Cushman OMC mechanical to vaccum fuel pump conversion

[CapnMorgn]

Greetings all,
I have a Truckster with an OMC (not sure if 18 or 22 horse) with gasoline in the oil. The mix is blowing out the breather as well. Following other threads, I am assuming it is a diaphragm failure in the original mechanical pump dumping fuel into the crankcase, or a float/carb issue. I should note there is no smoke whatsoever, no sluggishness or stutter whatsoever, but it doesn't seem like it is producing enough power. Turns about 2800-3k rpm at max pedal. I bought the recommended vacuum operated pump from Rupp. I figured that would be more simple to start with and less time consuming than yanking the carb and rebuilding it. So to hook up the vacuum pump I need to tap a line on the intake side that operates the pump correct? Can I tap that line into the rubber Tee intake manifold? Just screw a nipple into it or what? I would remove it first before tapping so no shavings fell into the intake tubes of course. Also, do I just leave the mechanical pump mounted where it is? Do I need to remove it and put a plate with an RTV gasket over the hole? If I leave it, and the diaphragm is leaking, do I need to plug up the hose barbs so it doesn't suck air into or shoot oil out of the motor? Anybody attempted this before? How did you go about it? Thanks in advance, and all advice will be well received.

Regards,

CapnMorgn

[junkandcattle]

There are at least two members who these cushmans pretty well. I know because they helped with mine. In the mean time search this forum for omc fuel pump. There is an older thread that may help.
Btw welcome to BGW

[CharleyL]

I doubt that you will get a vacuum operated fuel pump to work correctly with these engines. Most of the time your throttle is at or near wide open when in use and the vacuum level in the intake manifold is very low at that time. I bought an automotive electric fuel pump and replaced the mechanical fuel pump on my OMC 22 hp engine with it. I made a flat plate and a gasket to block the original mechanical fuel pump hole and then mounted the electric fuel pump on a flat plate attached to the frame down near the outlet of the fuel tank. It can go just about anywhere, but should be at or near the level of the outlet on the fuel tank for best results. One wire from the fuel pump gets grounded to the frame and the other wire is connected to the wire coming from the key switch to the ignition circuit that is powered on whenever the key switch is turned on. Rubber fuel line connects from the fuel tank to the inlet of the fuel pump. Another rubber fuel line connects from the outlet of the fuel pump to the inlet of the carburetor. The fuel pump does not require a bypass fuel filter, since the fuel pressure is regulated internally by the electric fuel pump, so the return connection to the fuel tank is plugged by a short piece of rubber fuel line with a bolt pushed into it. I used the stainless hose clamps at every fuel line connection (6 total). Total cost for the parts was about $80 from the local auto parts store and installation was about an hour. This was a cheaper, easier, and a more reliable way of dealing with a fuel pump problem than trying to repair or replace the original mechanical fuel pump.

If your fuel tank has any rust inside it you should use a fuel filter between the fuel tank outlet and the inlet to the fuel pump. My tank was very rusty and I purchased a fuel tank reconditioning kit from KBS Coatings, ordered through Amazon, and used it to remove the internal rust and then coat the inside of my fuel tank with an epoxy like coating. Although this kit took about 3 hours of labor spread out over about a week to do, the results were nothing short of spectacular. I don't expect to ever have rust problems with this fuel tank again.

Charley