3 position fuel switch

Im sure this question has been answered numerous times already, but my searches didnt come back with anything specific.

I think a lot of my wierd engine problems have been due to the fact that my fuel switch has been in the PRI position since I got it. I saw an immediate improvement once I put it into the ON position (that and some Seafoam in the tank). Obviously the ON position is the ON position, but can I assume that the PRI stands for prime? If so, when do you use this setting?

 

PRIME bypasses the vacuum diaphram in the petcock to allow you to fill the fuel bowls of the carbs otherwise your bike won't start. The ON position is not "on" unless there is a vacuum signal from #3 carb which pulls on the diaphram and allows fuel to flow. This feature prevents a bad carb float from flooding the engine (oil) or airbox with gasoline.

 

yup, ON is ON the fuel switch is acting on vacuum created by the running enging.

PRI bypasses this vacuum line and let the fuel flow. This is used when the carburator bowls are dry-empty so the engine wont come on...

Once running switch to ON or RES

 

What this could mean is that your floats are farked.

In the "On" position fuel flows to the carb, only when called for... well - when there is vacuum.

In the "Prime" position fuel flows until the floats stop it. If the floats don't stop it, the carb bowls over fill. It then flows into the cylander. Past the rings and into the oil. When you check your oil level it'll be over the sight glass. You start the bike and fire oil/gas from the crankcase, out the crank case vent, into the air box.

Basically it makes a big mess on prime, when a mistake is made.

When the petcock fails into "prime mode" and the floats stick - you get the same effect.

Now - I'm not saying that all of this bad stuff will happen, but keep your eyes open for stuck floats.

 

What makes you say that my floats are bad?

Do I need to put it in PRI everytime I cold start the bike, everytime I start the bike, or just when it has sat for a couple of days?

 

PRI is for when you run the carbs empty...

So pretty much only winter storage - or a really hot day that evaporated teh fuel somehow

 

If it runs like crap on prime, and runs better on "On" then it could be a carb issue - like the floats aren't fully stopping the fuel and it's running an overly high fuel level in the carb bowls.

 

But shouldent it run better for longer periods of time on ON anyway? If Im was running it on PRI for the past week, then the bowles would be getting too much fuel and it would surely run worse then on ON when it is only running on vaccum.....

 

It should be better on "on" yes.

PRI is used for 30 seconds to get it started and that's it.

Think of the "on" position as a second line of defense. If the floats stick wide open and the petcock is in the "on" position, and the petcock is working properly... no fuel will flow into the carbs unless the bike is running.

If the bike is left on prime, and the floats are working - no fuel will flow once the carbs are up to level. There is no safety net though - if the floats stick, the fuel flows, and you have a mess.

I think I've tangented the topic.

 

To clarify:

There should be no difference between ON and PRI with the engine running. The vacuum fuel valve is a safety device so fuel doesn't run through if the float valves aren't working properly.

With properly functioning floats it could be left on PRI indefinitely. Don't do this because if a float sticks you could contaminate your oil or start a fire.

So, either the fuel valve isn't working properly on PRI (not enough fuel flowing?) or your floats are "farked".

 

In regards to my floats being "farked", I going to hope that the Seafoam running through and my finding this switch in the "wrong position" is a coinsidence and my bike is running better due to the seafoam....I hope.