Q-jet running lean?!?!

[76-77-78]

Ok so I just recently acquired an electric choke Q-jet off an 83 305 chevy. And just rebuilt it with new needle, seat, check ball, and gaskets. And it is running 72 primary jets and either 49b, or 50b, primary rods. And at idle I have the idle screws out almost 5 turns which seem excessive to me. and that brings the air fuel ratio to about 13.7-14.3 which seems a little high but I want a second opinion. And at part throttle acceleration I’m at about 15.5-17.0 which I know is way to high so I want some of your opinions on if I have a vacuum leak or just need to do some more tuning

 

[76-77-78]

I have also sprayed ether around carb base gasket and intake gaskets and it didn’t change idle at all thanks in advance

 

[Snoots]

Before offering any suggestions, perhaps you could enlighten us on a few other things:

What engine displacement
what cam
what lifters
what timing
what device you're using to determine A/F ratio

Not trying to be a smart-arse, the info would be helpful.

 

[76-77-78]

Oh whoops that would be helpful info sorry about that. it is a stock L31 Vortec 350 higher miles in the 170k range. Has long tube headers, AEM wide band a/f gauge, timing is at 8 degrees btdc not sure what it’s at fully advanced.

 

[1987 GMC Jimmy]

Hmm, the mixture setting seems okay to me, but I appreciate your concern when referring to the AFR gauge. I’m at about 3/4 of a turn in from you. Anything less than 3.5 turns out will make idle and off idle performance crappy. I think it’d be good to get your vacuum reading to see if you’re where you need to be or if there’s something wrong. The needle behavior will further tell you how your engine is responding to your mixture setting, and the gauge as a whole can tell you a plethora of other things about the health of your engine. You should have a fairly stable vacuum reading of 18-21” of Hg. 17” will pass you in my book, but anything less is sub-nominal. That’s with a stock or close to stock cam, by the way.

 

[76-77-78]

Hmm, the mixture setting seems okay to me, but I appreciate your concern when referring to the AFR gauge. I’m at about 3/4 of a turn in from you. Anything less than 3.5 turns out will make idle and off idle performance crappy. I think it’d be good to get your vacuum reading to see if you’re where you need to be or if there’s something wrong. The needle behavior will further tell you how your engine is responding to your mixture setting, and the gauge as a whole can tell you a plethora of other things about the health of your engine. You should have a fairly stable vacuum reading of 18-21” of Hg. 17” will pass you in my book, but anything less is sub-nominal. That’s with a stock or close to stock cam, by the way.

Ok I will try and hook the vacuum gauge up tomorrow and see what I’ve got. It always was around 20” of vacuum with my Edelbrock 1406 so I’ll use that as my base line for troubleshooting.

 

[1987 GMC Jimmy]

A warped carburetor body is a worst case scenario that would cause a major vacuum leak, and a high mileage primary throttle shaft will have worn through the body and caused a vacuum leak. Don’t forget about the brake booster also. The rest is pretty much vacuum leak 101.

 

[76-77-78]

Today’s update

So I fired her up this morning and let her warm for a few minutes. then took the truck for a few minute drive around town to make sure the choke was off and the engine was up to temp. But I could almost immediately tell some thing was different this morning. It was idling around 10.5-11 on the a/f gauge. And at part throttle was at about 14-15 on the gauge. Acceleration was better too, at about 13-14 depending on how hard you got on it. So I brought it back home hooked the vacuum gauge up to the carb and was at 18” hg. so I adjusted the idle mix screws and got it up to a high of about 19”hg at about 13 to 1 on the a/f gauge. So I’m really confused at about it’s mysterious self fix but as long as it keeps on working I’ll be happy I guess. I will do a longer highway test later if the roads get better. It’s been snowing and raining off and on here in ND the past few days and the roads aren’t very nice right now. But thanks every one for the quick replies and the advice. It’s really nice having a site like this to go to for info being only 19 years old. I’ll update this again if any problems show up in the next few days.