Is it common for petcocks to leak fuel into the crankcase?
Gee, that's a loaded question, and the answer is a lot more complicated than you might realize. Better grab a beer and prepare to do some serious thinking before you continue reading this.
OK; you asked us:
"Is it common for petcocks to leak fuel into the crankcase?"
Actually, the problem most likely ISN'T being caused by the petcock itself. It is usually caused by a defective carb float valve, which I'll explain in more detail shortly. For now, just realize that this happens sometimes, and it is one of the problems you MUST resolve right away. Whatever else you do, do NOT start the engine; you can cause a LOT of damage from the diluted gasoline/oil mixture in the crankcase. Put the motorcycle key in a safe place, NOT in the ignition; then, drain the oil, change the filter, and put in fresh oil.
Next, make sure that your fuel lines from the gas tank to the carbs are routed properly, and that none are obstructed or pinched. If you're not POSITIVE that everything is hooked up correctly, ask for help here; I believe Kerry has some graphics available to show you the proper routing.
After that is resolved, you really need to make sure you're not using the PRIme setting on the fuel switch; turn it to the ON position, which will prevent fuel from flowing to the carbs when the bike isn't actually running. Then, you'll need to check your carb float valves; one is probably not fully seated. This could be caused by a defective float valve seat, a defective float valve, a defective float, or a small piece of grit or crud on the float valve seat, blocking the float valve from working properly.
Unfortunately, many people use the PRI setting when they have a problem with fuel flow / fuel starvation, which I disagree with. IMO, it is MUCH better to fix the fuel starvation issue, than to risk the problem you've posted here.
What I'm about to describe is pretty much the worst-case scenario, but many motorcyclists encounter something similar to this at some point in their riding careers, and a lot of them have no idea why it happens, how to prevent it, or how to go about correcting it. Hopefully, you'll be better informed than most after you read this.
In order to understand why this problem is happening, you need to think about how the gas could possibly get all the way from the fuel tank to the engine crankcase. Since the fuel petcock isn't directly connected to the crankcase, you have to follow the path of the gasoline from the fuel tank, through the carbs, into the cylinders, and from there into the crankcase.
Here's an explanation of what is probably happening to your motorcycle: When you use the PRI setting on the fuel switch, gas is ALWAYS being supplied to the carbs, whether the engine is running or not. MAKE SURE YOU UNDERSTAND THAT PART; when you use the PRI setting, gas is ALWAYS able to flow to the carbs, EVEN WHEN THE ENGINE IS SHUT OFF. Don't forget that, OK?
When the gas reaches the carbs, it fills the float bowls. Under normal circumstances, that isn't a problem; you WANT the float bowls to be full of gas, or the engine might not run too well. The carb float valves are supposed to STOP the flow of gas, once the float bowls are full. If a float valve sticks open, instead of shutting off the flow of the gas when the float bowl is full, the gas OVERFILLS the float bowl. When the float bowl overfills, the overflow tube (which hangs down below the float bowl itself) is supposed to drain the fuel onto the ground; for a variety of reasons, that doesn't always happen.
Now, when the gas is filling the crankcase, it is because even more things happen at the same time. For example, if the intake valve in the cylinder being fed gasoline from the defective carburetor isn't fully seated (CLOSED), that allows negative pressure (engine vacuum from inside the cylinder) to draw gas from the float bowl, through the open intake valve, into the cylinder. Remember, the engine vacuum has this effect when the intake valve is not fully seated. You have no control over the position of the intake valves when you shut the engine off; they simply stop moving when the engine stops. Since there is no way to know if one of the intake valves stops in the OPEN position (again, meaning the intake valve isn't fully seated, as I've already mentioned), you're at the mercy of fate, which might seems to be conspiring against you to make all of these circumstances occur simultaneously.
OK, let's review this for a minute; we now have constant gas flow to the carbs, we have a defective carb float valve to KEEP the gas flowing, and we have an open intake valve, allowing engine vacuum to suck the gas from the carb float bowl, past the intake valve, into the cylinder. Once the gas reaches the cylinder, it flows down the cylinder wall, past the piston rings, and into the crankcase, where it dilutes your oil.
That isn't all; not only does the gas dilute your oil, making it too thin to lubricate the engine, clutch, and transmission properly, in addition, if the gas flows long enough, it can fill the crankcase completely (depending on how much gas was in the tank to begin with). Of course, while the gas is trickling down the cylinder wall, past the piston and piston rings, it is also washing away the light coating of oil that normally protects the cylinder wall from the scraping action of the piston rings. That means that when you first start the engine, there will be little or no oil to protect the cylinder wall from damage from the metal-on-metal contact of the piston ring.
Doesn't sound good, does it? Now you know why you have to fix a problem like this right away; as I've mentioned before, don't even start the engine. You must have the self-discipline to resist the temptation to start the bike when you encounter a problem like this. Drain the oil, change the filter, and then fix the fuel delivery problem.
Well, another long semi-technical explanation from someone who should be sleeping. I hope I've answered your question, and explained the whys and wherefores well enough that you'll understand just how serious a problem this can be, along with the basic phyics behind why it happens, and what you can do to prevent or correct it.
If you have any questions about this, or need help with schematics, hose routing diagrams, or just want something clarified, start typing, and I'm sure someone will be happy to answer you.
Good luck with this; let us know how things turn out.
Great write-up Hugh! I just have a couple of questions.
First, you mentioned an "overflow tube"...
Quote from: Hugh JardonWhen the float bowl overfills, the overflow tube (which hangs down below the float bowl itself) is supposed to drain the fuel onto the ground
The only tube I am aware of that might fit that description is the
Airbox Drain Hose ... it is definitely below the float bowls! I just wanted to mention it in case folks go looking for a tube in the
immediate vicinity of the carbs or float bowls.
Second, is "negative pressure" [...] "from inside the cylinder" a necessary condition for fuel to flow down the intake manifold boots? I haven't looked real closely, but the
Fuel Hose Routing diagram makes it look like that is a downhill path. I wonder what conditions determine whether the fuel flows to the airbox vs the intake valves? Parking on a hill, certainly. Anything else?
actually i don't believe i have this problem...but it seems a really common problem so i was curious as to why it would occur.
I've noticed some gs owners have replaced the petcock completely with just on/off type valves and i was curious as to what they woudl do if they needed the prime position?
How does the prime position even work that it makes the gas come down without the engine pressure sucking the fuel in?
Again thanks for the great article and help hugh and kerry.
The simple answer is, the PRIme position provides a straight path for the gas to flow to the carbs. IOW, it is an ALWAYS open valve; gravity does the rest. The gas flows downhill into the carbs, regardless of whether the engine is turning over or not. As I tried to explain in my original reply, that isn't a very good idea, because there is nothing besides a puny little float valve (in each carb) to STOP the flow of gasoline into the carbs. If the float valve can't cut off the fuel, you have a serious problem and a lot of work to correct it.
One type of the ON / OFF valves you've referred to actually work as normal "PRIME" (ON all the time) and "ON" valves (OFF unless the engine is turning over, creating engine vacuum), with no REServe position.
Thanks for clarifying your situation; the truth is, I wrote that reply for ANYONE having that type of problem. I'm actually relieved to know that you DON'T have to deal with that headache.
Kerry:
I'll try to answer your questions and fix the GS-specific issues in my original post later tonight; thanks for your patience. If I hit a snag, I'll PM you and wait for your response. Thanks for all the help. Hugh J
ok guys, I was searching for some info on fuel valve and stumbled upon this great thread. and it looks old enough so I hope I am not hi jacking this thread. Anway, I think I understand how the valve works but I never realized the danger of running on PRI. After the write up in this thread I am not even sure if I should continue to type or run and get new oil... my bike was in PRI for about a month now and I was riding ocasionally (just a few miles to the university 1-2 times a week and about a 100 miles rides on weekends for the last three weeks). I put it in PRI cause the bike died on me at some point and when I looked in the tank there was gas... so I figured there's something wrong with the valve, but I thought that unless my bike dies or I see fuel leaking from carbs I am ok in PRI, now the above write up made me doubt how wise that was... so now the question is how much damage is already done and do I need to replace the valve to fix it? if so, what's the best place to get it. Thanks!
Quote from: Soaring... so I figured there's something wrong with the valve, but I thought that unless my bike dies or I see fuel leaking from carbs I am ok in PRI, now the above write up made me doubt how wise that was... so now the question is how much damage is already done and do I need to replace the valve to fix it? if so, what's the best place to get it. Thanks!
The success rate of buying a used fuel switch (aka petcock) seems to be very low. A common failure is the rubber diaphragm inside the silly thing that can provide the "I have fuel but it dies" symptom. New ones are pricy, but IMHO they are worth it if you are planning on keeping the bike more that two weeks.
Here's your 98 cents change! :roll:
You could just do away with it altogether.I did mine about a month ago works great.Straight from the res to the carbs just have to reset you trip every time you get gas and also know about how many mile per tank you get.
now I got two different opinions on how important the damn this is... anyone else? what will happen if I ride on PRI, but go to RES or ON when parked? will that do the trick? I might eventually replace the valve, but at this point I am looking for the cheapest solution possible
http://www.gstwin.com/racers_corner.htm#pingel
Look at this.
wow, that's an interesting page, but it kind of lost me there. I don't quite understand what the first thing does and how you install it... do you still need frame mounted valve? second, second valve now has just one tube, what happened to the RES tube? is there no RES anymore? is the tube that is left ON or PRI now? if it's PRI, what about this whole overflow issue?
I am sorry for so many questions, but I am totally lost here :dunno: :oops:
Well the fuel line comes from the Res you plug off the on or pri.That put one line straight to the T on the carbs. I put and on off vaule inline so I could turn off the gas.You would do away with the frame petcock.
I still don't get how does it solve the overflow problem. do I have to disconnect the line every time I park? if so, it seems I could just use frame mounted thing and just ride in PRI and change to RES/ON for parking... now, I got a feeling that I am missing something here... I guess I could use just a little more help :) also do I need to just install this lockartphillips thing or do I need pingel as well?
As I understand it, your vacuum activated valve is knackered.
Now you effectivly have 3 positions, 'pri', 'off' and 'off'
In this case running in 'pri' and using either 'on' or 'res' when your parked will be fine,
although you will no longer have a reserve on your tank as running in prime will allow you
to completely drain the tank before you get any warning about low fuel levels.
You can normally tell if you've got much fuel in your oil, just stick your nose next to the filler
hole next time the dipstick out. If your uncertain what it should smell like, sniff a fresh bottle of
engine oil 1st.
One cheap option for a fix is to look for a petcock reconditioning kit.
These contain all the required o rings and a fresh diaphragm. That ought to fix your problem.
Also a fresh vacuum line to the carbs could help.
//www.wemoto.com/?type=fuel+tap&flag=19 stock these kits, but are based in the UK.
I believe they will do international delivery, but you'd be cheaper finding somone local.
Oh, if the Diaphragm is really knackered you get fuel running down the vacuum line and into the
neck of the carbs completely bypassing the float needles. I just had to deal with this on a 1980 GSX 400.
You may wish to check for fuel in the vacuum line, if there is any then you
either need to fix your
petcock, replace it, or cap both ends of where the vacuum connects and run with your petcock as is.