Intermittent Misfire-327

[72step]

I have been experiencing an intermittent miss-fire on my truck which I don't drive a whole lot. The truck has a high Perf 327 with about 4k miles since being rebuilt about 14 years ago. In the last two years I've replaced the carb with a new Quickfuel dual feed, I've replaced the plugs, and wires. The last time I messed with it earlier this year it would seem to help to mess with the dist. cap but I'm not sure it really changed anything. I started it up 2 weekends ago after about 6 months of sitting and it sounded like it was ready for the drag strip. Then last weekend I started it and moved it around to do some other stuff to it and it sounded good with a very minor hint of a miss. Then today I pulled it out again to work on some more odds and ends and it sounded horrible. Back firing with barely enough power to move around my property. I'm just trying to figure out what it could be. I suspect the ignition system which is a Mallory Unilite with a promaster coil. My second guess is a loose wire or bad ground somewhere. My third and more dreaded guess would be a flat cam? I appreciate any advice yall can offer to try and pinpoint this. Could it just be bad gas? Thanks....

It has a Comp Cams 280H hyd cam, roller rockers, SS valves chrome moly push rods, 9.5:1 comp ratio. geardriven-pretty stout engine and has always ran solid with no problems or smoke or anything like that.

[PHAT TONY]

Could your fuel have gone bad from sitting? Gummed up the plugs?

[72step]

Would bad fuel cause it to backfire like crazy and plain old sound like crap?

[Stocker]

Not 100% positive, but yes, I believe it can...

[PHAT TONY]

If plugs aren't firing and that gas is collecting in the exhaust system it can go boom! This can start from a carb full of bad fuel "varnish" in/on the passages, bits and pieces causing a SUPER rich running condition. Poor idle, black smoke, fouled plugs followed by cussing and head scratching

[mr48chev]

Yes, I would be inclined to think that the gas that we have today would go bad if it sat that long. Unless you use a gas stabilizer in it there is a fair chance that it will go bad if left for more than a couple of months.

Also unless I don't have a choice I only buy gas from newer name brand stations or Costco or Safeway. I don't go to old stations nor do I go to off brand or no brand stations.
My brother-in-law drives a fuel truck in the Seattle area and
delivers to all kinds of stations. He says that Costco will not let drivers "stick" the tank because they don't want potential contaminants in the tanks.

Enough of that but as I said before if nothing changed I would suspect fuel that was contaminated or had gone bad from sitting.
If it has rained a lot or the humidity is high you might have condensation inside the cap. I've had to deal with that more than once with customer rigs in the past.

[70duramax]

is it backfiring through the carb or exhaust? Through the carb its a spark/timing issue through the pipes its a fuel issue, this applies to any 4stroke, with out getting my hands on it anything else is speculation. hope it helps, good luck

[unclewatts]

Could be bad gas but misfiring is typically caused by a bad coil.

[MrBeast]

fuel going bad is a probable cause, also water in the fuel, if you live in a humid area and the tank was not full it could have condensated a bunch of water in, also might check your plug wires.

On my 85 I had a set of accel wires go bad that were brand new less than 6 months ago, drove the truck about 10 times in that ammount of time.

[2tons of fun]

i just fought a bad misfire ona ford 302.

turned out the the very clean, very new looking cap was wasted.

i used my timing light on each wire to id the dead cylinders, swapped the wires and the same cylinders sucked. new cap and it runs like a champ.

[72step]

Thanks guys. It was running quite rich with a bit of smoke. I had a 750 Carter AFB that always ran rich but this quick fuel carb pretty much took care of that. It is backfiring through both the carb and the exhaust. I suspect the fuel, but I still don't trust the old Mallory system. I'll try what one of you said with the timing light. The wires are brand new but I'm not crazy about the way the coil wire hooked up to the Mallory Promaster coil. What would be a not too expensive but reliable ignition conversion for my truck? I'm thinking just a plain GM HEI then I just have to get the right curve deal for it right? I don't know about these high performance Mallory or MSD units and long term reliability. As far as the fuel, shall I just run it dry then put fresh fuel in it and that's it? or do I have to get some carb cleaner, maybe run some sea foam in it? Thanks. You guys are great; it's good to know I'm not alone trying to chase an annoying problem.

[jim zag]

Check to see if you have a nice BLUE spark. I bought a spark tester at spring Carlisle and I use it all the time. I had a new Accel distributor that acted strange and found out the module was bad. The spark tester showed very week spark. You can not beat an HEI for dependability. You can buy an MSD Streetfire HEI at a reasonable price.

[72step]

Cool, does that MSD street fire take a control box? I saw it on Jegs, not a bad price. How's reliability? I've never ran an HEI on my truck, so it's one unit with nothing but wires to hook up right? I'm going to seriously consider that. I'm just concerned about reliability at that price.

[72step]

8548201C - Mallory HEI Distributors

This Mallory looks pretty decent too.....

[jim zag]

You do not need a box with the streefire it is basically a new replacement distributor. One wire to make it work and a wire to make your tach work. It is a better quality unit compared with the cheaper ones on the market.

[pat 70/71]

I had the same thing happen to me my afb carb was running fine & motor started to miss/ cough I worked all the angles& what happened was I found low fuel pressure (real lean condition) it was a small split @fuel line . tank under bed. this is the strange part,motor only ran lean on the right bank. (pump was cavitating) air was getting into the fuel line @ the tiny split1/64"hope this helps. ...Pat

[hayhauler71]

Drain the tank and add new. Fuel lasts about 4 months before going south. Recommend finding non oxygenated fuel has a longer shelf life.

[72step]

Thanks for the ideas. It could be any of those things mentioned. I guess I guess starting with the fuel would be a start. I also have a new fuel line kit from the pump to the carb from jegs, I'll plumb that in and check my ignition system. It's hard when all your components are 14 years old! I don't trust my mech fuel pump either it is a Holley mechanical pump that's been on there 14 years but I do have a fuel pressure gauge on the fuel line into the quick fuel carb. I can check that too. THe thing is when it's hitting good it really hauls a$$ with the setup I have! I'll have to make it a winter project to get it running good by spring. I plan on entering it into the super nationals show in Feb. I guess my priority is to assure it's nothing major. Thanks....