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I recently removed the M74B from my '47 FL and installed an M35. The M74B ran extremely well affording the ability to run the engine at a ridiculously low, but consistant RPM when idling with the spark retarded. After installing the M35, I now experience what I believe to be a lean low speed circuit. The behavior is as follows:
1. With the carb adjusted at 2 turns out (high speed) and 5 turns out (low speed) the motor starts very easily.
2. After initial warm-up and at an elevated rpm of about 1,300 or so, and with no choak, the low speed can be screwed in about 1.5 turns to peak rpm.
3. When I return the throttle to the fully closed position, the motor pops back through the carb intermittantly until stall.
I'm using the larger venturi from an M74B. I'm also using the larger main jet, but this shouldn't affect the idle circuit. I have one of Cotton's Durable floats which is correctly adjusted.
The carb was fully rebuilt by me with very careful attention paid to cleaning and to the idle circuit, especially in cleaning and protecting the small idle slot in the bore.
The nipples in the head were untouched during the carb swap, and I'm using Cotton's peek manifold seals which have not been over-tightened. Spark timing is spot-on.
Aside from placing the incorrect carb back on - does anyone else have any suggestions before I remove this unit and strt disassembling it (Maybe Tom Cotton)?
1. With the carb adjusted at 2 turns out (high speed) and 5 turns out (low speed) the motor starts very easily.
2. After initial warm-up and at an elevated rpm of about 1,300 or so, and with no choak, the low speed can be screwed in about 1.5 turns to peak rpm.
3. When I return the throttle to the fully closed position, the motor pops back through the carb intermittantly until stall.
I'm using the larger venturi from an M74B. I'm also using the larger main jet, but this shouldn't affect the idle circuit. I have one of Cotton's Durable floats which is correctly adjusted.
The carb was fully rebuilt by me with very careful attention paid to cleaning and to the idle circuit, especially in cleaning and protecting the small idle slot in the bore.
The nipples in the head were untouched during the carb swap, and I'm using Cotton's peek manifold seals which have not been over-tightened. Spark timing is spot-on.
Aside from placing the incorrect carb back on - does anyone else have any suggestions before I remove this unit and strt disassembling it (Maybe Tom Cotton)?