head-scratching carb problem

[Guest Tishabet]

I have a really puzzling (to me, anyway) condition right now with the carb on my '38 model 41. The carb is a Stromberg AAV-1 which I recently rebuilt with a kit. Ignition timing checks out with a timing light and the car starts and idles very well. Once I get the engine up to upper RPMs it seems to run well there. The problem I'm having is in the <span style="font-weight: bold">transition</span> from idle to upper RPM.

With the engine idling, as soon as I opened the throttle a litle bit, the carb makes a significant "sucking" sound and the engine slows and starts to die. If I actually go from idle to perhaps 1/3 throttle or higher without nursing the engine, the engine will stall entirely. After some experimentation, I discovered that with more choke, the problem is less pronounced and I can get the engine to make the transition from idle to upper RPMs much more smoothly (though still not as smoothly as I would like).

Based on this observation, I surmised that the mixture at the point of acceleration was too lean, since I was making the mixture richer by giving it some choke and that seemed to help the problem. I figured lean mixture on accel = bad accelerator pump, so i decided to check mine out last night.

Here is where things stopped making sense to me.

I disconnected the arm that connects the throttle to the accleration pump. Then, with the engine running at fast idle, I gave the pump a little squirt. This caused the "sucking" noise and also caused the engine to stumble and almost stall, which is not what I was expecting. Even when I got the engine up to higher RPMs, the accelerator pump caused the same symptoms.

This got me thinking (always dangerous). So here is what I did: I had previously had the accelerator pump set to the hole in the throttle which would give the pump the most "throw," which I believe equates to the most gas when you hit the throttle. After my little experiment, I installed the accelerator pump arm to the hole with the least throw, and tired accelerating the engine again. This did not totally cure my problem, but it was a <span style="font-weight: bold">significant</span> improvement.

So what gives? The choke experiment seems to say that my mixture under accel was too lean, but the accelerator pump experiment seems to imply my mixture was too rich. Any ideas???

[jeff]

My '38 has a AAV-167 or something, which is a later replacement for the AAV1. It also has an economizer valve, which works a bit like the accel pump, but uses vacuum instead of the mechanical linkage, if I recall. I think its purpose may be to help with these throttle transitions???

Just throwing it out there as something to check. Someone else may be able to educate both of us further.

[Guest Tishabet]

Interesting...maybe I misunderstand the workings of my pump. My only past carb experience has been on aircooled volkswagens, which used the downward movement of the pump piston to shoot a fine spray of gas into the throttle body. Perhaps the AAV-1 does not work in the same manner?

[Guest ZondaC12]

i too adjusted my accel pump last year and it helped with a little stumbling that happened when i tried to rev it up. mine too is an AAV-167. i adjusted it to the one with more throw actually.

the only flaw i do see in one of your tests is stroking the pump at any rpm being held. it only needs that when its revving up, needs additional fuel to increase speed (as far as i know) so if youre just holding rpm and you suddenly richen the mixture, engine-no-likey. <img src="http://forums.aaca.org/images/graemlins/tongue.gif" alt="" />

symptoms of rich/lean mixture can be confounded. check your spark plugs. if theyre really sooty there you go too much gas! also watch your tail pipe if you can when you rev it, again if it could confuse the layperson into thinking you have a diesel engine, better lay off the fuel goin' in.

also when you're rev-testing this thing, pull off the air filter and actually watch the accel. pump jets (WITH SAFETY GLASSES!!!! i pumped it real quick once with the engine off to see where the jets were, and that stuff splashed RIGHT in my face off the carb throat wall! didnt get my eyes but i almos soiled myself i was so scared) and see what they do, like how much shoots out when you roll on the throttle lightly vs. a quick hard stab etc...

mine makes a sucking noise too with the air filter off. with it on you barely hear it, but i think thats normal, doesnt relate to rich/lean.

after reading your post though, if what you changed made it better, well then logic would tell me to stick with that! "if its stupid and it works, it aint stupid". friend told me thats an old slogan of the military. i like it!!!!

[Aaron65]

Have you checked for vacuum leaks???Any junk clooging a jet or passage? Missing check ball or needle in the accelerator pump? Just a few ideas...

[Guest Tishabet]

Aaron65,

I have not checked for vacuum leaks, that seems like a good place to start. It almost seems like actuating the accel pump is opening a new channel for air and is leaning out the mixture.

there were no parts left when i reassembled the carb with the kit, but I think my next step might be to closely examine a cutaway schematic of the carb and see if I might be missing something obvious.

thanks for the suggestions!

[Guest Tishabet]

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">symptoms of rich/lean mixture can be confounded. check your spark plugs. if theyre really sooty there you go too much gas! also watch your tail pipe if you can when you rev it, again if it could confuse the layperson into thinking you have a diesel engine, better lay off the fuel goin' in.</div></div>

The plugs have a healthy color and the engine seems to run great at both high and low revolution... it's the transition from one to the other that's cuasing my problems. Actually, once I nurse the engine through the transition fromm idle to throttle, reving it up further is flawless. So I'm guessing (dangerous!) that my actual mixture as set by the needles is OK.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">

also when you're rev-testing this thing, pull off the air filter and actually watch the accel. pump jets (WITH SAFETY GLASSES!!!! i pumped it real quick once with the engine off to see where the jets were, and that stuff splashed RIGHT in my face off the carb throat wall! didnt get my eyes but i almos soiled myself i was so scared) and see what they do, like how much shoots out when you roll on the throttle lightly vs. a quick hard stab etc...

</div></div>

that's a good idea, I will see if I can observe the actual gas delivery. Don't even get me started on the times I've come close to doing damage to myself with gas... a particular incident with a pressurized gas filter on a Porsche 944 comes to mind. Talk about almost soiling myself!

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body"> "if its stupid and it works, it aint stupid". friend told me thats an old slogan of the military. i like it!!!! </div></div>

Hear hear! I'm a fan of the KISS principle <img src="http://forums.aaca.org/images/graemlins/grin.gif" alt="" />

[carbking]

Does this vehicle have a vacuum advance unit on the distributor, and if so, have you checked it for leak and function?

Jon.

[Guest Tishabet]

It does have a vacuum advance, and I have not tested it to see if it advances properly (although it is hooked up). I'm going to work on the car for a few hours tonight, and I'll make a point to check if the unit is advancing correctly.

Are you thinking that a vacuum leak from the unit would cause my problem at the carb? Or are you thinking that a malfunctioning advance would screw with my ignition enough to get the engine to stumble?

If your hypothesis is that I could be introducing a vacuum leak via the vacuum advance, could I test this by removing the line at the carb and plugging it with my finger while I accelerate from idle?

Thanks!

[Guest Tishabet]

OK, I worked on the car last night and have some results to report:

Vacuum advance is not functioning properly, speark is advancing slightly at high RPM but not consistently so i guess I need to rebuild the advance unit. If I remove the vacuum line from the carb and plug it with my finger, there is no appreciable effect on the acceleration problem.

With the engine running at idle, I observed the effects of the acclerator pump when actuated by hand. Each time I pumped it, I could see a few drops of gas hit the throttle butterfly valves and rapidly evaporate. There was less gas than I assumed there would be. I tried to observe the effects of the pump with the engine off, but having just run the engine I discovered that pumping the accelerator pump resulted in a lot of gas vapor in the carb, making it hard to see what's going on.

I'll keep working on the problem this weekend and let you guys know what I figure out.

Thanks again for all of the suggestions!

[Steve_bigD]

If you disconnect the pump and pump it by hand with the throttle still at idle the mixture will become very rich and the engine will stumble and perhaps stall. In normal operation when the throttle is suddenly opened the air flow increases but the drop in vacuum delays the control signal at the carb so the mixture becomes momentarily very lean. The accelerator pump provides a direct discharge of liquid fuel into the manifold where it will atomize and prevent the mixture from leaning out until the carb can catch up.

The fact that closing the choke reduces the problem indicates that the mixture is still too lean on transition even with the accelerator pump at max discharge. You can prove this by trying to open the throttle with the pump disconnected. It should stall immediately. The pump should give a nice healthy squirt of fuel into both throttle bores. A few drops is not sufficient. The cover of the AAV1 can be removed in a few minutes and the pump inspected. There is a rod with a spring and a leather seal that rides in s vertical bore with a one way check valve at the bottom. Check that the leather seal is in good shape and the spring is good. Even though it was rebuilt it could have been damaged by careless installation.

Assuming that the timing, plugs and ignition are OK and there are no large vacuum leaks, and if the pump looks good and gives a healthy squirt, the problem is likely in the transition circuit from idle to main. There is an idle port and a transition port that progressively supplies the mixture until the throttle is opened enough to activate the main jets. Your problem can occur if these are clogged or have burrs. Adjusting the idle mixture richer may help a bit but if the problem is severe enough to stall the engine this will probably not be the solution. A low float level could also contribute to this condition.

I rebuilt my carb by boiling it in an alkaline solution followed by a thorough cleaning in solvent and then blowing out with compressed air. Never run wires through passages to clean them. I then used a rebuilt kit and carefully adjusted the float level and drop. I have no hesitation at all even when cold. A thorough re-inspection and adjustment of the insides if the carb may reveal the problem.

Good luck,

Steve D