Need help with Carburetor

[79dentside]

Hey all,

I had this thing tuned in pretty fine, ran fantastic, but yesterday and today it stumbles off the line and doesn’t seem like it’s getting the appropriate amount of fuel. It’s rocking a Quadrajet that I’m pretty sure is original. It has tarnish all over it, I’m sure ALL the gaskets are bad, but that hasn’t changed.

Before I start messing with it too much, what do you guys suggest? Truck has a newer fuel pump and a newer fuel filter less than 3 years old/~2,000 miles.

I have zero interest in rebuilding it, if it comes to that I will buy a replacement Quadrajet. If I can skip buying a new one, that would be preferable.

Thanks!

 

[78C10BigTen]

Did you change the filter in the carb or was it the inline filter you changed?

 

[79dentside]

Did you change the filter in the carb or was it the inline filter you changed?

Just the in line one located in the passenger frame rail.

 

[78C10BigTen]

Just the in line one located in the passenger frame rail.

There is a small one in the carb located at the fuel inlet, check that for clogs

 

[79dentside]

There is a small one in the carb located at the fuel inlet, check that for clogs

Will do. That’s for the idea.

 

[75gmck25]

If it was working fine before but now has the stumble off the line, it may be the accelerator pump. Older ones used a rubber seal material that does not hold up well to ethanol, so newer ones have a different seal. The replacement pump mechanism is cheap, but you have to remove the top of the carb. Once you get that far you want to check the float and other parts, and replace the upper gasket, so you are about 1/3 of the way toward a rebuild.

Bruce

 

[79dentside]

If it was working fine before but now has the stumble off the line, it may be the accelerator pump. Older ones used a rubber seal material that does not hold up well to ethanol, so newer ones have a different seal. The replacement pump mechanism is cheap, but you have to remove the top of the carb. Once you get that far you want to check the float and other parts, and replace the upper gasket, so you are about 1/3 of the way toward a rebuild.

Bruce

I see, I may tear into it today. Would this be something that is more pronounced as the truck warms up? I notice that first few accelerations from a stop; while cold, are just fine.

 

[79dentside]

Another thing to add, as I figure it’s possibly related.... yesterday it was having choke issues. The choke stuck even after a long drive idling at about 1,800 rpm, then it idled WAY DOWN and became really rich when I was trying to free up the choke linkage. Here’s a video of how it was running.

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I have checked the plugs and they are okay. I have since unplugged the electric choke, but I am still having the hesitation and the lack of fuel as previously mentioned. Thanks.

 

[75gmck25]

If you unplugged the electric wire to the choke you will now always be running rich.

Bruce

 

[79dentside]

If you unplugged the electric wire to the choke you will now always be running rich.

Bruce

I plugged it back in... good to know. Still have an issue with the choke engaging. Just drove it and put it in park and it was idling at 1,400 rpm. I had to slap the choke off (it was unplugged at the time) I have since plugged it back in and zip tied the choke linkage tabs down because I have to keep slapping them down after every drive... until I fix this, that’s how I gotta roll. I’m not gonna pop the hood every time I drive the dang thing.

 

[79dentside]

A little update. I’m not sure what the flip happened, but it’s apparent that this thing wasn’t as dialed in as I thought? I am not an expert, but I thought I had it pretty good. I decided to re-tune the idle/air mixture screws. I had them 2 1/8 turns out each and rolled with that previously. I hooked the vacuum gauge up today and the vacuum was low and erratic. It was probably fluttering between 15 and 16 or so pretty bad. I must have retarded the timing a little at some point in the last few months, so I ended up advancing the timing a hair, but my fix of the day was in the idle mixture screws... I adjusted them until I was happy with the vacuum that the engine was producing and holy smokes does this thing run great. I guess 2 1/8 was way too lean for my set up? I’m not even going to put an accelerator pump in it. No more lurching, no issues. Here’s vacuum at idle now. Steadier than its EVER been. Things runs amazing.

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I’m still not sure what went on here in the last 3 days... maybe it was choke related? I think I’m going to try to figure out how to do a manual choke so that I can remove my zip tie. Thanks for all the help.

 

[Backfoot100]

FWIW my QJ from the factory was unmolested when I got it. I rebuilt it and had to remove the factory anti-tamper plugs. The factory setting was turned out four full turns on the mixture screws.

Being a Holley guy, they normally get set 1-1.5 turns so my thinking for the QJ was it had to be super rich. After the rebuild I set them at 2 turns and it ran OK but like you, I was amazed when I readjusted them with a vacuum gauge. End result was 20" vacuum and the mixture screws set at exactly 4 turns each.
The factory setting was dead nuts. They obviously knew what they needed.

 

[79dentside]

It’s really funny you said that, because I got about 4 turns out on each side... I just thought it was a lot. I appreciate your advice, that aligns with what I found too.