3461S AFB Carb information Request

[Stuart F.]

Could anyone having an original 63 L-76 340 hp equipped with a 3461S AFB Carburetor please check their carb and see if it has air bleed holes next to the Primary Metering rod/vacuum piston hold down covers (right at the edge of the air cleaner gasket surface)?

The reason I ask is that I am having a bad problem on my 63 with fuel percolation. I am using replacement carbs, either a 3720SA or 3721SB with the same problem on each. Early AFB's had a circuit called "Anti-Percolator" which consisted of 2 high speed air bleeds that "also act as anti-percolator vents when a hot engine is stopped or at idling speed. This will help vent fuel vapor pressure in the high speed and idle well before it is sufficient to push fuel out of the nozzles and into the intake manifold" (a quote from an early Carter AFB Instruction Manual).

Both my carbs were specified for 64 L-75 300 hp applications and, it is possible, that there are more differences between them and the 63 L-76 applications than just jetting (as previously thought).

Stu Fox

[Duke W.]

Re: 3461S AFB Carb information Request

They aren't "air bleeds" - just 1/8" vents to maintain air horn pressure above the metering rod pistons. They are on the vertical wall between the primary and secondary throttle bores, facing the secondary air horn, just below the plane of the air clearer mounting pad.

Check the operation of the hot idle compensator.

Duke

[Gerald L.]

Re: 3461S AFB Carb information Request

Yep, just as Duke states, on my 3461S.

[Stuart F.]

Re: 3461S AFB Carb information Request

Duke and Jerry;

Thanks for checking your 3461S's for me. After your input and some further analysis, I think it is safe to assume the only basic differences on any AFB model used on either 63 thru 65 L-75 or the 63 L-76 is in the jetting (fuel and air), as well as primary throttle levers.

The manual I got from Cecil covered mainly some models used on 58 thru early 60's "W" engines, and these have some significant differences. The air vents (bleeds) I was referring to are actually the bowel vents for those models. They did not have the 4 angled vent tubes above the venturis as on our models. One thing that is consistent is the air bleeds found in each venturi cluster are referred to as "air bleeds/anti-percolating vents".

I am now looking at the top plate gaskets provided in the aftermarket rebuild kits to determine whether they are allowing adequate venting between the upper chambers and the fuel bowls.

Incidently, Duke, I noticed that the 1/8" holes from the secondary to the metering rod piston chambers, that you referred to, are drilled through to the upper chambers on the primary side (as seen with rods and pistons removed). Those air connections would be open during Idle and low speed operation (high vacuum), but would be effectively closed off at shut down or WOT (low/no vacuum) with the pistons FULL UP.

Bear with me guys, I believe I'm just trying to find a way to make an old 63 L-76 live with today's gas with the 10% Ethanol (or ethynol - whatever). I'm not positive that that is the total problem, but it is the only main factor that has changed in my world. Living with fuel diluted crankcase oil and potential hydraulic lock are not palatable options.

Stu Fox

[Joe R.]

Re: 3461S AFB Carb information Request

Stu,

I know this is late to ask this question but you do have the correct aluminum manifold for the application and have blocked the exhaust passages as Clem suggested haven't you?

And have checked off everything on the earlier list?

JR

[Stuart F.]

Re: 3461S AFB Carb information Request

Joe;

Thanks for your response.

I have not blocked the heat risers (manifold exhaust crossover passages) as yet. The manifold has never been off the engine, but I intend to remove it come the cool season here. I want to check the baffle/shield on the bottom and, I'm embarrassed to say; do a little repair to the oil filler tube boss. Last time I changed the oil fill tube (about 1982), I drove the new tube in with hammer and wood block too zealously and cracked the manifold at the front of the hole. You can't see it with the naked eye as I hammer pulled it back slightly and the crack closed up. I know it is there and it should be heliarched and ground. As such, I'm waiting to do any other mods at that time.

I realize you can probably do it (SS sheet stock inserts) by loosening the bolts, but these gaskets are original and I am afraid they are really heat glued tight and I may break a seal at the ports or water passages.

Stu Fox

[Stuart F.]

Re: 3461S AFB Carb information Request

Just as a little followup info:

I inspected a new #1406 Edelbrock Performer (AFB) at the local Advance Auto Parts store today and noticed the following;
- The primary air bleed/anti-percolating vent tubes from each primary venturi cluster protrude through/above the top plate of the carb within the throat opening to the air cleaner.
- the bosses next to the covers over the primary metering rods/pistons (which appear to only be there to locate/center the air cleaner throat opening on our carb Models) are hollow and open clear through into the upper chambers (above the top plate gasket) to serve as the main bowl vents (no angled tube vents are used as found our Models).

In discussion with the sales clerk, I discovered that we are not alone with fuel percolation problems. He has an RX-7 with a "Nitto" 4-barrel carb on an aluminum intake manifold. He said he too was getting dilution of his crankcase oil due to fuel dumping out of his secondaries after shut down - interesting.

Stu Fox

[Stuart F.]

Re: 3461S AFB Carb information Request

I finally got smart, thanks to my NCRS friend Gerry who put me on to the book offered in our own Driveline: "Carter/Edelbrock Carburetors"

I'm sure this book will be a welcome addition to my library besides being a timely reference for my ongoing AFB project.

Funny how we can't see those things right in front of our face sometimes. Thanks Gerry.

Stu Fox

[Gerald L.]

Re: 3461S AFB Carb information Request

You are welcome, Stu. Wish I could be of further help with your carb, something is amiss in her.

[Michael H.]

Re: 3461S AFB Carb information Request

I finally got smart, thanks to my NCRS friend Gerry who put me on to the book offered in our own Driveline: "Carter/Edelbrock Carburetors"

I'm sure this book will be a welcome addition to my library besides being a timely reference for my ongoing AFB project.

Funny how we can't see those things right in front of our face sometimes. Thanks Gerry.

Stu Fox

Here's one more item to kick around. Horizontally mounted fuel filters seldom fill completely with fuel. There's trapped air at the top.
When the engine is shut down, the temp of the fuel and air in the filter rises, building pressure that's over and above what was in the line/filter from the fuel pump. Could be that the increase in pressure is forcing the needle/seat to fail and over fill the carburetor bowl.

Many cars built a few years later with external filters have the filter mounted vertically.

I wonder what would happen if you somehow temporarily insulated the filter and fuel line from radiant engine heat? Might be an interesting experiment.

[Stuart F.]

Re: 3461S AFB Carb information Request

Michael;

I would love to do that, but I'm still trying to stay in touch with originality, at least to the naked eye. That filter mounted right on the water manifold in a horizontal position is, well, just asking for trouble. I want to do some running and shut down temp comparisons with my I.R. gun, but I need to change the oil again first and I ran out of PF-141 filters. The one time that I changed the carbs out after a one day shutdown, I still had pressure in the line at the carb. That only means the n/s's were holding at that moment. My fuel pump pressure test under operating conditions (engine warmed up) yielded just under 6 psi. Back many years ago, I used to run a fuel hose from the pump to the carb with an inline "clear" filter. Of course I had no problem then, but the gas was gooooooodd.

Thanks for your ideas. It helps me plan my attack.

Stu Fox

[Michael H.]

Re: 3461S AFB Carb information Request

Michael;

I would love to do that, but I'm still trying to stay in touch with originality, at least to the naked eye.

Yup, I understand. It was only meant to be a temporary test to isolate the problem.

I wonder how much the temp of the fuel filter/lines rises after hot shut down?

I also wonder just how much pressure increase there is at shut down?

[Stuart F.]

Re: 3461S AFB Carb information Request

Michael;

I will update my notes and plan out my test procedures for when I get the oil changed. I too really want to know what's happening there (pressure and temp wise).

I have to fill up again soon. Should I stick with the same gas to try and duplicate the scenario? My problem is I do too many changes at once sometimes (even 2 is too many), but what the heck! Nothing has helped yet!

I'm going to put back on my better (newer) carb which only has slightly lowered floats in it so I don't expect any great change from it. But I want to get my older piece off so, if I do any major mods, I want it to be with that one as it is my major experimenting unit.

Thanks again for your input. I'll keep you posted about what I learn.

Stu fox

[Gerald L.]

Re: 3461S AFB Carb information Request

The more I think about Michael Hanson's horizontal filter can idea as the root cause the more I think it very plausible. It does not involve the closure of multiple bowl vents which would be required in order to generate a heat soak pressure in the bowl to push liquid gas out into the throat. The liquid gasoline under some pressure is coming from the filter can heat soaking (by means of fuel vaporization there in a closed system, ie, the filter can), thru the inlet n/s and fulling the bowl with liquid gas. This could total several ounces of fuel. I assume that far too much gas in a static, already full carb bowl will find any usual or unusual passage into the throat. And, Stu, the n/s could still hold some of the pressure back in the line that you saw on one knock down.

How much pressure could be generated? Look to the Reid Vapor Pressure test of gasoline quality for a reality check. RVP is run in a bomb at only 100* and it generates values of about 6-9 psi (summer) and stepping up to 15 psi in the winter in the north. As heat soak temps increase above 100* the pressure in a closed filter can (even without a vapor space) will be much higher. I don't know how much higher but I suspect a lot since an atmospheric distillation of gasoline will vaporize 50% of the gasoline volume at around 170 to 180*. Filter can heat soak at 180* could generate enough pressure to push liquid thru the carb inlet n/s until vapor escapes up the filter outlet and relieves the filter can pressure.

If this is true what don't we all have this problem. Maybe we do in varying degrees, we just live with it.

Stu, try this. Blow compressed air on your filter for 5 minutes after shut down. See if bowl stays cool and no gas spits thru throat.

[Stuart F.]

Re: 3461S AFB Carb information Request

Gerry;

It sure seems plausible, but why has the situation become so acute here just lately?

1) Is it due to the change in the gas formula?

2) Is it possible that the filter is becoming plugged (diminished capacity)?

3) Could it possibly have something to do with my putting inlet filter screens in either carb? (neither carb had them before, and my 3721SB had never been apart.)
Note: I ran a thread some time back asking if others had the inlet screens - most said they did, but a number said no.

How can I effectively insolate that filter from the water manifold without replacing it with an aftermarket design?

Interesting. Thanks.

Stu Fox