AFB starves on the secondary side

Hey everyone:

I'm having an issue with the 1966 AFB on my 383. To begin, this is the original carburetor to the engine, and it is one of the early-style small pattern AFB carbs with NO COUNTERWEIGHT. It never had one.

The issue is: the engine starts to starve for fuel when you stay in the secondaries any length of time. It does not bog, stumble, backfire engaging the secondary side. It opens up just fine, then after a couple seconds the engine starts backing off and surging. If I back out of it some, I can still partially stay in the 4bbl without it acting up.

I've been into this carb at least twice. The first time I overhauled it since it had sat about 30 years unused. The second time, I opened it up to make sure the jets weren't swapped front to rear-they weren't. I rechecked the float adjustments and they were spot on. The secondary butterfly adjustment is also spot on. The only modifications I have done to the fuel system is a 1/4" spacer underneath because it kept percolating fuel in hot weather. Otherwise, the carb has all it's stock parts in it and the fuel system is brand new front to rear. The fuel pump is an offshore cheapo.

I'm not sure what I could have missed, but I'm open to suggestions as always. Thanks!

Perhaps an inline fuel filter is plugged or the pump is not putting out enough.

Starting from the tank to the pump with good psi readings to the filter to the needle and seat, if all that is good, the carb should be good fuel wise.

Did you add any high performance parts to the 383?
That early AFB carb is only 600 cfm.

If you have been inside the carb, did you make sure the clusters are clear in the brass tubes & the air bleeds are clear?

When it starts to back off, kill it and coast to a stop, Then check the fuel level. I can imagine the fuel level is low causing a lean condition. Maybe a restriction/filter issue or pump issue. I had a carb do same thing (sort of) and it had a glass fuel bowl (nikki carb) so I shut it off when it bogged and looked, the fuel level was down 2 inches to the jet. Faulty electric pump.

Question: How does the AFB secondary door stay shut with no counterweight..or no secondary door at all?

It's only a 600 "if that". What are we workin on here? A stone stock engine? ALL that info will help you get the best possible answers.

pishta said:

Question: How does the AFB secondary door stay shut with no counterweight..or no secondary door at all?
View attachment 1715424814

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There is no air door on the early AFB carbs.
The butterflies simply close and that’s that.

Do you have any step up springs you can play with...Suggest you go lighter if possible?

pishta said:

Do you have any step up springs you can play with...Suggest you go lighter if possible?

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For what reason?

Is he bogging because he is too rich? Lighter spring would keep him in the primary longer and get him into the power enrichment later, maybe past that flat spot?. May help him over that condition. Stronger would send him right to the power enrichment with the absence of the required airflow. If this has just started, the springs could have fatigued and become 'lighter' and are now preventing the power enrichment at all. Send for a A/F meter!

Ahhhhhh, where did you read that? How do you come to this conclusion? His complaint is about the secondary side at higher rpm.

If all is good in the carb, the only thing I can think of fuel pressure and/or volume not able to keep up.

Look at the theory of operation of an AFB. At idle the vacuum piston are pulled in by manifold vacuum, sinking the rods to their deepest,widest point into the jets- restricting their flow to minimum. When manifold vacuum drops, the springs lift the rods to their smaller diameter, increasing the area of the jet exposing more fuel to the boosters giving more fuel to the manifold, and...........Uh..ooh secondaries........only rely on velocity through boosters to pull additional fuel through a non restricted jet...and metering rods are really not in play.....So this reply is completely off base.... Just listen to Rumblefish!

Im pulling my foot out of my mouth right now...well, maybe the fuel level post was worth something....ouch.

@pishta LMAO! Oh that was hilarious my friend! OK, your back on track. That’s all that counts.

fuel pump volume test, and looking for bubbles; is where I'd start. Don't care if the system is brand new. Maybe somebody left a foreign body in the fuel line; you never know. The volume test will separate the carb from the rest of the system, so you don't chase your tail for the next three weeks. And make sure the fuel tank and float-bowl,are vented.

I know this thread is a few months old. But, it’s often over looked and hasn’t been mentioned in reference to a full throttle/secondary bog/hesitation. If your kickdown is not operating properly at the correct speeds. You will have these symptoms of a bog/hesitation in the secondaries. Because the carburetor needs the higher rpms from 2nd gear when the tranny kicks down near full throttle. In 2nd gear or 2nd gear rpms the fuel pump is pumping more gas that the carburetor needs to sustain or build more power near full throttle. When the tranny is in Drive at a much lower rpm and you hit the gas near full throttle at that lower rpm it leans out the mixture. Because the valve fully opens suddenly and introduces a rush of sudden air. There isn’t enough gas to follow.

The bog or hesitation goes away as you let off the throttle because you are slightly closing the throttle valve allowing the fuel pump to catch up, pumping more gas and reducing air. Drop the tranny down into 2nd gear manually at 35-45 mph. Mash on it and hold it there for a good distance. See if the problem goes away. I bet it does. Your rpms are too low for that throttle response. The only way you can over come that is raising the rpms. That is part of the kickdown function. You can’t correct that problem doing anything to the carburetor. Jetting, etc won’t fix it. The rpms need to increase. If the throttle is increased sudden enough. That’s the transmission’s job if it’s working as intended. This is one of a few reasons why it’s so important that the kick down works properly at the correct speeds. Not only for the transmission‘s health. But, the engine needs it to work properly under certain circumstances. It affects how well the engine actually runs on a street car. It affects the carburetor getting the correct amount of fuel under certain throttle load circumstances. 3/4 to full throttle. Low or high rpms. With a manual transmission your increasing the rpms, amount of fuel pumped manually. A manual transmission doesn’t have this problem. If a manual transmission bogs in the highest gear at 3/4 to full throttle. You down shift manually. This over comes a bog hesitation because you are increasing rpms and fuel as a result. Assuming your pump is efficient enough.

Make sure the fuel pump push rod isn't worn...

I had the same carb on a 273 commando and it acted in the same way. tried everything to fix the issue but had no luck. gave up on it . had it apart several times.