Another AFB Question - Sorry.....

[Timothy B.]

Re: Another AFB Question - Sorry.....

Joel,

Is this car a 340 with aluminum manifold or 300hp and cast iron manifold..

[Joel T.]

Re: Another AFB Question - Sorry.....

Joel,

Is this car a 340 with aluminum manifold or 300hp and cast iron manifold..

Tim;

This is a 340HP car with a stock 129 Winters manifold....

Joel

[Timothy B.]

Re: Another AFB Question - Sorry.....

Joel,

Ome way to see if it's the gasoline verses carburetor mechanical problem is to try some race gas. Is the timing and V/A working properly.

[Stuart F.]

Re: Another AFB Question - Sorry.....

Bill;

The 340 hp L-76 does not have the exhaust passage beneath the carb like a 300 hp L-75, and that is why the Stainless Plate is not required (aluminum vs. cast iron manifold).

I bought my Phenolic spacer from Dr. Rebuild because they have good individual base gasket diagrams in their catalog. Based on the port arrangement of the aluminum L-76 manifold, I concluded from their diagrams that the standard 340 hp gasket would seal the manifold to the spacer best, and a second one matches the carb base as well. That's why I used them in a sandwich for this application (one above and one below the spacer).

Remember, this is a departure from stock. I would not recommend anyone do it that wants their 340 hp L-76 to appear stock for judging. I drive my car and wanted it to run as best as I could get it to using the available premium pump gas (w/Max 2000 additive), and it does. So well, in fact, it surprises me. It never ran this well in the previous 46 years before I made the changes.

Stu Fox

[Joel T.]

Re: Another AFB Question - Sorry.....

Joel,

Ome way to see if it's the gasoline verses carburetor mechanical problem is to try some race gas. Is the timing and V/A working properly.

Tim;

There is a private airport near me and I have purchased avi gas before, for the big block... I could try that. As far as the timing and v/a goes... all is correct there.. I am running a B28 can per Duke's 2" rule.

Joel

[Joel T.]

Re: Another AFB Question - Sorry.....

By the way, the phenolic block and stainless steel part ARE by the book, (Check the AIM) specifically to avoid the type of problem you are seeing. Your exhaust heat crossover is putting hot exhaust gas directly onto the base of the AFB, and the AFB needs a system to keep the hot exhaust off the aluminum bottom as the heat transfer will boil the gas. That was in the 60's with good gas, Much more important today.

Bill;

Wow, I never realized that... I'm on the road today but will check the AIM tonite when I get home...

I checked the LI Corvette site and they show the SS shield and spacer for the 300 HP AFB... Would that be the same as for the 340 car?

Thx,
Joel

[Stuart F.]

Re: Another AFB Question - Sorry.....

Joel;

Is your carb a 3461S? Not that it matters, but just to be on the same page with you. I can only hope to find a correctly dated 3461S someday to replace my original that was stolen. My current carb is a 3721SB replacement that I have tried to bring back to 3461S specs. I began with the 3461S main fuel jetting, then sweetened it a bit beyond that. My venturi clusters are different, along with their air bleeds and low speed jets, and the pump squirter is different. If anyone would care to know, I could quote the piece part numbers as they are all from other stock model AFB's in the series (I did not drill out any jets or air bleeds).

Just some other notes in passing; I chose the electric choke conversion primarily to eliminate the sticking choke pull-off problem I had for 46 years, and it works. Eliminating the clean/hot air tube keeps the piston free from white corrosion. I determined the mere 1.0 to 1.5 hg air pull through that tube to be insignificant. The thermo spring captures the link to both close the choke and open it. In the course of the warm up cycle, the pull-off piston (for the most part) is just along for the ride, but it still provides an assist.

Lastly, don't apologize for this thread. As you can see it is very interesting and important information for all of us older C-1 thru C-3 owners who have to deal with todays gas.

Stu Fox

[William C.]

Re: Another AFB Question - Sorry.....

Sorry, I thought you were working with a 300, my error.

[Jack H.]

Re: Another AFB Question - Sorry.....

For what its worth, one classic Carter carb restorer (Dick Katter) packs a note with his finished WCFB carbs (don't know if this holds for AFB's) to the effect that warranty is void if fuel line pressure exceeds 4-5 lbs...

[Duke W.]

Re: Another AFB Question - Sorry.....

One way to prevent/mitigate percolation is to block just ONE side of the heat riser passage. This will allow some exhaust heat under the carb, but considerably reduced from having the passage completely open to allow flow between the banks.

If you do this the heat riser valve MUST be blocked open, and it doesn't make any difference which side you block with the choke hot air system used on AFB and WCFBs, but block the LH side with Holleys or Q-jets that have the choke therno-coil mounted on the inlet manifold above the RH side of the heat riser passage.

When I blocked both sides of my heat riser on my '63 L-76, the engine had a bad off-idle stumble until it had been running for at least 20 minutes around town, even in Southern California's mild climate.

Duke

[Clem Z.]

Re: Another AFB Question - Sorry.....

One way to prevent/mitigate percolation is to block just ONE side of the heat riser passage. This will allow some exhaust heat under the carb, but considerably reduced from having the passage completely open to allow flow between the banks.

If you do this the heat riser valve MUST be blocked open, and it doesn't make any difference which side you block with the choke hot air system used on AFB and WCFBs, but block the LH side with Holleys or Q-jets that have the choke therno-coil mounted on the inlet manifold above the RH side of the heat riser passage.

When I blocked both sides of my heat riser on my '63 L-76, the engine had a bad off-idle stumble until it had been running for at least 20 minutes around town, even in Southern California's mild climate.

Duke

duke your carb must have been calibrated wrong as i blocked all my heat risers crossovers on every aluminum intake manifold car or corvette i ever owned including a 63 340 HP corvette and even here in Pa winters once the water temp came up the cars ran fine. i think there was a TSB on some AFB carbs around that time where they changed the boosters,jets and rods.

[Stuart F.]

Re: Another AFB Question - Sorry.....

If I read Joel's post correctly, he said he was using an F.I. spacer block in place of the heat riser butterfly, so I don't think excessive heat under the carb is his problem. I'm about to change mine as well, but have taken a new one and hogged it out round, i.e. no flats on the sides of the opening. Probably just a minor amount of restriction there, but might as well do it right.

Stu Fox

[Duke W.]

Re: Another AFB Question - Sorry.....

duke your carb must have been calibrated wrong as i blocked all my heat risers crossovers on every aluminum intake manifold car or corvette i ever owned including a 63 340 HP corvette and even here in Pa winters once the water temp came up the cars ran fine. i think there was a TSB on some AFB carbs around that time where they changed the boosters,jets and rods.

Probably one of the reasons I had this problem is that I am pulling a 3.08 axle through the CR transmission, and I don't like slipping the clutch.

Knowing you, I'm sure you had AT LEAST a 4.11.

Duke

[Clem Z.]

Re: Another AFB Question - Sorry.....

Probably one of the reasons I had this problem is that I am pulling a 3.08 axle through the CR transmission, and I don't like slipping the clutch.

Knowing you, I'm sure you had AT LEAST a 4.11.

Duke

4:11 for sure and i even ran the corvette in our corvette club gulf oil sponcered gas milage runs with the enrichment springs out from the power pistons with no problem.