Pontiac 400 2801 cam stock quadrajet tune
 

Car seems to run fine under acceleration however it want to die when pulling up to stop. ( Kinda suspecting the choke) car is warmed up having issues maintaining a good idle I’m running the stock quadrajet carb for 1977 400 4 speed which previously before doing a head gasket worked fine. I smoked the upper half and didn’t see anything that would indicate a leak. I sprayed it with car cleaner and didn’t notice the idol jump or anything. I just had a little bit of a loss. I did notice, however, that the primary air flaps never seen to fully open with the car at 180°. The flaps don’t even appear to have changed from when it was cold.

You mean the choke flap is not firmly held wide open when the motor is up to temp?

If so and it’s just flapping in the breeze so to speak then that’s the source of your stalling out when coming to a stop.

Not to mention if your choke is on this far, your gas mileage and performance are going to suck.
Also, because of the secondary lock out, even when you floor it, you’re only running off the primaries. IOW, your 4 barrel ain’t 4 barreling.

No the choke is open just a little bit pretty much the same as when it just starts. I’m going to run it some more tonight and see what’s up

Sounds like your fast idle cam follower is not pivoting freely on its shaft.
Some carb spray and then a bit of auto trans fluid might be all it needs.

Also so that where all on the same page so to speak, in a different post you mentioned that you had adjusted your idle vacuum to 17 lbs, but vacuum is measured I. Inches of Hg not lbs.

Steve I haven’t adjusted it yet with the vacumm gauge it’s just bouncy all over as far the Vacum gauge plugged into the emissions port

If your gauge is hooked up to read full manifold vacuum it should not be moving around , it should be steady if all is well.

With what you have had to repair on the motor and the bouncing vacuum reading your getting at this point I really think you need to do a compression test with only one plug in at a time.

In doing this I would bet that you find two cylinders right next to each other with low compression because the head gasket is leaking between them .

It would be 30 minutes well spent to check this possibility out.

Steve I had it hooked up to the pvc line at the carb base. But you are correct with the compression test. I’ll probably ought to do that just to make sure it’s not valvetrain or head gasket issue. I’ll test the cranking pressures tonight and report back what I find

Steve
I just completed compression check I only did it 1 plug at a time and all pegged 120 I’m sure with all plugs out everything would have been above 140. Considering the 2801 cam bleeding some compression as well. I guess we can at least rule out head gasket or valve hanging open. As normally

My 2801 in a low compression 400 and a Q jet idle pretty good.

Hey Skip do you recall what your cranking compression might have been on your 400 with the 2801 cam.

The OP stating that he’s got only 120 psi with number 62 heads on the motor sounds low , and even considering that the 1977 pistons have a chamfer on there OD.

His erratic vacuum reading makes me wonder if his Cam is retarded.

Mine degreed in right on the money dot to dot. Not sure what cranking psi is. I can try and check maybe this weekend -in the 81 TA and at the shop.

So your saying the choke is not opening after it heats up? If so wire the sucker open and try it.

Steve
I did the compression check with all the plugs in and took one out each time. The dial pegged 120 in every cylinder. I’m assuming would have gone higher but with all the plugs in I think they stole some of the compression. The 2801 is basicly the ram air with more lift and duration so there is a little loss from that as well. The car ran 13”s before the head gasket went. On the compression test I did at that point it had two cylinders at 0 cranking pressure

My 69 400 with 62 heads, 2802 cam pumps 160 to 170psi, every time I've checked it. Same heads with smaller cam should pump at least on 150. Something is up. Was it spinning over very slow? That's why you do compression test with all the plugs removed.

Also keep in mind that a stock '77 Quadrajet is set up pretty lean, especially the idle circuit. You may need to enlarge the idle feed tube restrictions, the idle mixture screws and other orifices in the idle circuit to provide enough fuel.

Late Q jets are really lean in idle tubes and down tubes. Even with a little cam my stock 180 HP 400 and the 78 Qjet would nozzle drizzle at an idle. Fatter 73 Q jet was fine.

I think the OP has to pull all the plugs and take another cranking compression test just to verify that it’s a sound engine then continue tuning from there.

I had one where someone (me) hooked up the needle and seat incorrectly. It just hangs from a hook. I cant recall the exact scenario but you arent supposed to put it through a hole I think and I did?

Long story short, it was totally fine after the rebuild and then after a few weeks it started doing exactly what you said. It would flood and die and a stop after it was warm, but would be okay as long as you were cruising. I too thought I had a choke issue, but eventually narrowed it down to the needle and seat. It just worked its way into not seating after a period of time.

Took it back apart and moved the little hook/clip deal for the needle and its been fine ever since.

Lean is a big part of things here with any Q-jet from 1975 and up since factory duel exh set up’s where history and then cats where stuck in to further make things worse!

Motors making a as installed factory 180 to 220 HP did not need much fuel.

To get a new 400 Q-jet combo with headers set up I would start with a .073” jet and a 43 rod on the primary side and that should keep the motor fat enough to not overheat and if the total mechanical timing is no more then 34 with 93 in the tank the motor should not ping when running just on the primary’s .

I had a gauge from a brand new mighty vac pump that I was using to read manifold vacuum and it had me thinking my engine was messed. Right when i was learning about manifold vacuum needle readings. Loved it. Anyway I'm sure OP is using same gauge as before.

My 9.25:1 400 with a 2801 tested 164-170 psi same 400 years earlier with a 280H cam tested 150-156 psi. Keep in mind the 2801 has 53 degrees overlap while the 280H has 60 and a lot more intake duration.