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I just got another older bike (1988 ninja) with "on" "off" and "res" on the fuel switch. Is there anything detrimental about leaving it in the "on" position when not riding, 'cause sometimes I forget.

I believe that leaving it on can flood the engine

Im not sure, My own bikes both have On/res/Pri on the fuel switches, so theyre never really off to begin with, Probably nothing bad would happen, but if you do remember to turn it off, it cant hurt anything (Although Im willing to put money on you forgetting to turn in back on once or twice and sitting there going "Why wont it start??!!" )

If the petcock diaphram is working properly, it don't really matter. The petcock won't open until the bike starts, unless you have it on 'prime'. Even if it does leak, as long as the needle valves in the carb seal properly, you still won't dump fuel when not running.

But lots of folks don't feel right unless they turn that thing to 'off'. I'm not one of them, but I've never had a bad petcock or leaky needle valves......or if I did, it wasn't that way for very long.

A matter of habbit for me is to turn it off. Just something that I was told to do when I got the bike, neve with expllanation. Who knows what condition the petcock's internals are in, considering i'm hade to knock mud and dirt off of it after numerous... run-ins with the ground me and mine have made.

Your petcock is a whole different ballgame, Geoff. It will flow fuel in the on position, running or not. The needle valve should still stop the fuel from draining into the motor...

On my dirtbikes, I would only turn the fuel to off when I was hauling it in the truck. And then only because the airflow around the carb would create a low pressure zone there and suck the fuel into the floatbowl, where it would drain out the overflow vent, leaving a big puddle in the truck box and wasteing gas I should be useing to ride with. And making a mess outta the bike......

On an older bike there's a good chance the gas could leach down into the lowers and dilute the oil.

Instead of guessing about what it might do, it's better safe than sorry. I'd say get in the habit of shutting it off.

.02

I've had similar non-diaphragm petcocks on most of my bikes and have never bothered to turn the fuel off. As long as the float needles are in good shape, it's not a problem. If the floats do get stuck open, you'll get a puddle of gas out of the overflow tubes. That's your indication that it's time to pull the carbs apart, because if they do that sitting still, you're probably going to have problems when the engine is running. Carbs are very sensitive to the height of the fuel in the float bowl.

My last bike had one... and everytime I turned it off I forgot to turn it back on and would almost stall out so I just left the darn thing on all the time

My bike has got one as well. What I found was the problem was not so much a serious issue as a money issue. Leaving that valve open allows fuel to fill the bowl in your carb. the fuel evaporates and the line fills the bowl again...and so on and so on. So you're bike won't quit on you, but unless you want to kickout a refill everyday...close the line. Turn it to the off position....you'll ocasionally, as I do, ride off and forget that thing is off. no problem...it is funny though.