rough idle

[AaronW]

My son has a 75 GMC with the 350. All of a sudden, it will barely idle. Problem started shortly after it ran out of gas. We replaced the fuel filter up in the carburetor, and I went ahead an put a different fuel pump in it, too, since it wasn't very much $$. Still same problem. It starts fine, but sounds like it's barely running, and it shakes pretty bad. If you give it some gas, it does start to even out once you rev it up a ways.

So would the next logical step be to pull the carburetor? I've tried spraying some carb cleaner down it, but that almost makes it die as well, at idle. I'd wondered about replacing the plugs and plug wires, and maybe I should do that anyway, but that doesn't seem like it would be right for this particular problem (correct me if I'm wrong). It's the stock carburetor, so I think it's some version of a rechester, though I've got to look back at the manual to check for sure.

Thoughts?

Aaron

 

[Matt69olds]

If you ran it out of gas, and the problem started immediately after refueling and restarting, it’s probably got some trash in the tank or lines that got stirred up after refueling.

You might be able to clear the crap out of the float bowl by pinching closed the rubber supply line to the pump. Start the engine, wait for it to die. Unclamp the line and restart. The sudden rush of fuel past the needle/seat in the carb might clear out whatever is holding it open, causing flooding. Or it might be a waste of time.

If the truck ran well before, you might be able to pop off the top of the carb and clean out the junk. Watch a couple YouTube videos on Q-Jet disassembly. DO NOT PRY OR BE AGGRESSIVE WITH THE METTERING RODS OR POWER PISTON!!!! The proper way to get the power piston out is to push it down, and let the spring force pop the piston out of its bore. If you bend the metering rod hangers, you will have fuel distribution issues (one half of the engine running rich or leaner than the other) or make the piston stick in its bore.

The biggest reason people think the Q-Jets are junk is because they screw them up taking them apart or putting them back together. In other words, they creat the issues.

 

[AaronW]

If you ran it out of gas, and the problem started immediately after refueling and restarting, it’s probably got some trash in the tank or lines that got stirred up after refueling.

You might be able to clear the crap out of the float bowl by pinching closed the rubber supply line to the pump. Start the engine, wait for it to die. Unclamp the line and restart. The sudden rush of fuel past the needle/seat in the carb might clear out whatever is holding it open, causing flooding. Or it might be a waste of time.

If the truck ran well before, you might be able to pop off the top of the carb and clean out the junk. Watch a couple YouTube videos on Q-Jet disassembly. DO NOT PRY OR BE AGGRESSIVE WITH THE METTERING RODS OR POWER PISTON!!!! The proper way to get the power piston out is to push it down, and let the spring force pop the piston out of its bore. If you bend the metering rod hangers, you will have fuel distribution issues (one half of the engine running rich or leaner than the other) or make the piston stick in its bore.

The biggest reason people think the Q-Jets are junk is because they screw them up taking them apart or putting them back together. In other words, they creat the issues.

Thanks, Matt. That's my assumption as well, that we got some crud sucked in out of the tank. The truck mostly sat for quite a few years, for it was given to my son. I'll try those suggestions.

Aaron

 

[fast 99]

Engine running dribble fuel into the front venturies one side at a time. If one picks up and smooths out that side idle circuit is restricted. Take the idle mix screw out of that side, squirt carb cleaner and or air into the hole where the screw was. Doesn't always work but worth a try.