Petcock, Octopus Questions??

[fxrer]

While troubleshooting the cold start problem with my ‘80 SG, (see previous thread, idling/starting issues), I looked at the petcocks and octopus. My original owner’s manual, AND Clymer’s repair manual, both say that when the petcock is in either the “ON” or “RES” position, fuel should NOT flow from the petcock.

The Clymer procedure to remove the fuel tank says to turn each petcock to “ON” or “RES”, and disconnect the fuel line. The implication is that fuel will NOT flow from the tank.

When I follow those instructions, turn my petcock to “ON”, and remove the fuel line, gas pours from the petcock. I can quickly stop the flow by turning the petcock to the “OFF” position. (The manual says there is NO “OFF” position, however my petcock plate has four, clearly labeled positions of “OFF”, “RES”, “ON” and “PRI”). Are the petcock positions labeled differently between Standard’s and Specials? Different between years?

I removed both petcocks, o-ring seals were excellent, all were spotless clean, the filters were perfect, and there were no leaks anywhere.

I also disassembled the diaphragm (octopus), all looked fine. I performed the “suck” test, as described in the manual, and I could hear the diaphragm clicking, and the fluid flow through the delivery line stopped.

Questions - How does the vacuum, when applied to the rear vacuum tube of the petcock, cause the fuel to flow out of the forward fuel line? And question #2, if the fuel and vacuum lines are disconnected (taking the diaphragm completely out of the equation), how can the petcocks be passing fuel?

Also, lines for the diaphragm and vacuum advance both connect to separate nipples on the #2 intake. On my bike, the diaphragm line connected to the intake port closer to the engine, and the advance line is hooked to the port on the carb. Is this right?

[Ken Talbot]

Yes, you've hit it right on, Rick. The Specials and Standards have different petcocks. The description you quote from Clymers talks about the Standard.

Go back to the fuel line routing diagram and plumb your bike to what is says. For sure, only the octopus should be connected to one of the vacuum nipples.

If you have a vacuum connection to any of the nipples on the petcocks, you could be sucking fuel directly into the cylinders, bypassing the carbs altogether.

[fxrer]

Hi again Ken,

It seems like you are the fuel system expert on this forum! I carefully followed your explanation, and diagram, of petcock & diaphragm operation which you posted as a reply to Bkush, on 4-06. The fuel & vacuum lines on my bike are connected exactly per the diagram.

It seems that my confusion arose because the descritpion in my Clymer manual is for the Standard, and does not differentiate between the Standard and Special petcocks.

And so it is normal, with fuel and vacuum lines disconnected, for fuel to flow from my tank’s petcock when it is set to “ON” or “RES”? And thus, for a Special, the only way to stop fuel flow when removing the tank, is to set the petcock to “OFF”?

If this is so, shame on Clymer for not making it more clear.

But thank you (again) for clearing it up.

So, I am now convinced that my fuel/vacuum lines are connected proerly, and the diagphram and petcocks work as they should.

[hollywoodxs11sf]

I had it explained to me once before and it goes like this:

When the petcock is set to "on" or "res", fuel flows out of the rear brass fitting only. This fuel is then routed to the octopus' innermost hose fittings (#1 and #2). If there is no engine vacuum applied to the other side (the Taiyogiken side) of the octopus, this is where the fuel stops.

When vacuum is applied to the octopus, the fuel is allowed to flow from the inner fittings (#1 and #2) to the outer fittings (#3 and #4), which are routed to your carbs.

The forward fitting on the petcock runs to a tee in the fuel line between the octopus and carb. When the petcock is set to "prime", fuel flows from this fitting only, bypassing the octopus, directly to the carbs.

[fxrer]

Hey Ken, Hollywood,

Back from vacation, and there is still a small leak from the carbs when the bike sits. It looks like it’s coming from #1 carb, but since the bike leans that way on the kickstand, it may be from another carb (too).

So, based on the explanation of the petcock and octopus operation for a SPECIAL, I must have two problems, right? The octopus must be leaking which allows fuel to continue to flow to the carbs, even with the motor off. AND, there also must be a float valve/seat leak in one or more of the carbs?

I checked the octopus, the diaphragm appears OK, with no leaks (When I suck on it, it holds a vacuum). I examined the little o-ring seal at the tip of the valve in the octopus - it looked fine, even under magnification. I flipped the orientation, re-installed, and I still have the leak. Is there a “rebuild kit” for the octopus? Or a source for a new one?

With the carbs still installed, I was able to pull the outside carb float bowls, floats, valves and a valve seats out - there was some minor gunk which I cleaned, re-installed everything, and still the leak. So, I will try the air pump technique (previous thread) to blow out the fuel lines, and hope that might work.

Otherwise, I guess I can live with turning the petcocks to “off” position when the bike sits; OR, buy rebuild kits for the carbs, with seats and needles.

Unless you have any other suggestions?

Thanks,

Rick

[TopCatGr58]

Hey Rick,

There are rebuild kits for the Octopus, IIRC in with the petcock rebuild kits I've seen. However, if you don't mind turning the petcocks to OFF, then just pull the Octopus and save some $$. As for the carbs, you may be getting repeated exposure to fine dirt/rust particles from the tank, stuck in the lines, etc.. You probably need to pull the petcocks and lines, clean them thoroughly, and put INLINE filters on it to catch the rust so it won't make it to the carbs and get stuck in the float needle valve!

Your petcocks may still be leaking, and if so, then the fuel could be getting into the PRIME circuit which bypasses the Octopus, and goes directly to the carbs.....so.....your Octopus may be functioning properly, but your leaking Petcocks could be the main problem along with the dirt!
T.C.

[fxrer]

Hi TC,

I’ll hold off on the octopus rebuild kits for a while, I can live with turning the petcocks to OFF. After all, I did it (with BSA’s) for 15 years before Yamaha introduced the vacuum operated octopus system.

The better idea is those inline filters. I've checked my tank and there is only the slightest bit of light rust. Nothing I ever was concerned about, but I guess some “fine” particles could be causing problems. Do you recommend any specific type or brand of filter? I’d guess that I’d only need two filters, one each for the fuel supply lines from the tanks to the octopus? But they would have to be small (in diameter) to fit in that maze of fuel lines under the tank.

Thanks for the tips.