Engine running very rough

[kcsa75]

Typically the 350 in my truck runs like a sewing machine, but over the last couple of weeks, it's developed a couple of issues.

A few times times when I've been driving it'll cough or spit once or twice then be fine. This has been going on very intermittently for a couple or three of weeks. Then this week it just began running very rough, like it's not hitting on all cylinders.

The ignition module and coil in the HEI distributor were replaced back in the spring and I've check to make sure all the plug wires are connected. The Q-Jet was rebuilt in the spring as well and has performed flawlessly since.

My next thought is to check the fuel filter, but after that I'm out of ideas.

Any help would be appreciated.

[BIG69GMC]

when was the last time you replaced your spark plugs? wires? Sometimes just the one will go bad and then they all start going if they havent been done in a good while.

[bigmac73]

when you downshift and accelerate, dose it spit , miss , sputter?

if so i would change the plugs

my opinion on the fuel filter is either they are clogged or not,i would change it just to keep it fresh, but i dont really think that is the overall problem, i would get rid of the small rock or paper filter in the carb and get a inline filter that yoou can see through . like this one


Amazon.com: Wix 33003 Complete In-Line Fuel...

[kcsa75]

I pulled the 1 and 8 plug tonight after I got home from work. No. 1 looked OK, but 8 was fouled really bad. I ordered a set of plugs along with a new cap and rotor. That will make all the disti parts new in the last six months. The plug wires are about three years old, so I think I'm going to take a wait-and-see approach with those for the time being.

[68gmsee]

If it still acts up after replacing plugs, cap and rotor check and/or replace all fuel filters. Then check the fuel pump output into a plastic container for about a pint of fuel in 30 seconds or less.

If all that checks out and it still running rough, check for possibility of contaminated fuel. Buy a 2 foot section of fuel line the same size as the one going to the fuel pump from the tank. Hook it to the inlet side on the pump and the other end into a one gallon container filled with fresh gas placed under the truck.

[Steeveedee]

HEI rotors are famous for burn-through if the secondary circuit (pugs and plug wires) are high resistance (wires cracked, plugs worn out). In my experience, 14k miles is about as far as a rotor will make it. If you are changing the parts anyway, at least check the rotor for burn-through. The evidence is tracking (charring) on the bottom part; you may be able to see light from the top side of the rotor in especially bad cases. A couple drops of gasoline may flow right through the hole. Amazing that some of them manage to limp into the shop.

[Steeveedee]

Originally Posted by kcsa75

I pulled the 1 and 8 plug tonight after I got home from work. No. 1 looked OK, but 8 was fouled really bad. I ordered a set of plugs along with a new cap and rotor. That will make all the disti parts new in the last six months. The plug wires are about three years old, so I think I'm going to take a wait-and-see approach with those for the time being.

#1 and #8 are on different runners from the carb. Make sure that #s 1, 7, 4, and 6 (one runner) and #s 2, 3, 5 and 8 don't have a lot of difference. If so, the problem is in the carb. This is extremely uncommon in Q-Jets, as the metering jets and rods are a pretty robust design. I've seen it in older Rochester 2 and 4GC carbs where the metering equipment has come loose inside.
Make sure you didn't break a plug wire when you pulled it off the plug. The HEI systems' wires can really stick to the plugs and it is easy to break the plug connector off the wire when you pull the wire off the plug.

[kcsa75]

When I got it apart, the cap and rotor were pretty well used up. I also found a spark plug boot that was split, which may have been the cause of the sudden change, so I went back to the part store for a new set of plug wires.

It started up and ran smooth, but I think I still have some issues with the carb. If I punched it from an idle it wanted to "backfire" back out the carb and there was not much power on the low end. I had the carb rebuilt less than a year ago, so I think I'll take it back to the same guy and see if he can get things straightened out.

Thanks for your help.

[buddylj1990]

thought check your vacuum advance on distributor

[msgdsrf]

I'd test the no. eight cylinder for compression. Possible cam lobe or burned valve issue still lurking there.

[kcsa75]

It turns out the Quadrajet gave up the fight. When he rebuilt it back in February, he told me there were some issues with pitting in the bowls. Instead of messing with it or trying to find another Quadrajet, I opted for a brand new Holley 4160, 600 CFM. We also did some work on the distributor. The end result is it looks really cool and runs like a champ.