dancing on the throttle plate. Any advice or direction would be greatly appreciated. I now return the thread to it's rightful owner.

 

Actually, if my car ran like this from now on, i would be happy. But hearing this periodic miss...I know there is some lost power to the mmotor.

 

You'll see it right away. It would be a grey or black line down the inside of the boot. I make lots of money off it when guys change plugs for a misfire and leave the old wires.