Accelerator nozzle leakage on shut down

Tech question posed via messenger.

Pictures:

1. The stated issue

2. Old style accelerator pump check ball

3. New style umbrella style check

4. New style umbrella check with bleed

5. Standard metering block

6. 4180 style block with pump channel bleed off

Explanations:

1. Heat often boils fuel in an accelerator pump on hot shutdown.

2. With the old style checkball, this is rarely an issue as the ball falls off the seat and any pressure can be vented back into the bowl. The negative of this system is you typically lose a small amount of pump shot before ball is seated.

3. With newer style umbrella check, 100% of fuel is available to pump shot. The negative is if fuel boils in pump, there is nowhere for it to go except out the pump nozzle.

4. A common builder trick to avoid this is to drill a tiny (like .011) hole in the pump area. This allows boiling fuel to escape to the bowl. Obviously this also causes a small loss of pump volume.

5. With a standard metering block there are no breaks between pump and nozzle.

6. The OEM saw this issue on 1980s era 4180 style carbs and inserted a tiny bleed at the high point in the pump channel. This allowed any excess pressure a place to bleed off.

The person asking wanted to know why another carb didn’t have this issue.

Hard to say exactly, his 4781 likely has a 50cc pump, more volume more problems.

There are a million variables for why it may happen now and didn’t happen before.

Either way, you have the info on why it happens, is for you to figure out how you wish to address the issue.

Good luck