350 Acting real sick, But why?

[T-ROD]

So today I get in my truck to take a load of things to storage, in the process, I decided to go ahead and get gas, but then as I was pulling in, it started acting real strange almost sounded and felt like a dead miss on every other cylinder, it was barely running. But then when I got the truck turned off, it started back like new but then when I pulled up to the storefront to grab a drink, it started back again, it would feel very gritty and rough running, and sounded like its exhausts was run through a tin can, and when you rev it, it ran decent, just a very small hesitation and a pop. I've heard of lots of things doing similar things, but it's very intermittent and it has brand new pickup coil, coil, plugs, plug wires, and it's getting around 14.2 volts when started up instantly, so I'm trying to figure it out, because when the motor runs right it will have you in the edge of your seat, but then it just acts like a sick dog within a matter of seconds, getting proper amount of fuel and air also, it's real strange, it's a fairly new engine, timed almost perfectly, and runs real nice most of the time, just real aggravated about the intermittent issue, could it be moisture in the cap? Maybe module? Or the factory est that's still wired? I can't say if they would be doing that exact thing, because they usually recreate similar issues but not exactly the same, and the distributor seems to be timed well, and the ESC does work when it's running right, you can pull vac. And it retards timing a bit, and I also thought about the advance sticking, could it do that?

 

[Snoots]

sounded like its exhausts was run through a tin can

Bad catalytic convertor maybe?

 

[crazy4offroad]

Sounds like the ignition module is the only thing you didn't change. They're cheap so I'd give it a try. Snoots' suggestion is also valid, except for the pop. Backfire through intake or exhaust?

 

[1987 GMC Jimmy]

How low on gas did you run it? As far as gas goes, you may have gotten some bad gas, but if you ran through the tank with no issues, it’s probably not a serious contender for your problem. How are you gauging proper fuel? If it was fuel, the symptoms sound more like a lean condition than a rich one so I’d have my eye on the fuel pump and accelerator pump diaphragm. The only thing is that a flooded engine will make tin can sounds too. I’d pull a plug just to verify that they’re not getting soaked in fuel. If it’s not fuel, I’d either have the ignition module tested or watch the timing more closely with a light to make sure a problem didn’t come up with the timing set. With the timing connector unplugged, the timing mark should be steady, and if you blip the throttle, it should move up and down steadily. It shouldn’t hunt around or wobble.

 

[legopnuematic]

I dealt with a failing distributor in my 76 for a few months. It all started when I went to QT to get a drink, come out and try to start it up, wouldn't start. No spark. Get my parents to bring me a new module, installed it in the parking lot, fired right up. A few days later I started to get a pop through the exhaust and if I'd mat it it would stutter and shudder while popping through the exhaust. New plugs, cap rotor. It would run like a banshee and then start it again. Went almost the whole summer like that, real light acceleration and stayed off the highways. Looked at the mechanical advance one day and it had froze up, freed it up and put new plug wires on it and a curve kit. Ran the best it had ever. Then it started it again. Threw in the distributor I had purchased for my 79 and it hadn't done it since. After 42 years and outliving the original engine it just got tired I guess.

 

[T-ROD]

Sounds like the ignition module is the only thing you didn't change. They're cheap so I'd give it a try. Snoots' suggestion is also valid, except for the pop. Backfire through intake or exhaust?

It's mainly through the exhaust, sorta like when your battery is going dead if the alternator isn't charging, but it has good ground and voltage

 

[T-ROD]

Bad catalytic convertor maybe?

They were removed when I had the engine built

 

[T-ROD]

How low on gas did you run it? As far as gas goes, you may have gotten some bad gas, but if you ran through the tank with no issues, it’s probably not a serious contender for your problem. How are you gauging proper fuel? If it was fuel, the symptoms sound more like a lean condition than a rich one so I’d have my eye on the fuel pump and accelerator pump diaphragm. The only thing is that a flooded engine will make tin can sounds too. I’d pull a plug just to verify that they’re not getting soaked in fuel. If it’s not fuel, I’d either have the ignition module tested or watch the timing more closely with a light to make sure a problem didn’t come up with the timing set. With the timing connector unplugged, the timing mark should be steady, and if you blip the throttle, it should move up and down steadily. It shouldn’t hunt around or wobble.

It did go through a tank of gas but I was wondering if maybe it could have had water in it, it just ran so much like an electrical problem due to how it was so hit and miss, one minute it ran like a brand new truck, very throttle responsive and all, the next time you'd turn it off and fire it back up it run like it was on four cylinders, but then randomly it would come back to life, it only ran fine when it was over 1000 RPMs, it still had a little pop and hesitation but run fairly strong, just mainly at idle it ran like pure crap, I was wondering if it could have been a fuel issue though, because I actually didn't check the fuel at that very moment, I was just going by the fact that I checked it not long before I took of and the carb was adjusted almost perfectly and the exhaust when it acted up didn't smell too rich or that it was starving of fuel, it seemed real strange because it would go from running great to all the sudden cranking it over and barely getting it to stay running, it would run at like 200-250 RPMs, and just sounded like it was every other cylinder running, and it had that gritty Grundy feeling you get sorta like when your timing is way out of whack or like those separate coil cars when a pack is out, it felt like a dead miss on every other cylinder until you got it past around 1000 RPMs then it felt like it would cut in and start catching up, but I've never even seen an advance get stuck that far outta whack, can they? I've had a few weird things happen on vehicles but the way it ran was just simply hard to explain, it almost felt like a timing issue, it's just that it is so random that you can't get it to recreate it to continue checking, and it does seem to do it once it's been drove a while but then again it done it after it was parked overnight the other morning too, and it's almost exactly the same thing, then back to normal, it's driving me crazy.

 

[1987 GMC Jimmy]

If you’re ever worried about water in the gas, a bottle of Heet will be good insurance. I think it’s a bottle for twenty gallons. As far as the distributor goes, I would think a mechanical issue in there would be more to blame than the advance mechanism. I don’t play with these unfortunately since I don’t have a vehicle with good ole HEI, but you might try testing it with the cap and rotor off and pumping up the canister with a vacuum pump. A timing light might also reveal something when the issue is manifesting itself. I wish I had something more tried and true, but maybe someone else will. If you can’t find somethint definitive with the timing, I’d check fuel flow and make sure the carburetor is discharging an appropriate amount of fuel - two healthy streams of gas starting the instant you start to move the throttle.