Won't start

[LoOoD]

Whats up.. new to motorcycles(besides the MSF classes, I've only riden like 10 miles), even though I owned on for two years. Anyways I got the bike fixed and was able to ride from the shop(carb was rebuilt) and back to my house (15miles away). Then I left the bike outside for 3 weeks. I started a few times w/o problems but didn't drive it anywhere.

But now, when I try starting it up I can hear it trying to start but it dies when after a few seconds ( 10 - 15 seconds) of it running if I'm lucky enough to even start it. Then I hear something blowing off when it dies.

petcock was on reserve.
choke all the way down.
I can see gas in the tank, looks like enough.
battery is new and fully charge (had it connected to a batter tender for the past 48 hrs)

any ideas? thanks!

[Andy Pandy]

Fill up with gas (i know it looks enough already) so you don't need to be on reserve.

Then you will eliminate one possble cause at least.

Andy

[GRU]

every time you want to start your bike put it on PRI for like 10 sec and then try to start it


fill the tank all the way up because it may look like enough but the gas may be lower than it looks...

if your bike runs good on prime and then turns off when you put it on ON or RES then you could have a vacuum leak or not enough gas

[Frost]

what is PRI (prime) for anyways?

[GRU]

when on PRI the fuel flows to the carbs by gravity (not by vacuum like ON and RES)

[Kerry]

what is PRI (prime) for anyways?

Think of PRiming an old-style well pump.  Sometimes you had to get a leather washer or something wet so it would expand and keep a good enough seal to allow the pump to pull water up from the well.  And as water flowed through the pump, it kept the washer thingie wet and maintained the seal.  Other types of pumps have different PRIming requirements or procedures.

I think the choice of the word "PRIme" for the petcock position represents one situation above all others: the need to fill the carburetors (float bowls, actually) with fuel when they are empty.  When might this happen?

1) You ran out of fuel on the REServe position.  The vacuum from the carbs isn't sufficient to pull more fuel in, even though the fuel level may be above the outlet inside the fuel tank.

2) You had your carbs worked on.  When the carbs are reinstalled in the frame, they are empty of fuel and need to be refilled before the bike will run.  But the bike won't pull fuel through in the ON and REServe positions unless it IS running.  Catch-22!

The answer in both variants is to turn the petcock to PRIme for several seconds, and allow gravity to pull fuel from the tank and fill the float bowls.  Then (if you have enough fuel in the tank) you can turn the petcock to ON and start the bike.  In turn, the running bike will resupply itself with fuel.

Which is what makes the "PRIming the pump" analogy work!