vacuum diaphragm

[stal871]

On my 81 special, exactly what role does the vacuum diaphragm play in the proper deliverance of fuel to the carbs? Mine currently leaks like a stuck pig when the motor is turned off and the pet-cocks are left on. I know I need to replace it, but I don't know how much they cost, or what their availability is at the local Yamaha shop. I've got a lot of riding planned in the next couple weeks. What would happen if I just by-passed it for temporary, and hooked up both hosses from the pet-cocks directly to the carbs, just like the primer one is already?

[TopCatGr58]

Hey there Stal87,

What would happen is that you would be able to do lots of riding! Just have to remember to turn off the petcocks when stopped, which is probably what you are having to do now!!!

A rebuild kit is available which will allow you to rebuild the petcocks and the vacuum diaphragm. It's purpose is to allow you NOT to have to turn off your petcocks when stopped. It's suppose to provide another link of protection against gas flowing into the carbs when off, especially if your float valve needle seats are not working properly which is why many folks get the flooded carbs syndrome, fuel into the engine when parked!!

But, I just bypassed mine, as have many others. Get a couple of 1/4" "Y" shaped connectors(less bend angle than "T") so that you can link the two outlets from your petcocks so you won't loose RESERVE, and then run the single out to your carbs, and that's it!! ENJOY.

[uk1.1s]

Diaphragm rebuid kit

TC - is that a rebuild kit for the octopus you refered to? Not come across one of those being available..... replaced the petcocks seals and now have solved the same problems Stal87 was having (they were not expensive) - so recommended.

I noticed someone recently posting (Ken Talbot maybe) the cheapskates guide to petcock repair....Stal....if you can't wait for parts or, like me, there is sometimes "too much month at the end of the money" then try this first. Can't find the message to link too - which is a pity as it had pictures and everything (must have been Ken!)....so I'll describe it.

First drain the tank. On the Petcock - remove the screws on the face and the plate it holds on, this should then allow you to remove the middle of the valve - ie the circular center. Behind this in the petcock body (or sometimes stuck to the back of the part you just removed) is a rubber seal with 4 (I think) holes. It is the rubber edges of these holes which wear and are mostly the cause of fuel leaking. If you carefully take this out and turn it over it should be symetrical and can be used the other way around. Replace it this way around and you should get some modest use from it...maybe 10 - 15....20 years!

Wish Ken had posted 6 months earlier before I bought my replacements!!

[stal871]

I put new seals in the petcocks a couple years ago, I know they are working fine. The leaking fuel from the vacuum diaphragm must just be the fuel that is left in the line between the petcock and the diaphragm.

It's nice to know that the only purpose for the vacuum diaphragm is an extra protection against carb over-flow. Before I read your replys, I went ahead and took it off and found that the 2 bottom screws were loose and it simply needed "pinched." I also put an thin layer of RTV gasket sealer around the edges between the rubber gaskets and the plastic piece in the middle, just for good measure. Now that I know it has no performance purpose, and as long as you keep up on your petcocks and float-needles, I may jerk it off all together. I like the thought of that simply because of less hose, fewer clamps, and no chance of it giving me problems in the future.

Thanks a ton.
stal87

[Underdog]

Never ever use silicone RTV wnere it will come in contact with fuel. It swells up like a baloon and is sure to find its way into the system & plug something you don't want pluged. Like a jet or needle valve.

[RS Thornton]

My understanding is the putting the petcocks to the PRI position does the same thing as taking out the diaphragm. Seems to work to fill carbs after maintenance work.

[xs1100yam]

Hi


I had same problem.
I found out the gasket ( coin size with 4 holes) was damaged
And that's why the crankcase was full up whith a mix of oil and gas
...and need to change oil 3 times



Patrick

[Czekus21]

Hey stal87, if you havn't got er figured out by wed. Here's what you do. On wed. Go to the liquer store by hy-ve on ST. Pat and Campbell and get some beer. answer your Email I sent. and we'll figure it out.
ETA, sometime by noonish or so.