Carter AVS 4737S 1970 GTX 440 4BBL

[Tony 70GTX]

The assumptionTheThe is the carb worked fine before rebuild. Had it rebuilt than did it work for a period of time? Or have this issue as it came back from rebuilder? If the latter, you need to disassemble and inspect. Despite reliable or reputable... folks have gotten carbs back that were not right.

I have seen wrong gasket used on the venturis. Wrong rods/jet combos, etc.... If it did work fine at first, could be a clogged fuel or air bleed. First suspect is the accel pump as discussed. Then clean out the idle mixture port to see if it is clogged. After that might have to disassemble to inspect clean primary venturis

The assumption is the carb worked fine before rebuild. Had it rebuilt than did it work for a period of time? Or have this issue as it came back from rebuilder? If the latter, you need to disassemble and inspect. Despite reliable or reputable... folks have gotten carbs back that were not right.

I have seen wrong gasket used on the venturis. Wrong rods/jet combos, etc.... If it did work fine at first, could be a clogged fuel or air bleed. First suspect is the accel pump as discussed. Then clean out the idle mixture port to see if it is clogged. After that might have to disassemble to inspect clean primary venturis.

I had the carb rebuilt because of the same issue.The issue was fixed but I had an issue where the car after sitting for a week or so wouldn’t start intermittently unless I tapped on top of the float well. Took the carb off,fixed the issue and the old issue reoccurred! Checked the accelerator pump and it has a good strong stream. The issue only happens taking off from a dead stop and shifting from 1st to 2nd. 3rd and 4th is fine with a lot of giddy up

 

[Tony 70GTX]

I presume the car in question is the 70GTX in the OP's avatar. That engine is over 50 yrs old & is stock, according to the OP. The odds are pretty good that a 50+ yr old engine has been rebuilt.
Maybe the OP is the original owner......or, if not, has documentation to prove the engine is untouched.
So my question/suggestion is: how sure are you OP that the engine is totally stock?
Need more info on the flat spot. Is it occurring with just a small amount of throttle? Just off idle? OK at higher throttle openings?
If the flat spot is occurring just off idle with a small amount of throttle opening, it could be worn throttle shafts.

The car is number matching, no motor work. Owned the car for 30 years and knew the previous own. Car has 70K original miles

 

[Tony 70GTX]

Possibly its the accelerator pump adjustment or the accelerator pump plunger assembly (the diameter of the plunger in your rebuild kit may be incorrect) or the accelerator pump inlet check valve (a brass body ball check valve in the bottom of the accelerator pump well) may not be seating, causing the accelerator pump to not provide the shot of gas when the throttle is actuated. Another possibility is the accelerator pump disc discharge check weight not opening or the discharge nozzles may be partially obstructed......just a thought....others may provide additional information......
BOB RENTON

Checked the accelerator pump and it has a good strong stream. The issue only happens taking off from a dead stop and shifting from 1st to 2nd. 3rd and 4th is fine with a lot of giddy up

 

[Tony 70GTX]

Is the engine totally stock? Any deviation from stock with these tightly controlled emission carbs can lead to flat spots.
With the carb full of fuel, activate the throttle while looking into the pri bores for acc pump squirt.

Is the flat spot with the pri or sec or both?

Checked the accelerator pump and it has a good strong stream. The issue only happens taking off from a dead stop and shifting from 1st to 2nd. 3rd and 4th is fine with a lot of giddy up

 

[Dragon Slayer]

I would try running some good carb cleaner in the tank, or possibly a tank full of good ethanol free marine gas. Maybe some varnish build up from sitting. Especially if it made the needle stick in seat.

 

[Geoff 2]

I have seen these carbs that came with a small link that hooks onto the collar of the needle [ needle & seat ] & hooks onto the float arm. It prevents the needle from sticking when the fuel evaporates; the float drops & pulls the needle with it. You could probably maker something out of a staple or fine wire. Just make sure it does not cause any bindin.

 

[Dragon Slayer]

You could call rebuilder and see what they think. Based on carb working fine when returned, then redeveloping this problem after sitting it seems it has to do with the idle transition circuit. The AVS have an idle transition air valve in the lid, that lean the mixture out when throttles first open and manifold vacuum pulls the air valve open. Factory set. Without trying to modify or fully disassemble the carb your only options are carb cleaner, remove idle screws and shoot carb cleaner into transition port to clean the outlet hole and slot if they gummed up. I would put a lint free cloth in the bore to catch dirt and liquid. Make sure you know what idle screw turns out were when you started.

Then reset idle mixture via service manual. Go a 1/8 to 1/4 rich and see if symptoms change. After that you have to take lid off and pull primary venturi to check idle jet for clogging. Again, the combination of varnish/gum build up is going to lean out the quality of the mixture, and if outlet ports clog, reduce fuel volume to primary bores at idle and transition to mains. Double whammy. I think this is the most likely issue, based on running fine at higher rpm/load.

 

[68-500]

For what it’s worth, I was struggling with a condition that sounded very much like what your describing. I’ve been following this thread closely. Last night the car was running good other than the off idle stumble than suddenly started barely running then died. Almost like out of gas. I instantly diagnosed it as a fuel pump failure. I replaced it this morning and my off idle stumbling is gone. Might though a $40 fuel pump in that thing and see what that brings you. Funny thing was mine ran good wide open all the way up to 75mph when I run out of legal speed limit And let out of it. But stumbled off idle.
Edit: After driving around yesterday the stumble is still there, but is not happening as often IDK? I hope you can figure yours out. Im watching.

 

[Geoff 2]

Off idle stumbles are often referenced on forums like this with factory lean calibrated carbs. Everything has to be 'just right' to avoid the stumble.

 

[PlymouthMakesIt]

Leather must be different in different countries......
Maybe your superior knowledge can explain why having the check valve at the base of the pump well is a bad thing...Are you suggesting it should be at the top?
I'll bet the 4 bbl Holleys that were factory fitted on a few BB engines also had 'ridiculously lean idle' ccts because they had to meet the emission targets...just like AVS did.
And..er....the squirter is not a 'pull over squirter'. You have obviously never taken a close look at it. It is a veeeeeeeeery clever design that atomises the fuel BEFORE it leaves the squirter for better burn characteristics & less emissions. You will not find one on a Holley or clone....
So much BS in post #10.

Must be that Australian kangaroo leather, I've had no luck with the leather pumps. The best ones I've found are the rubber ones that Daytona parts company sells for Carter carbs.

 

[Geoff 2]

And I have had rubber pumps roll over & you get no pump shot. I am not going around testing out all the brands of rubber to see if one works........when I can use leather which works EVERY time.

 

[Geoff 2]

I think it was Dragon Slayer who posted some time back that there were 4 or 5 different sizes [ diameter ] in leather pumps, because they go back decades until the first AFB came out in 1957.

Guess what! If you got the wrong diam pump, it won't work &....that is not the fault of the leather....

 

[RJRENTON]

Something to consider with the Carter AVS 4737S 1970 GTX 440 4BBL carb. Because it's an emission controlled carb, in order for cleaner emissions, tge carb has a special OFF IDLE AIR BLEED festure. Whrn thd throttle is opened slightly, manifold vacuum opens a spring loaded check valve in the idle mixture passsge and additional air flows in diluting the idle fuel mixture slightly to achieve better off idle mixture and emissions. The adjustment is located in the front of the top air horn assembly and is underneath a pressed in cap. Removing this cap will expose an adjustable restrictor. To reduce the effect of the addional air introduced, screw the adjustable restrictor clock wise until it bottoms out, to eliminate the function. Count the number of turns required to bottom the adjustment to allow it to be returned to the original position if wanted/needed. By eliminating this addional off idle air bleed's function, the off idle mixture will be richer and will correct an off idle stumble condition. This change has nothing to do with the idle fuel metering source in the primary booster venturi assemblies or the position of the off idle transfer slot relative to the throttle plate position. Something to consider......i've done this several times and is effective.
BOB RENTON

 

[Geoff 2]

Bo, I have seen that cap/plug. So removing that exposes the screw adjuster?

 

[RJRENTON]

Bo, I have seen that cap/plug. So removing that exposes the screw adjuster?

Yes....carefully remove the cap...the treaded adjustment is underneath the cap.....
BOB RENTON

 

[Dragon Slayer]

I did mention off idle bleed. It is not meant to be adjusted and that is why factory seal. Easier to plug the inlet port in the top to see if that helps. BUT the implication is the carb did work, then started to act up. So idle mixture and bleed along with cleanliness of the transfer port/slot would be where I look first.

 

[68-500]

Yes....carefully remove the cap...the treaded adjustment is underneath the cap.....
BOB RENTON

Pics? Under this circle? If so a small drill bit and pry it out? Sorry Im not trying to high jack the thread. It seems we have the same stumble.

 

[Dragon Slayer]

Yes. Look on the vertical sides you will see the inlet hole.

 

[RJRENTON]

I did mention off idle bleed. It is not meant to be adjusted and that is why factory seal. Easier to plug the inlet port in the top to see if that helps. BUT the implication is the carb did work, then started to act up. So idle mixture and bleed along with cleanliness of the transfer port/slot would be where I look first.

The adjustment serves a two fold purpose. First, it controls the off idle air bleed function during initial throttle plate opening and second, it performs a deceleration enleanment function (adds air to the idle fuel mix), during coast down (closed throttle) to prevent an overly rich idle mixture and higher exhaust emissions during this condition.
BOB RENTON

 

[Tony 70GTX]

I had the carb rebuilt back in April of this year. The issue I’m experiencing is in 1st and sometimes 2nd when I give it gas the carb dumps down and then picks up where it left off. Fuel pump pressure is strong, gas float in the carb is correct specs, adjusted the timing, adjusted carb air to allow more gas into the carb. I’m at a loss. I’m really don’t want to replace the carb bit I’m at the point of thinking it with an Elderbrock. The no modifiations to the engine the car is all original matching # car.

Any suggestions is welcomed

Issue resolved; changed meter rods and change main jets to provide a little more fuel.