Rebuilding Yamaha Vacuum Operated Fuel Valves (Petcocks.)

[Paul75]

I did think the same......which one would be correct? If it's the larger I'll drill out the smaller to suit!

 

[TimeMachine]

Mine are almost .145 in or 3.75 mm. Thats a 4mm ( 3.78 measured dia) screw shown in the pic. So the holes are the bigger size. Can't see the hole size mattering too much. Be sure the vacuum line is rigid enough to Not collapse under vacuum even after being heated by the radiant engine heat.

 

[Paul75]

On another note, when I screw down the petcocks, is it okay to use fibre washers like these pictured?

 

[Paul75]

Guys,

Both of my diaphragm blocks are identical meaning that the passage in the diaphragm block on one will point down and towards the rear and on the other will point down and towards the front. Is this incorrect?

 

[Paul75]

Another question, I've screwed down the faceplate onto the wave washer and it's pretty tough to turn the lever now, this is after though cleaning polishing and lubing. Is this normal or do people back off the screws a quarter turn? If I back off they turn beautifully! Also, is there any way of testing the petcocks before installing on the tank? I guess I want to test the Diaphragms are working correctly......

 

[weekendrider]

The wave washer sometimes slips over and gets caught between the face plate and body. The result is the selector is hard to turn. A little film of grease on both sides of the selector doesn't hurt either. Look carefully to see if the plate is sitting flush all round, things may be in a bind.
You might try putting a short piece of fuel line on the outlet nipple and see if a lite suction will pass air to test off the tank?

 

[TimeMachine]

The petcocks lever movement is rather tight feeling by design. Move that lever slowly always ! And WER is advising light grease , I second that ! As for the outlet direction, it is clear by your previous pics that my 78 standard has different cast components and both my outlets go forward. After seeing many XS owners posting problems about the "dreaded vacuum petcocks" I now believe that the Standard model tank came with superior vacuum petcocks than the Special model . Am curious if others know more about this and perhaps interchangability ? Having owned 2 78 Standards with no petcock trouble whatsoever, I'm still a fan. -RT

 

[weekendrider]

TimeMachine your '78 standards are different. My reference is a '79 F and later specials.

 

[TimeMachine]

weekendrider, do you know of "interchangability" ? Between the different models ? Curious now -RT

 

[weekendrider]

The mounting hole spacing is the biggee. 78 (IIRC, maybe 77) on is different than earlier models. Other than that no. I am trying to use a set from an 80 (converted to manuals by swapping the selectors) on a 79 standard tank and the hold up is the sealing o-ring between petcock and tank. Unobtanium from Yamaha. gggGary found a regular o-ring that worked for him. I'm still looking. . . .

 

[Paul75]

See photo, regarding the passage in the Diaphragm blocks, one faces down and back and one down and forward, I can only assume this to be normal as it cannot be fitted differently.

Also, I'd still like to test these rebuilt petcocks prior to installing on the tank, is there a solid method?

 

[650Skull]

Thought i would add this.
78E, and going by the brochures and pics of bikes the 79F also has the same petcock as the 78E.

Standard and Specials have different Vacuum operated petcocks. The different petcocks interchange between the specials and standard, the mounting holes are the same spacing on the 78/79 standard tanks and all 78 and later Special tanks.

 

[gggGary]

Bump of an old thread cuz it's still the best petcock how to.

 

[xjwmx]

Hi Guys,

Great thread. Does anyone know why there is a larger bore on the right image? It's a '79 Special, vacuum type.

Many thanks

PaulView attachment 101309

Does the bigger one get reduced farther in? Mine has a restriction to do some things to the vacuum, de-pulse and weaken it I think. I was thinking it was an actual jet screwed in, but maybe not.

 

[gggGary]

There is an orifice, one way valve in that cap, fitting, checking that it's free and working, part of an overhaul. I agree function is to ensure consistant fuel flow, reduce the pulsing nature of vacuum from a single cylinder port.

 

[member3480]

i may have a couple Gary if you need them for parts

 

[TedBell]

I figured this would be a good place to put this question: I have an '81 with the vacuum petcock, and the bottom of the plastic filter where it seats in the petcock has a male projection on one half and an oblong hole on the other half. The petcock has two holes just inside the filter mount, and the filter fits with the male end in either hole. Is there a right way and wrong way to orient the filter? Does the male side go on the left or right when looking at the petcock from the lever side, or does it not matter?

Thanks,
Ted

 

[5twins]

It does matter because one side of the plastic filter is for "Reserve" and the other side has a higher pipe in it and is for the "On" position. Get it wrong and you'll be running off the "Reserve" pipe when in the "On" position and when you run out of fuel, you'll be all out with no "Reserve". I can't tell you off hand which way to install it is correct. You'll need to figure out which of the two holes in the top of the petcock feeds the "Reserve" lever position and which one feeds the "On" position.

 

[TedBell]

To be clear, it is the bottom of this filter that I'm referring to.

Thanks,
Ted

 

[Jim]

The holes in the oval cutout are offset slightly....

The screen will only go one direction without being forced into place......

I J B Welded mine in place so it doesn't come loose and leave you without the reserve side.