Accidentally left on PRIME

[GER144]

Even if you had it on 'prime' the float needles are suppose to still do their job and not allow excess fuel out of the bowls, filling your crankcase/air box. If you do have fuel in the crankcase or airbox, the float needles/seats/seat o-rings are leaking and need to be repaired/replaced. Check the float heights too.
Todays crap fuel is hard on the o-rings around the float needle seats, and the needles themselves if they are rubber tipped.

[chuck hahn]

In a way yes AND no GER144. They ARE supposed to shut off the flow of fuel if floats heights are properly set. BUT.. They arent machined so precisely to not leak ever so slightly even when properly set up. In cases where the petcock is left on PRIme, the gravitated weight of the fuel pressing down will push fuel past them...regardless. Its just the way it works.

For this very reason, the petcock is designed to shut off the fuel as soon as the vacuum from the engine is gone..thus preventing the pressure of the fuel in the tank from even getting to the float needles in the first place.

[GER144]

I hear what you're saying Chuck and I understand how the vacuum petcock works. Vacuum petcocks are great. With the vacuum shut off, then there should be almost no way that the fuel can overflow the carbs. BUT, there are plenty of bikes out there that do not have vacuum petcocks.
Some stock bikes and ones with aftermarket taps (pingel) with just on/off and they have no trouble keeping the fuel from leaking out when left ON. (as long as everything is in good shape and adjusted correctly).
If the needles, o-rings, float heights are all correct, I still believe the fuel should not overflow if left on prime.
I ran my stock '83 katana for years with a pingel on/off petcock using stock carbs and VM33's and never once had a fuel leak/overflow issue leaving it in the 'on' position. But with todays fuel, that may be a different story!
With a vacuum tap, best to still not leave it on 'prime' just to be safe.

[shinzon1]

Most of our bikes have metal tipped valve needles. a small amount of fuel will leak past them if there is any pressure pushing past them like the weight of the fuel in the tank. Most ATVs and lawn mowers use a rubber tipped needle that is designed to hold the weight of the fuel because they don't have vacuum operated petcocks. Most of them are either on or off if that is even an option for off. The reason for this is that metal tipped needles and seats aren't machined to close enough tolerances to prevent all fuel from leaking past. The rubber tipped can form a complete seal as long as there is no damage to the needle or seat.

[sedelen]

I've accidentally left both my bikes on "Prime" on the sidestand and yes they leaked.
I have since left one of my bikes on "Prime" again but this time no leak, the only difference I can think of was it was on the centerstand.
So, I'm thinking the level of fuel in float bowl being at an angle while bike on sidestand might not allow enough height to completely shut off the fuel resulting in a leak.

[sedelen]

I've accidentally left both my bikes on "Prime" on the sidestand and yes they leaked.
I have since left one of my bikes on "Prime" again but this time no leak, the only difference I can think of was it was on the centerstand.
So, I'm thinking the level of fuel in float bowl being at an angle while bike on sidestand might not allow enough height to completely shut off the fuel resulting in a leak.

I'd like to update this, yesterday I left my 1000 on prime for a couple of hours, it was on the centerstand, and it did leak. So, with the two bikes I have one leaks on the centerstand and the other one doesn't. Oh well, so all that mumbo jumbo about the float bowl being at an angle is probably just mumbo jumbo.

[chuck hahn]

On gthe leaky one..raise the float heights 1/2MM and see if it stops.

[fastbysuzuki]

Sticking float/needle valve or wrong float height surely has to be the problem I would think how else can the fuel rise to the overflow? I ran a gs850 for two years with the vac pipe to the petlock blanked off. So on was off and every other position was on never had a fuel leak.

[Agemax]

Sticking float/needle valve or wrong float height surely has to be the problem I would think how else can the fuel rise to the overflow? I ran a gs850 for two years with the vac pipe to the petlock blanked off. So on was off and every other position was on never had a fuel leak.

reserve relies on vacuum as well so the only position that would work without a vacuum line would be prime. the other 2 would be "off"