Round Mikuni Fuel Pump

Customer Asks: 

• My round mikuni fuel pump has a tiny weep hole right in the center of it, I guess that's what it's for... And it is leaking a very small amount of fuel like a few drops.... Is that good? Or show a leaking diaphragm? What is the difference between the different models of mikuni offered now??

• Another question, if an electric fuel pump is pushing fuel toward the Mikuni from the tank, And a diaphragm in the pulse pump fails, can it push fuel up the pulse line into the  engine?? I've heard its best amount the mikuni pulse pump, above the pulse line, with a loop in the pulse line, mounted horizontally and rubber mounted to limit vibration and heat. Do these matter?

• What is the best model of the current mikuni options for dual carb for a 503 engine. I see several models.. Thank you

Answer:

All the answers to your questions can be found in the Rotax 503/582 installation manual. (See attached)

• The weep hole at the back of the Mikuni fuel pump is designed to allow oil and fuel condensate to drain from the back side of the diaphragm. The impulse line connects to a port on the crankcase, and it is the piston pulses that drive the diaphragm to operate the pump. It is normal for some fuel/oil mixture to travel up the impulse line, as this is the same mixture the engine uses for combustion. This is not an indication of a leaking diaphragm, low fuel pressure is.

• Mikuni has updated the design of their fuel pumps. The internal diaphragm now includes more plastic material compared to rubber, and the newer design tends to last longer than the older version.

• In the event of a Mikuni fuel pump failure, an electric fuel pump can push fuel through it. The Mikuni is a free-flowing pump, meaning fuel will continue to reach the carburetor and the pump will not obstruct flow even if an internal failure occurs.

• Everything you've stated regarding the mounting location and positioning of the fuel pump is correct. The impulse line must be made of a stiff, fuel/oil-resistant rubber and must not exceed 20 inches in length. This DOES matter and must be followed for proper operation.

• The correct fuel pump part number for your Rotax 503 engine is P/N 994 486.

For more information on the 503 fuel system design, please watch the following video: https://www.rotax-owner.com/en/videos-topmenu/2-stroke-videos/681-2sdkfsd-01