Running cold and rich. Idle mixture screws all the way in!

[Grandpas_Truck]

Hello All!
I have recently acquired a 1986 GMC 2500 with the 350 and a Quadrejet carb from my grandfather which he purchased new in '87. Only modification he had done to it was an emissions delete. (Which was done close to when the vehicle was new). First thing I noticed when I got the truck is that the engine never gets above 150 degrees Fahrenheit. While driving, I also noticed when you give it throttle, the truck would hesitate, before finally catching up with itself. To fix this, I rebuilt the carb and while rebuilding I found one of the enrichment tubes had fallen out. After the rebuild, the truck runs a lot better (no more hesitation). However, when tuning the carb with a vacuum gauge, I had to screw in the idle mixture screws all the way till they wouldn't go in anymore. The vacuum raised to about 17 in Hg, but I never got to the point where the vacuum would start to go back down. Black smoke can be seen coming out the exhaust, especially when I give her the gas Indicating it's still running rich, even with the idle mixture screws in all the way. Could this be caused by the truck never getting to operating temperature?? Any help would be appreciated!

 

[Ricko1966]

If the choke is fully open the rich condition is not from the engine being cold.If it's idling with the mixture screws all the way in you have a problem,probably more than 1. First make sure your choke is getting all the way open and the fast idle screw is no longer in play. Now make sure the throttle stop screw is almost all the way backed down as in back it all the way off,not touching,screw it in until it touches then give it 1/2 turn or so. See what that does for you. Still messed up look hard for vacuum leaks. Post progress. Hopefully that gets you somewhere and we dont have to diagnose/ dissect your carbureator rebuild.On the running cold issue put a new thermostat in it. I'd tell you to test the old one but,it's gotta Be out to test so might as well have a new one,drill a tiny hole in it 1/16th or so. Not normally what I'd tell you but with this project I'm afraid you are going to end up with an airlock by the time you get all your future issues fixed.

 

[squaredeal91]

Sounds like you did some good checking. It should not idle with those screws in if base idle is set and choke is off. Is it dribbling fuel into venturi while running? Maybe float is out of adjustment or needle and seat are passing fuel.

 

[Grandpas_Truck]

On the running cold issue put a new thermostat in it. I'd tell you to test the old one but,it's gotta Be out to test so might as well have a new one,drill a tiny hole in it 1/16th or so. Not normally what I'd tell you but with this project I'm afraid you are going to end up with an airlock by the time you get all your future issues fixed.

What temp thermostat would you recommend? 195 F?

 

[fast 99]

195 is fine. won't go into possible carb problems until previous suggestions are checked.

 

[Matt69olds]

Look down the throttle bore while it’s running, so you see fuel dribbling out the venturi? If so, you might have the float set too high, or the clip that hold the needle to the float hanger wrong. Some people think the clip goes thru the holes on the float, it actually just hangs over the float. Some carb experts suggest eliminating the clip.

How did you remove the metering rods? It’s really easy to bend them if you pull them out. The correct textbook method is to compress the spring and let the rods snap up, you may have to do that several times until they come out. Once they are reinstalled, they must move freely and evenly. If they aren’t even, you will have the fuel mixture screwed up on one half of the engine.

Are the throttle blades fully closed at idle( especially the secondary side) sometimes they hang up on poorly made gaskets.

Spray carb cleaner around the throttle shaft bores. Unless it’s a fairly low mileage carb, more than likely the bores are worn out and leaking air around the shafts. They make bushing kits to repair the throttle body.

Don’t listen to the idiots that tell you to install a Holley. The Q-Jet is the best all around carb ever made.

If you have Facebook, look up the Quardrajet group. Cliff Ruggles and Ray Klemm are very active there and know what they are doing. They both sell quality parts and have some excellent tech articles.

 

[Ricko1966]

I guess it's time...Sh!t can it, LS Swap and a holley.

 

[Grandpas_Truck]

Update. So I let the engine get up as hot as it'll go. (Around 150) and the choke was only at about 45 degrees. Measured 14 volts at the choke. I adjusted the choke and now it is fully open. There was a loud hissing and sure enough, when I sprayed starter fluid near the primary throttle blades, the RPMs changed. When I did the rebuild I did notice that the shaft was a little sloppy but I didn't think it was too bad. Guess I learned my lesson. But I do have a question. Wouldn't a vacuum leak cause a lean mixture? Not rich? I'll keep updating with anything else I find.

 

[Ricko1966]

A vacuum leak many times will create a Rich mixture because at low vacuum the carb thinks you are accelerating or in a heavy throttle condition so it adds fuel from the power circuit. A lit of people have a hard time understanding that. But your car uses vacuum to determine fuel requirements. Another fun fact a bad plug wire will create a lean condition even though you can smell unburned fuel. Because the dead cylinder is pushing 14 parts air but only 1 part fuel. Make sure the leak is actually throttle bushings not a hose off or broken base gasket.