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Old November 12th, 2019, 12:09 PM   #1
tpelle
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Ninja 250 Gas Petcock Use

A little background - I'm thinking about getting back into motorcycling after a hiatus of around 50 years! My last experience riding a motorcycle was as a teenager on a friend's 2-stroke "scrambler", which we rode on his grand-dad's farm. My plan is to take the MSF course, then, presuming I pass, go buy a low-powered motorcycle. I've sort of narrowed my selection down to either a 2008 - 2012 Ninja 250, or a 1988 - 2007 Ninja 250. Honestly, I'm leaning towards the pre-gen, as there are several features that I prefer over the newer model - center stand, valve adjustments by setscrew/lock-nut, temperature gauge, and the traditional gas petcock. (Really, the only thing that the newer ninja has going for it, in my opinion, is the styling of the bodywork.)

I've been looking at a BUNCH of YouTube videos, and have yet to see anyone explain how to use the fuel petcock on the pre-gen Ninja to manage the gas supply. Universally, everyone extols the virtue of the gas gauge on the new Ninja, and complain about visually opening the fill cap to verify their remaining fuel.

The gas petcock is intended to provide the means to keep track of fuel usage, so if you don't know this works, it goes like this:

1. Before you start the bike up, presuming it has a full or mostly-full tank, turn the petcock to the "run" position. In "run", the taller of the two fuel pickup tubes in the tank is selected.

2. As you ride, eventually the level of gas in the tank will go down, until finally the top of the "run" fuel pickup tube will begin to uncover (intermittently due to the sloshing of fuel within the tank). At the time this happens the engine will begin to "sputter" as the supply of gas to the carburetor float bowls can't keep up with the demands of the engine.

3. Now, reach down by your left knee and switch the petcock to "reserve". This will de-select the "run" fuel pickup tube, and will instead select the "reserve" pickup tube, which is shorter. Now the engine can draw on the remaining fuel, maybe a quart or more, that is below the "run" pickup but still above the "reserve" pickup. This should be worth maybe 15 or 20 miles of riding - enough to get you to a gas station.

It's good practice, whenever you shut off the engine, to switch the petcock to "off". Yes, I know that the petcock has a vacuum valve that will shut the fuel off when the engine stops, but this might fail and possibly overflow the carburetor float bowls, which could cause gasoline to eventually contaminate the crankcase oil and cause serious damage to the engine. So getting in the habit of switching the petcock to "off" on shut-down is a good habit to acquire.
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Old November 12th, 2019, 12:22 PM   #2
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Sweet post; welcome back to two wheels
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Old November 12th, 2019, 01:34 PM   #3
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On mine ZZR250 (EX250H7) I have no way of completely turn the petcock off as it seems. There are 3 positions On, PRI, Res. The first and third will allow the fuel through when the vacuum is present. The second will let it through without vacuum (priming). I am really missing the manual on, off, reserve thpe of valve.
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Old November 12th, 2019, 07:52 PM   #4
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I use fully automatic new-gen petcock on my pre-gen. No gas-gauge, no reserve!
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Old November 13th, 2019, 01:10 PM   #5
tpelle
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Quote:
Originally Posted by SibSerge View Post
On mine ZZR250 (EX250H7) I have no way of completely turn the petcock off as it seems. There are 3 positions On, PRI, Res. The first and third will allow the fuel through when the vacuum is present. The second will let it through without vacuum (priming). I am really missing the manual on, off, reserve thpe of valve.
Well, that' different. I have never seen such a petcock. I suppose that they are relying on the lack of intake vacuum to take care of the off function. At least you have the On and Reserve settings to allow for fuel management.

I could see where the Prime position would be handy - just don't ever leave it in PRI after the engine starts.
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Old November 13th, 2019, 02:18 PM   #6
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Quote:
Originally Posted by tpelle View Post
Well, that' different. I have never seen such a petcock. I suppose that they are relying on the lack of intake vacuum to take care of the off function. At least you have the On and Reserve settings to allow for fuel management.

I could see where the Prime position would be handy - just don't ever leave it in PRI after the engine starts.
To be honest I have not seen this either before I got this bike. Yeah the PRI help to make sure the carb bowls are filled up initially but after that they recommend to keep it in ON.
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Old November 14th, 2019, 04:33 PM   #7
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On my 2011, the petcock is has a setting for on and pri. If you turn it to pri, the petcock will bypass the vacuum and use the vent hose for air. You would do this to initially fill the bowls after draining them. In my case I have the carbs out to clean them because I let the bike sit too long. When I put the carbs back in I will fill the bowls using this method. After just a few seconds on pri (the fuel comes out pretty quick) switch to on. If the vacuum every failed, the carb has an overflow vent hose (the same air hose used when on pri) to vent the fuel out to the ground. I don’t see how it could end up in the crank case using this method. Also if you want to drain the tank (as in your are taking the tank off and you don’t want to manage the weight) remove the supply line from the carb and route the hose to a catch container. Switch the petcock to pri and out comes the fuel from the tank. If you open the gas cap and really let it breathe the fuel will come out pretty fast.

Hope this helps.
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Old November 15th, 2019, 09:27 AM   #8
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Huh?!???

Quote:
"If you turn it to pri, the petcock will bypass the vacuum and use the vent hose for air."
Petcock is not connected to or related to carb vent-hose in any way. PRI position flows fuel regardless of anything else. Try this test:

1. remove tank from bike
2. disconnect ALL hoses from petcock, vacuum & fuel
3. turn petcock to PRI

What happens? It flows fuel out fuel-port no matter what! No hoses, no vacuum needed.

What Tom and Sergey was pointing out is that in cases where float-valves leak and not stop fuel-flow, having petcock on PRI and flowing fuel constantly can cause fuel to leak out carbs and down into crankcase. That's how my 2008 got hydro-locked and snapped couple of teeth off starter ring-gear. https://www.ninjette.org/forums/showthread.php?t=300194
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Old November 15th, 2019, 02:52 PM   #9
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Quote:
Originally Posted by Gregg_VA View Post
the carb has an overflow vent hose (the same air hose used when on pri) to vent the fuel out to the ground.
Gregg are referring to element 92190 ?

If so I do not think it's an overflow vent hose. It is the atmospheric pressure sensor and it's level is above the jets in the venturi. If the petcock flows gas and the float valves are messed up the fuel will flow through the jets (pilot and needle) into the intake, then down into the cylinders and into the crankcase. I could be wrong but I am not aware of any alternative overflow vent hoses.
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Old November 15th, 2019, 04:22 PM   #10
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In the Kawasaki Service Manual for my model (2011), under the appendix for house routing pages 17-28 and 29 it states “Run the carburetor vent hose between the air cleaner box and battery”. That is my reference for it being a vent hose. If it isn’t venting fuel to ground, what is it venting?
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Old November 15th, 2019, 06:07 PM   #11
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It’s vent hose for pressure and vapour. But liquid fuel can’t possibly flow out of that hose unless bike is upside down. It’s similar to tank vent hose where it’s well above fuel level and its main purpose is to vent away fuel-vapour on hot days to prevent pressure build-up. And that vent-hose doesn’t connect to petcock in any way.
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Old November 15th, 2019, 07:48 PM   #12
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Quote:
Originally Posted by Gregg_VA View Post
In the Kawasaki Service Manual for my model (2011), under the appendix for house routing pages 17-28 and 29 it states “Run the carburetor vent hose between the air cleaner box and battery”. That is my reference for it being a vent hose. If it isn’t venting fuel to ground, what is it venting?
Gregg, like DannoXYZ mentioned it's venting gasoline vapour out. But basically the idea behind it is to equalize the pressure above the gasoline surface in the carburetor bowls with that of the atmosphere to make it constant and not dependent on the pressure of the gasoline vapour. Remember the way the fuel is delivered through the jets is by the pressure differental between the gas surface in the bowls and the vacuum created by the pistons through the intake.

If the was no equalizing hose provided the pressure above the bowls would depend on the temperature of the gas a lot. That would mess up all the tuning of the carb and the richness of the mixture would go up due to temperature increase and down with the temperature decrease. The carb tuning would be all over the place.
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Old November 16th, 2019, 02:01 PM   #13
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Got it. That makes a lot of sense. Thanks for setting me straight.
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Old November 17th, 2019, 07:06 AM   #14
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in most simple terms....the vent hose is venting to atmosphere.....think of your finger over the end of a liquid filled straw, liquid unable to flow with gravity. Same basic concept.

Also doubles as "overflow" hose too, a more extreme duty....we won't even go there.
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Old November 17th, 2019, 02:06 PM   #15
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Mechanical computer, heh, heh...

Hundreds of years later we developed technology to actually build Babbage's machine... and it worked perfectly!
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Old November 18th, 2019, 10:23 AM   #16
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Quote:
Originally Posted by tpelle View Post
Yes, I know that the petcock has a vacuum valve that will shut the fuel off when the engine stops, but this might fail and possibly overflow the carburetor float bowls, which could cause gasoline to eventually contaminate the crankcase oil and cause serious damage to the engine. So getting in the habit of switching the petcock to "off" on shut-down is a good habit to acquire.
The first point you made actually happened to me after a long ride with my friend. We parked our bikes in a parking space that was set at a slight incline and went into a McDonalds to get some grub. We came back out and I tried to start my bike - it kept cranking but didn't start. My buddy said the bowls were flooded and told me to turn the petcock to "Off" before starting it again. Luckily it started and I was able to ride off without issue.

I don't understand your second point about oil contamination though. The only way it could get into the oil is through a combination of bad rings/seals and gas accumulating in the cylinders via a chronically over-rich condition, but the latter would be unlikely as the excess gas would get blown out of the exhaust at the completion of each power stroke. If the former happened, the bike would run poorly and would show outward signs of that. Maybe I'm missing something though.
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Old November 18th, 2019, 08:05 PM   #17
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The second effect takes place over time when bike is not running and I’ve seen it numerous times.

With petcock not off for whatever reason, combined with leaky float valves, petrol level fills up float bowl and continues to rise upwards. It flows out of jets, fuel-circuits and out into venturi of carbs. It dribbles down throat of carbs and down intake tract of engine.

Due to extreme duration of cams, it’s very likely intake valves will be slightly or fully open. Petrol is much, much less viscous than oil, so it has no problems getting past rings. And since there’s no combustion-pressure pushing rings outwards, ring-seal against cylinder wall isn’t that tight in non-running engine. So overnight, or over course of days, petrol continues to flow from carbs past rings and into crankcase. It continues to rise and displaces air until entire crankcase is full. You’ve got hydro-locked engine because pistons can’t move down and compress fluid.

There’s many cases of this happening, just do web search. I had it happen to my 2008 250. I picked up an ‘86 Interceptor for free in ‘88 because owner though he seized engine. Starter just buzzed and complained. Pushing-starting just dragged back wheel and left skidmark, yup seized engine. I started removing engine to swap in another. When I opened oil-drain to make it lighter, over 3-gallons of oil & petrol mix rushed out and overflowed my 3-gal drain pan.

So yes, leaky petcock combined with leaky float valves can hydro-lock engine. If you’re unlucky enough to have cylinders halfway down, they may be able to complete 1/2 compression stroke and actually ignite. Then they stop suddenly halfway down and snap some rods and shove remnants through side of engine!
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Old December 10th, 2019, 02:15 PM   #18
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Quote:
Originally Posted by DannoXYZ View Post
I use fully automatic new-gen petcock on my pre-gen. No gas-gauge, no reserve!
The new gen petcock bolts right up? Is it vacuum operated like the pre gen? Any benefits?
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Old December 10th, 2019, 03:04 PM   #19
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New-gen petcock is vacuum-operated just like pre-gen. Has PRI position for draining tank easily. It's also compact and fits fully under tank. Doesn't have knob to bump into tank like aftermarket petcocks that are too short to clear side of tank.

However, lack of reserve means you don't get warning when running low. I use mileage of 230-250 miles as time to fill up depending upon how hard I've been riding.
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