Petcocks and the Vacuum

[marker2001]

I have looked around but have not found much on how the petcocks work.

My problem is this -
The last time the bike was started, it ran fine - 2 years ago. Now after finally fixing the tranny, I find that the black rubber disk in the petcock is damaged. It probably was stuck and I turned the lever anyway. Now the bike will not stay running and will not idle well.

My question is this -
From reading other tech posts I think what I thought were two fuel hoses leading from the petcocks are actually one fuel and one vacuum hose. Is this true, and if fuel gets into the vacuum hose will this affect the idle? Will having fuel in the vacuum hoses damage anything?

Thanks in advance!

[beechfront]

there were different setups on different years. what year/model do you have?

my bike is a 79 special and it uses a separate vacuum valve know as the 'octopus'. petcocks have two lines coming out, one for prime which flows directly to the carbs, and the other flows to the octopus.

[marker2001]

Just an update - My bike is an '82 XJ1100 and has the "octopus" as noted above.

[TopCatGr58]

Hey there Marker,

Both pipes from your petcocks are for fuel. The vacuum feed control is handled by the Octopus. What you did was mess up the rubber valve in the petcocks, and will need to get a petcock rebuild kit which will provide those parts. There is a fine rubber O-ring around the outside edge of the petcock valve/handle, you'll want to be careful to try not to tear it up, it can be reused.

If you're in a hurry, you can go to your local Hardware store, get a thick large neoprene washer, and cut one out yourself to fit. I did with a single hand held paper hole puncher, just eyeballed the location of the 5 holes needed, and then cut it sorta round shaped with a good pair of scissors, fit fine, just make sure you get one thick enough, take the old one with you to compare. If you get one too thin, the wavy washer won't put enough pressure against it to seal it properly!

Your idle problem may be related to not enough fuel getting to the carbs? Also, your octopus valve may be sticking, not providing enough flow with low vacuum pull on it at idle vs. running speed. Many folks have bypassed their Octopus, do a search for Octopus in the threads, you'll find diagrams on how the fuel lines are run, pros and cons of the Octy, etc..

PS...Beechfront....just a friendly FYI, many folks don't have their year/model in their sig, but have put it in their Profile, so you can quickly click on the "profile" tab/button on a person's message to check, but Marker already replied with the info.

[Jerry]

Petcock theory revisited

I have looked around but have not found much on how the petcocks work.

Do a search on "petcocks" or "octopus" and you will see a lot of discussions on this frequently-posted topic.

Petcock theory

Petcocks were designed to perform several functions. First, they shut off gas flow when you want to unhook the gas lines to remove the tank or carbs. (Off function.) Second, engine vacuum alows gas to flow when the engine is running, but as a safety feature they shut off gas flow when the engine is not running, preventing gas from flowing out of the tank in case of an accident. (On function.) Third, they provide a bypass operation to allow fuel to flow at any time, usually to fill the carbs after a cleaning or service (Prime function.) Forth, they allow you to access the reserve fuel in the tank when you start to run out of gas. (Reserve function.)

Off, On, Prime, Reserve: four settings on XJ petcocks and I believe those XS models that use the octopus.

There is a diagram for the octopus hose connections on a decal on the bottom of the XJ seat. The shop manual's diagram looks like this:



(Images are larger in the manual. If you need these, let me know and I'll e-mail the larger versions to you.)

As noted, petcock rebuilt kits are available and they include the octopus parts as well. You can remove the octopus; do a search on "octopus" and you will get tons of hits and they will include information on how to remove the octopus if you wish.

Biggest problems are torn vacuum diaphrams or wave washers (also known as spring washers) that go flat. A good clean and re-bending the washers is often all you need to fix the petcocks.

[fusionking]

Find a GOOD shop

Any good bike shop will have an assortment of those rubber discs. Thats how I fixed mine "Dirt Cheap"!!