Carb Help

[matthud]

I took my car out for a drive the other day, I got it nice and warm, it was about 25 minutes. On my way back home, it stalled when I had my foot on the clutch. Couldnt get it restarted. Had to push it around the corner since I was on a busy road. I had someone else try to start it while I looked down the throat of the carb. When the gas pedal was pushed the gas sprayed down the carb like it was supposed to. But when you let off the gas pedal the gas didnt stop flowing into the carb. After about an hour+ of sitting there letting it cool down, it seemed to be ok, and it started.

I realize now it stalled because it was getting too much fuel. Was this a stuck float or something else? And do you think it was caused by the hot engine temperature?

Its a Qjet on a 69 427.

 

[Vettehead Mikey]

"I realize now it stalled because it was getting too much fuel. Was this a stuck float or something else?"
Classic stuck float

"And do you think it was caused by the hot engine temperature?"

Nope, nothing to do with temps. Might be a good idea to replace the float, needle and seat before it does it again.

 

[matthud]

But even with the engine not running, no fuel being pumped, the gas was still being pushed into the carb, even after you took you foot off the gas. I thought the float just metered the fuel in the bowl.

 

[Nitronick]

It was probably something that got under the needle. All it takes is just one speck of dirt to keep the needle from completely closing.
The needle and seat functions just like the flapper valve in a toilet. If everything is ok, it will fill with water and shut off when it is supposed to. If something keeps the flap from shutting, it will run continually.
If you don't have one already, you might want to look into installing an inline fuel filter. Yes, there is the filter in the carburetor, but it is spring loaded, and if it captures enough dirt(which it is supposed to do), it will eventually have enough resistance that the fuel pressure will overcome the spring and bypass the filter. This is the way it was designed.
If this has just happened this once, I would change out the filter in the carb. Also look into getting an inline filter(about $5 at any auto parts store). You might want to pull the carb off the car, turn it upside down and drain all of the gas out of it into a clean bowl. That way you can see what is in the bowl. It won't hurt anything to flip it over.

 

[Nitronick]

But even with the engine not running, no fuel being pumped, the gas was still being pushed into the carb, even after you took you foot off the gas. I thought the float just metered the fuel in the bowl.

If something was under the needle causing it to stay open, gas is going to flow until there is no more fuel pressure.

 

[matthud]

How much work is it to replace the float and needle? Is that something someone who hasnt torn down a Qjet can do?

 

[Nitronick]

How much work is it to replace the float and needle? Is that something someone who hasnt torn down a Qjet can do?

I would suggest taking it to a shop. It is pretty involved if you've never been in one before. The float level is going to have to be reset. Anyone knowledgeable in Quadrajets should be in and out of it in 10 minutes.

 

[ruby76]

sure

How much work is it to replace the float and needle? Is that something someone who hasnt torn down a Qjet can do?

If you have an eye for detail - ie: remembering where parts go, then yes, I rebuilt my Q-jet with a rebuild kit from AutoZone, took about an hour including the time it took for the parts to completely dry from cleaning. But the needle and seat can be done while the carb is still on the engine if you wanted. I used one of my wife's old cookie sheets to put the screws and parts on so I wouldn't loose any of them. The carb was better than it ever had been after my do-it-yourself rebuild - including needle, seat, accelerator pump, float and all gaskets.

Jeremy

 

[matthud]

Well, I'm an engineer, so I like to think I have an eye for detail. How hard is retuning the carb after rebuilding it?

 

[ruby76]

Well, I'm an engineer, so I like to think I have an eye for detail. How hard is retuning the carb after rebuilding it?

The only thing to "retune" it would be idle mixture and idle speed. I'd set the idle speed at whatever rpm you want, then alternating between mixture screws, turn them to the right 1/4 turn each till rough idle, then back them both out alternating 1/4 turn each till you get the smoothest idle. If you have a vacuum gauge I hear you can adjust the idle mixture till you get highest manifold vacuum - but I have never done it that way. I've always just adjusted till I get a nice smooth idle. I run my idle speed at/around 800rpm, but folks like their's different based off of cam specs. after you adjust mixture, re-set the idle speed if necessary.

Since your an engineer, Id say you'll have no problem at all.

 

[Nitronick]

All carburetors we remanufacture are set to 4 turns out on the idle mixture screws. This is just a bench setting, to give plenty of fuel on start up. Once the car has reached operating temp, you should screw in the idle setting as low as it will go and still idle. This gets the engine drawing fuel on the idle circuit only.
Adjust an idle mixture screw in one at a time(it doesn't matter which one) until the motor starts to shake like it is going to die. Pull it back out to where the idle is smooth, and we usually go another 1/4 turn. Once you have set them, set your idle speed to where you want it. Like ruby76 said, 800 to 850 sounds good. You just don't want it to kick too much when you put it in gear.
Do a google on "rebuilding a quadrajet". There are several sites with pics to give you a guideline to follow.