Gas leaks, fuel pouring out of your carbs? Gas in the oil? How to fix list.

MotorbikeBruno

Over 1,000 Posts
Hello All, I recently checked in on the 1-800-CAFE-HELP board as I'm always up for helping others. Do I know it all? No, hardly, but I clean a lot of carburetors and fix a lot of old motorcycles so maybe something I've seen can help you stop that fuel from going where it shouldn't, or making sure it's going to where it should! This is a work in progress, because...well, I'm at work...ha!

Let's start with the basics:

Fuel enters your carburetor from your gas tank. First possible culprit is your fuel valve/petcock. There's really 4 main ways fuel can get from there to your carbs.
#1 Manual petcock. This has a setting for ON, OFF and Reserve (RES)
#2 Diaphragm petcock (automatic petcock) which has ON, Reserve and Prime (PRI)
#3 Manual petcock, that goes to a remote automatic petcock (see a 1981 CB750F model for this example)
#4 Manual petcock that has ON, OFF only which goes to a fuel pump.

Here's a shot of 3 of them. Hopefully you know what a fuel pump is/does :)

Find out what you have before you can figure out if it's got a problem.
#1 Manual petcock.
With the petcock set to OFF, does fuel come out? It shouldn't. If it does, you will have to drain the tank (mostly, or enough to angle the tank so that fuel can't reach the petcock) Now you can work on it. If you have a RIVET type petcock, there isn't much you can do. You can try turning the fuel to the ON position and blow backwards through it with compressed air, and then do the same with the Reserve to see if you can free up anything that may be blocking the fuel valve from closing fully. If it's as old as the bike is...you'll be replacing it.

On the flip side: If you turn your manual petcock to the ON position and you don't get fuel, make sure you have enough fuel in the tank for it to reach the "ON" position's fuel draw tube. Tank is full you say? Then you have something clogging the fuel valve. I've seen many of these with KREEM in them...since the previous owner sealed the tank with the petcock still on it! Reserve is the more likely one to get clogged because it's fuel pickup tube is shorter. If you get "poor fuel flow" please also check that your gas tank air vent isn't obstructed (typically located in the keyed gas cap)

#2 and #3 Usually those work a great deal of the time. Which brings us to #2 and #3. They are lumped together due to the fact that they are essentially the same thing, just one is attached directly to the petcock, while the other is separate. The idea of this type of petcock is awesome. You turn your fuel valve to the on position and just leave it! As long as you have enough fuel for the "on" position, you are good to go! When you turn the engine over, it creates a vacuum which is drawn through one of the carburetor vacuum sync ports typically (some carbs actually have their OWN port just for the diaphragm petcock line). This vacuum pulls on the diaphragm and moves a projected part with an O-ring that is on the opposite side of the diaphragm away from where it seats to shut the fuel off. When no vacuum is applied, there is a spring on the vacuum port side of the petcock that pushes the O-ring projection back into place and effectively seals off your fuel! Great! Wow! Awesome right!??? Until it fails. :mad: This unit is typically known for GAS IN MY OIL, and puddles on the floor habits compared to the other petcocks. Over time, the O-ring has shrunk or become brittle and doesn't do it's job anymore. Or, it's been sitting a while and fuel has varnished and is blocking it from returning to it's seated position. But wait, there's good news! These usually have rebuild kits available!!! For about $25 or so, you could have a brand new diaphragm and o-ring set for your petcock! If I was you, I'd find a replacement manual petcock for my bike, but that's not always available, so a rebuild kit is what you'll have to do. Videos and more to come...

#4: You can leave your fuel in the on position. Many systems like this only have an "ON" and and "OFF" for the petcock. Most early sportbikes like the CBR600's had this setup. Honda Magnas also had this setup under the right sidecover. You leave the gas in the on position because the fuel pump acts as an automatic petcock of sorts. It doesn't allow fuel past unless it's pumping. A lot of the early fuel pumps actually have a set of points under the plastic cover! Some times they have corrosion and just need a quick cleaning and they will fire right up and work!
 

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