Running the carb out of fuel at shutdown

[ndutton]

On the advice of a good friend and engine rebuilder for Mercury Marine from 40 years ago, I have not subscribed to the practice of running fuel out of the carburetor at shutdown. His reasoning was running the engine until it was starved of fuel did not run the carb dry, just low enough to spoil the fuel-air mixture needed for the engine to run leaving residual fuel in the carb bowl allowing it to fully evaporate more quickly leaving the nasty residue behind anyway. Sounded reasonable to me so shutting down was a mere flip of the ignition switch leaving a full carb bowl ready for an easy, no-prime start the next time. I admit it was not a popular opinion but it came from a respected source so that was my policy.

Not any more.

My own direct experience last year changed my policy. I now run the carb out when cleaning up the boat after a trip. Following a lengthy layoff I went to the boat for a father-daughter sailing day. I fired up the engine and got busy taking off covers, dock cord, etc. Then the engine quit. Ok, restarted and it quit again. Started it and feathered the choke to try and keep it running but no joy. That was it for the day, I was not about to spend Father's Day working on the engine so we found another way to spend our day together.

The following week I took off the carburetor very carefully, treating it like a land mine and brought it home for a forensic disassembly. The carburetor bowl was empty and the float valve was stuck in its seat shutting off the fuel supply. I keep a rebuild kit on hand so a rebuild was done, carb reinstalled and tested. All good.

I concluded that the long layoff with a full carburetor bowl pressed the rubber tipped float valve into its seat holding it there with float pressure for a prolonged time causing it to stick in place. No way would that have happened with the float valve open so a new shutdown policy was adopted. As a side benefit I now get an EWDS function test at shutdown too. Closing the fuel valve - I have one immediately ahead of the carb - gives me a buzzer and "Fuel" light (low fuel pressure), a minute or so later when the engine stops I get the "Flow" light (low raw water flow) followed shortly by the "Oil" light (low oil pressure).

[Dave Neptune]

Hi-jack

Neil, I have a question for you as you seem to be quite familiar with USCG regs.

As you know I moved to Ok. I also just bought a "power boat" for the lake. It is a 2017 O/B model, a Key West 239 FS with a 300HP Yamaha on the transom. In the mfg's specs it lists a std 100Gal fuel tank under the floor and is balanced very nicely. The question pertains to a below deck, under the motor well inside an access locker to the filters and thru hulls they (the factory) installed a "squeeze bulb" for priming the engine when the filter is changed. Was it OK in 2017, I doubt it but there it is installed and listed as part of the boat. What do you think? The boat does have a USCG placard.

Dave Neptune

[ndutton]

I would argue that in an enclosed space as your description suggests, it is not compliant. Does your boat have a ventilation blower system? If it does, the builder acknowledges the fuel storage space is enclosed. If it does not have a blower, the builder has deemed the boat to be an 'open' boat with different, much more lax, requirements.

[joe_db]

I have always run the fuel out of any of my outboards when done for the day and likewise my A4.
If I think I'll be using the engine again in the same day, I run it until the fuel pressure gets to 0, but not until it quits.
My current outboard is better behaved, but I had one that was guaranteed to have a stuck needle valve if you let it sit without running the fuel out The ones I have now seem to tolerate it if you forget.

[Dave Neptune]

Ever since I worked in an outboard shop back in the late 60's I have always shut down an OB whenever putting it away for a while, like a week or so. I would run the engine until it started to die then kill the engine with the choke or in the case of some Mercuy's by repeatedly pushing the enrichener button as there was no choke. This would wash the crankcase and cylinders with lots of fuel and oil, the fuel would evaporate and leave a nice coat of oil on everything moving inside.
On many OB's I worked on there was a problem with sticking needle valves. A lot of the problem in these carbs was the fact that the clearance between the OD of the needle and the bore of the guide to the seat. I modified many of these ONLY WHILE doing side jobs on my own. All I did was ream the bore out a bit more so small debris would just go ahead and pass and not bind to cause sticking. I also did this on my own A-4 and a couple of carbs I modified for friends and it worked well. Also if you have a choice go with the solid needle as the tipped ones seem to stick far more often.
When shutting down my A-4 and leaving the slip I would shut off the valve at the carb and run for a minute or so and then shut it down, I rarely ever ran the carb "out" of fuel.

Neil, the space is enclosed and no blower. The space includes bilge, bait, power steering and wash down pumps all at the end of the fuel tank. The access is through a sealed hatch in the motor well. The engine is fly by wire for the controls and a salt water series so everything connecting the motor to the rest of the systems is through a single flexible tube sealed at both ends. This tube enters the same "space". I don't see how they can advertise the "squeeze bulb" install it and still get the USCG sticker. I will do some checking.

Dave Neptune