AFB Concerns

[Stuart F.]

Re: AFB Concerns

Tim;

Thanks for reminding me. I have your former posts an E-Mailed items on my list of things to check and/or address to make sure I've covered all the bases this time. I'm a born tinkerer, but this bugger is sressing my limits. I'm already to send for some more base and top plate gaskets.

Thanks again.

Stu Fox

[Timothy B.]

Re: AFB Concerns

Stu,

Just a suggestion, disconnect and plug the pcv port in the rear of the carburetor and leave the engine side open to atmosphere so it can breathe. Go for a ride and see if the surge stops. By reducing the air the engine can draw, it has to pull from the idle circuit and main metering circuit so this may stop surge.

You may need to readjust the emulsion screws and curb idle.

[Stuart F.]

Re: AFB Concerns

Tim;

Thanks. I'll incorporate your suggestion into my next test procedure. A bit busy this week, but intend to put together a battle plan and take a big swing at it again next week. I previously swapped out my PCV valves from the 3721SB without any change. Also, I actually have reduced the air draw by virtue of using an electric choke which, in effect, seals the choke housing. I have also used starting fluid spray at all possible junctures around the carb exterior while on hot idle and could not detect any leaks (no idle speed change). I will; however, check out those holes below the air valve as you noted.

Thanks again.

Stu Fox

[Timothy B.]

Re: AFB Concerns

Stu,

One more thing I thought of to consider while we are on this lean surge topic. If your curb idle speed is set too high the emulsion screws will have less effect because the exposed transfer slots will be supplying the bulk of A/F emulsion at idle. This in turn will cause a lean setting of the emulsion screws when you go through the adjustment because they will not make a noticable difference in idle quality.

Make sure to slow the idle and adjust when the engine is good and hot then fine tune the idle speed. Once set the air to A/F relationship should stay about the same as the throttle blades are opened.

[Stuart F.]

Re: AFB Concerns

Tim:

I have always set my idle at, what I would call, 750 rpm. I say that because most specifications call for an 800 rpm idle setting with the L-76 engine. My dash tach has always wiggled or bounced at idle right in the middle between the 500 and 1000 rpm indications, i.e. 750. That's where this engine has always liked to idle regardless of which carb I have on it. During the course of setting and testing with the 3461S carb, I have for a short while tried it at 900 rpm, but it made no difference with regards to the surge problem. It did help off idle starts some what (3.36 rear, 2.20 1st gear, etc). It never came close to equaling my 3721SB as with it I can even make smooth second gear start ups (unintended).

I do recall I had both carbs turned bottom side up side by side and noticed the notches in the butterflys for the transfer and idle slots seem to be deeper on the 3721SB. I did not actually measure them, so maybe it was my imagination looking for answers.

Stu Fox

[Stuart F.]

Re: AFB Concerns

Upon closer examination of my 3461S carb body, side by side with my 3720SA, I noticed that there are two small holes above the throttle plates that go clear through to the outside - both on primary and secondary. In my old Carter literature I found them in a diagram and they are called "Vapor Vents". They are said to "assist in quick hot engine starting by eliminating fuel vapor that accumulates in both the primary and secondary bores above the throttle valves by venting it to atmosphere through these passages."

These vent passages are NOT present on either my 3720SA or 3721SB carbs.

I don't know or see exactly how these could affect the lean surge (herky jerky) problem that I have with this 3461S, but I can at least see how it might contribute to a more pronounced smell of gas in my garage after hot shut down. The 3721SB on the car now is not any where near so bad. They are; however, down stream from the fuel/air mixture metered out by the venturi clusters and right next to the transfer slots and emulsion (idle) screw ports.

Does any one have an opinion or some knowledge of this feature and whether it might be contributing to my problem? Perhaps I should seal them with RTV to see if there is an improvement - yes, I think I will, the more I think about it. Thanks for all your help. Will report back my findings.

Stu Fox

[Clem Z.]

Re: AFB Concerns

021.jpg rochester carbs both 2 and 4 barrel ones had these passages. most rochester carbs used slots from the bore to the outside in the main body to throttle body gasket to do the venting

[Stuart F.]

Re: AFB Concerns

I do recall them with Rochesters as they were the three sections, but I had not seen them with Carters. So, Clem, what do you think? Could they be upsetting my mixture? The back of the carb shows a lot of gas stain outside these vents to the secondary very quickly - I mean after only a few short hours of operation. I also wonder if my mounting of the carb using the 3/8" spacer and two gaskets (in a sandwich) could be altering the metering characteristics enough to cause this. I'm going to push my primary jet size to .106" from .104" which will give me more flexibility to further enrichen the primaries with the metering rods I have on hand to see if that helps.

Thanks for your response.

Stu Fox