1979 Special Fuel Issue

[Schming]

You have them mixed up a bit.

Left petcock:
Front port (prime port) goes to Tee, and Tee goes to #1 on octy and left fuel port on the carbs (this can be switched to the right port if you choose - makes no difference)
Rear port goes to #2 on octy - this port is vacuum-controlled

Right petcock:
Front port (prime port) goes to Tee, and Tee goes to #4 on octy and right fuel port on the carbs (can be switched to left if you choose)
Rear port goes to #3 on octy (vacuum-controlled)

I think (not 100% positive) that if you turn on the Prime port, fuel will flow from BOTH ports, which means that the ports are plumbed together inside the petcock. The two rear ports are definitely plumbed together inside the octy. This means that if fuel is being pushed out of the rear port on the petcock by turning on the flow of fuel, fuel will flow from the rear on one side, through the octy, INTO the rear port on the other side, then out the front port. That is why there is an OFF position on the petcocks - that is the only position where fuel is not flowing somewhere.

Hold on a minute, study post #5 in this thread:

http://www.xs11.com/forum/showthread...ght=octy+valve

The diagram in the thread is from Clymers and there is a discrepency where as the lableing on the No. 2 hose and No.4 hose need switched.

I do think you have it right that the fuel flows from one rear line to the other rear line and somehow bypasses into the prime port but that may be dependent on the position of the other fuel valve. The prime port is only fed from the reserve inside the petcock.

[CatatonicBug]

In terms of port numbers, I listed them starting left to right, as sitting on the bike, 1-4. Both hoses on the left petcock connect to the left 2 ports on the octy, and both hoses on the right petcock connect to the right 2 ports on the octy. No crossing there.

[Schming]

Here's a link with a pic of the octy valve, where you can see the order :

http://www.ebay.com/itm/1979-79-Yama...9292d5&vxp=mtr

And yes that is correct, lines don't criss-cross.

[MPittma100]

Fuel Issue

What I have concluded as a result of the posts in this thread is that there is a lack of needed (missing) instruction in the Yamaha/Clymer service manuals pertaining to the procedure for checking the performance of the "octy".

The procedure states that with both front lines removed from the petcocks, there should be no fuel coming out of those "lines" with petcocks in the "on" position when there is no vacuum applied to the octy. By applying vacuum to the octy, fuel should then flow out of the removed lines.

What is NOT stated (but should be) is that the front petcock nipples (where the lines have been removed) should be plugged/capped while performing this test. Apparently Yamaha assumed that we were smart enough not to need that particular bit of information?

Brain wrapping is now complete for this fuel issue.