How it Works.... Fuel pumps and crankseals

[JDT]

First off why do we need a fuel pump?

The tank is usually mounted as low as possible on a snowmobile's chassis to help keep the center of gravity low on the machine.
Being as it is lower than the carburator fuel must be pumped up to the carb. But that is not how it always was.

The very first snow machines had the tanks mounted on the engine, above the carb, and then fuel just gravity flowed down to the carb.
Tanks were small and need to be refilled often as the performance of engines increased.
Pouring gas into a small tank above a hot engine soon became an serious issue.

So tanks were lowered and moved away from the hot spots and the capacity became larger. And pumps became common.
Snowmobiles aren't always driven on flat frozen lakes. Sometimes we go up and down hills too.
If the tank was in the very front or very back of the chassis flow would be effected by angles.

Some carbs have the pump built right into them like our beloved Walbro's.
Others require an external pump like Mikuni's.

The basic fuel pump has an input that draws fuel from the tank, an output that supplies fuel to the carb and a pulse line which makes the pump actualy pump.
If you have a twin cylinder engine with twin carbs than you have a pump with two outputs. A tripple? Than three.
The type of pumping action that takes place inside a typical fuel pump can best be described as a recipicating pump.
A diaphram moves back and forth causing a pair of check valves and pump discs to osscilate thus sucking fuel in and pushing fuel out of the pump body.

So what is the magic behind the pulse line?
The pulse line is connected to the crankcase.
As a piston moves down in the cylinder the air inside the crankcase is compressed slightly and this causes the air in the pulse line to also rise in pressure thus pushing on the diaphram in the fuel pump.
As the piston raises in the cylinder this creates a small amount of vacuum in the crankcase and therefore in the pulse line which makes the diaphram in the pump body move in the opposite direction.
Now these changes from pressure to vacuum(pulses) are happening at a very high rate, same as the rpm of the engine, so they do not have to be very strong.
Infact I do not think you could even measure the pulse as it happens so fast.

Couple of things to note here. A twin cylinder engine may have two barbs or connection points on the crankcase(one on each side).
But they can not be tied together. And here is why.
When one side is going positive pressure the other side is going negitive pressure(vacuum) and they would cancel each other out.
The two sides are seperated inside the crankcase by what is know as the center seal. If this seal fails so does your pulse supply to the pump and there you sit.
Thankfully this is a rare occurance because it entails complete crankshaft teardown and rebuilding to repair.

But what is all this chatter about changing outer crankseals?

If the outer crankseal fails on the side that supplies the pulse to the pump the result will be a decrease in pulse strength or maybe even no pulse at all.
This will lead to starving the carb(s) of fuel and low performance. Plus that cylinder will be running lean.

If the opposite crank seal fails(the side not used for the pulse line) the fuel pump will continue to function just fine. But now the other cylinder will run lean.

Back to the fuel pump itself. The pulse line should be a clear line. That way you can see is there if anything moving back and forth in the line. If you see anything in this line you have a problem.
The diaphram in the pump body flexs every time it pulses. They do not last forever. Lucky for us they are cheap and should be replace every few years.
If the diaphram gets a pinhole it will leak. It may still pump but a decreased capacity. This can cause perforamce issues under load and draw fuel directly into the crankcase.
If it gets is a big hole or a tear it will cease to pump at all. And there you sit.

And then there is the dirt issue. It only takes a speck of dirt inside the pump to cause a valve to not seat completely and thus lower the output of the pump. Change that filter often and carry a spare.
Running without a filter on the input of the fuel pump is inviting disaster. If the dirt does make it through the pump it will find its way to the carb and cause havok there. And there you sit.

If there is a hole in the pulse line itself the pump will not perform up to snuff and air will be let into the crankcase again causing the dreaded lean condition. Common causes for impluse line failure are rubbing on the jackshaft or steering post. Also watch out for a pinched hose that will restrict the pulse.

Cleaning a fuel pump is a breeze compaired to a carb. Only a few screws and less than 10 internal parts.
Testing a pump is just as simple. All you need is two chucks of fuel line.
Put one piece on the pulse fitting. Now using your mouth blow into the other end of the hose.
Never use a compressed air line to do this or you will blow the diaphram out in an instant.
If you can keep blowing into the hose your diaphram is leaking or your pump body gasket is bad.

Now take that second piece of line and put it on the output fitting on the pump. Hold the end of that second line near your ear and with the first line on the pulse fitting suck and blow as fast as you can on the pulse line. You should hear a distint popping sound coming from the second hose near your ear. This is the sound of the valves opening and closing in the pump.
If you have a pump with two outputs move this second hose to the other fitting and repeat the test.
The two outputs should sound exactly the same. If one sounds different than something is amiss.

If you see fuel weaping out of the pump body you have a gasket issue. This may not effect the pump performance but you may find your sled on fire later.
While your are poking around in this general area under the hood look for problems with the input and output lines.
Pinched lines, loose clamps, cracks in the lines, lines rubbing on moving parts, cloudy sections in the line.
Fuel lines should be clear or translucent material rated for gas/oil mix.
Black rubber automotive vacuum line is not a good choice.
The rubber breaks down inside the line and black goo makes its way into the pump and the carbs.
And you may find yourself sitting again.

Crank seals do not last forever. Again they are not expensive. But if they fail and you lean it out consider yourself lucky if you only find the engine overheating and sized.
But what else can happen is huge. Meltdown!!! A hole in the piston. Aluminum coating the electrode on the spark plug. Pieces of piston dome rattling around in the crankcase.
Bearing failure. Scored cylinder walls, Broken rings. Yep, seals are cheaper.

Beware of NOS gasket kits that have 30 year old seals in them. Rubber parts that old are a wise choice. Same goes for NOS engines. The engine is probably just fine but the seals went bad just sitting on the shelf. And as always whenever you open up an engine remember dirt is your biggest enemy. And don't be afraid to pre-lube seal lips and bearing. It may smoke on first startup but it will not be starved for lube.

[JoeRainville]

Well done Todd. I would also suggest copying this into the FAQ section so it does not get lost.

Thanks,
-Joe

[Danzig]

[harleysportster]

Well done!!!

[80spitfire]

Nice job Todd! I second the motion to put this in the FAQ section.

-Brian

[S_Kyle]

Very well written, as usual.

[WinnipegStPaul9]

Todd, what a great article!!! Thanks.
DA

[JDT]

After reading all the posts this winter about guys trying to get sleds running that have sat for a decade I that this might help.

Note I did not even mention the role of crankseals in fuel transfer, bearing lubrication and engine cooling, ect.

Just trying to make a point that so may of these problems are related to air leaks.