Engine misfiring and stumbling

[jhv48]

Before you start messing with the advance mechanisms, replace the cap and see if it runs better. That looks like carbon tracks on two of the posts to me also.

If the engine ran fine before the backfiring started, then just find the miss and then it will run fine again. No need to start messing with the advance mechanism unless you want another can of worms loose all over your garage floor.

[Leecbr]

I'll start with the distributor rotor/dist cap due to the the carbon tracking. That's why I love this website and the knowledge pool available. Thanks for all of the great advice guys...

I was wondering if the carb could have something like scaling or debree clogging one of the jets causing it to run lean. I'll start at the distributor and then work towards the carb ( power valve ).


Lee

[Zaphod B]

This may be apples and oranges, but on my (now long gone) '89 'Stang with a 302, replacing the cap and rotor was an every-other-oil-change scheduled maintenance item for me. At the first sign of any stumble I'd install new ones and it would smooth things right out.

[Silverback51]

Had this happen to me a couple years ago, and I replaced the everything. By everything, I mean the cap, rotor, finally the complete distributer, plugs, wires, coil, MSD box, and it still ran the same.

I was ready to load it on a tow truck and have someone fix it and call me when it was done.

I was talking to an old friend of mine and he mentioned that the MSD unit is very sensitive to voltage input. He suggested that I disconnect the alternator and try it. I did and it ran fine. Put a new alternator on it and it did the same thing. Come to find out it was the voltage regulator. Replace it and it ran fine.

I did however end up with great collection of spare parts.

Take the belt off of the alternator and see how it runs.

[Gaz64]

Originally Posted by Leecbr

I'll start with the distributor rotor/dist cap due to the the carbon tracking. That's why I love this website and the knowledge pool available. Thanks for all of the great advice guys...

I was wondering if the carb could have something like scaling or debree clogging one of the jets causing it to run lean. I'll start at the distributor and then work towards the carb ( power valve ).


Lee

If you don't drive it often, the carb could have water corrosion inside it.

By the sound of it, you have multiple little problems.

[jhv48]

Generally, carburetor problems will not show up in only a couple of cylinders, unless you have a single plane intake manifold.

Stick with the cap replacement and then we'll go from there if that doesn't cure the problem.

[winnipaw]

Did you happen to mention if the plugs were fouled with carbon? Does the car idle OK? Choke mechanism? How about considering fuel pressure. Do you have a pressure regulator? What is your fuel pressure reading? When the car is turned off, is there any fuel visible in the venturi or throttle plate. I'm chasing all these issues at the present time, after rebuilding my Holley 2160. I've done this many times and its never a cut and dry fix. Everyone here has given good ideas. Trial and error will get results eventually.

[Leecbr]

Hey guys, I want start out by saying, thanks for all of the great tips..

I replaced the distributor rotor and cap myself..The carbon track was alot worse up close. It was a dark line bridging two of the terminals. Plus my terminals had some buildup on them....It was an easy fix swapping those parts. The new blaster coil was an excellent upgrade too. I left that wire on the advance for now since things have been fine...

I had the Carb rebuilt and cleaned.
The Carb shop said my Power Valve had failed...( like you guys had suggested ) The jets were dirty and I had them replace all of the gaskets and accelerator pumps. The accelerator neoprene was bad and leaking a bit of fuel.. New accelerator cover, lever and springs..

They saved some of the old parts to show how bad they were..

I bought the car used so I had no idea the carb was so neglected. The shop said it had most likely sat a long period of time without gas prior to me owning it. The gaskets were hardened, the Power valve gasket was shriveled. The jets needed cleaning and they blew out all of the air bleeds and jets.....Well all is good now...Until the next repair.

The cobra is back and running strong once again. It was very educational to say the least..

Thanks for all your help,

Lee

[Rwillia4]

this is a good read and the true reason I like this forum. There is plenty of knowledge out there that guys want to share to help there fellow friends. Awesome results.

[jhv48]

Thanks for posting the results.

It will help the next guy who does a search.

[KHBock]

Hey all - had the same misfire and stumble happen to me this past weekend. I noticed when it stumble the tachometer went right to zero. Then back to normal when the engine was ok. All other gauges where normal. The tach is wired to the 6AL box. I am thinking its the MSD 6AL box or the coil. I checked the rotor, cap, spark plugs, voltage (13.2) and all is well. Anyone have any other suggestions?

[greg schroeder]

I'd look at the metering blocks on the Carb. The coating gets pretty funky after a while. Just pull one off at a time. It's pretty obvious if that's the problem once you look. The passages get plugged up. Clean and scrape off the chunks carefully or replace the blocks if needed.

[KHBock]

Greg - the metering block - wow I would never think to check that. I was thinking more electrical with the the tachometer zeroing out??

[Rick Parker]

KHBock:

The tach dropping to "0" is indicative of an intermitant electrical open circuit in the primary side of the ignition system IE: bad module in distributor or its connections, coil or its primary connections, or power leads and switched 12 volt circuit to the Ignition Box.
Recently I had a tachometer fail. I sent it for repair and the circuit board had failed, perhaps vibration or it was just "Time to go". Ignition still was working well.

If you haven't already, suggest you mount the MSD box on the rubber Isolatord to help eliminate vibration:
Vibration Mounts, MSD 6 Series - 8823

Trouble-Shooting - MSD Ignition Tech Support Forums

[KHBock]

Thanks Rick - I'll start there with the troubleshooting tips. KB

[jhv48]

KHBOCK, Is your ammeter reading positive while driving?

[KHBock]

jhv48, my voltage gauge was normal at 13.7volts. I do not have a amp meter installed.