Of 63's, AFB's and Butterflies
Just as sort of a Final "Seasonal" Report on my efforts to overcome the crap gas percolation problems I had this past summer with my 63 L-76:
- I experimented using two AFB's; one 3720SA and another 3721SB, the latter being my newer carb and to which I applied my changes. I re-jetted both using main jets and metering rods to the specs of the original 3461S (which was stolen). I also used Tomco Inlet Valves on the 3721SB, but standard N/S on the 3720SA. I changed to a fuel hose from the carb to the fuel filter to isolate the carb from heat better and facilitate frequent carb changes. I also went to an electric choke to eliminate the choke tube heat and air bleed source, and make it easier to change carbs.
None of the above changes helped at all.
- I then applied a phenolic spacer under the carb sandwiched by two 63 340hp base gaskets and realized a significant carb temperature reduction (some 30 to 35 degrees).
That did the trick. However, as we too have ambient conditions here in Florida of typically 20 degrees cooler daily and nightly temps than during summer weather, I won't know for a few months yet if this will be enough.
- With this arrangement I did incur some other problems;
First, I had significant low speed lean surge. To compensate, I exchanged the primary venturi clusters from the 3720SA into the 3721SB which gave me the .037" idle/low speed jets (instead of the .035"). These clusters also have slightly larger air bleeds. That did away with the lean surge.
Next, I now had a bad low speed stumble which is not tolerable with a close ratio and 3.36 rear end. I had to feather the clutch just to get started from a stop. Today I cured that by; UNWIRING MY HEAT RISER BUTTERFLY!!! I have my wire done in such a manner that I can slip it off for, I guess, the winter. The engine runs real sweet now, practically idles off from a stop and has instant response at all speeds and in all gears, and she'll still pull red line. I'm tickled in debt to all who have given me input.
Stu Fox
- I experimented using two AFB's; one 3720SA and another 3721SB, the latter being my newer carb and to which I applied my changes. I re-jetted both using main jets and metering rods to the specs of the original 3461S (which was stolen). I also used Tomco Inlet Valves on the 3721SB, but standard N/S on the 3720SA. I changed to a fuel hose from the carb to the fuel filter to isolate the carb from heat better and facilitate frequent carb changes. I also went to an electric choke to eliminate the choke tube heat and air bleed source, and make it easier to change carbs.
None of the above changes helped at all.
- I then applied a phenolic spacer under the carb sandwiched by two 63 340hp base gaskets and realized a significant carb temperature reduction (some 30 to 35 degrees).
That did the trick. However, as we too have ambient conditions here in Florida of typically 20 degrees cooler daily and nightly temps than during summer weather, I won't know for a few months yet if this will be enough.
- With this arrangement I did incur some other problems;
First, I had significant low speed lean surge. To compensate, I exchanged the primary venturi clusters from the 3720SA into the 3721SB which gave me the .037" idle/low speed jets (instead of the .035"). These clusters also have slightly larger air bleeds. That did away with the lean surge.
Next, I now had a bad low speed stumble which is not tolerable with a close ratio and 3.36 rear end. I had to feather the clutch just to get started from a stop. Today I cured that by; UNWIRING MY HEAT RISER BUTTERFLY!!! I have my wire done in such a manner that I can slip it off for, I guess, the winter. The engine runs real sweet now, practically idles off from a stop and has instant response at all speeds and in all gears, and she'll still pull red line. I'm tickled in debt to all who have given me input.
Stu Fox