Rebuilding Yamaha Vacuum Operated Fuel Valves (Petcocks.)


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Baraboo, WI, USA
Copied with permission from Written by bigfitz52
A more or less generic petcock rebuild how to, very useful for your 650 petcock rebuild. gggGary

IF you are using a K&L petcock rebuild kit read here before you use the diaphragms and shaft part of the kit.

Photos all died in the photobucket FU of 2017, Here it is in PDF form with all the pics intact! Ian Vanore Thank you SO much!
Rebuilding Yamaha Vacuum Operated Fuel Valves (Petcocks.)

THEORY of OPERATION: The way these things are supposed to work is this:

When the petcock is in the ON or RES positions, fuel flow is held "off" by a spring-loaded diaphragm seal. When vacuum from the intake manifold is applied to the smaller port (see pic below) the diaphragm lifts the valve seal off the seat and fuel flows (out the larger port.) When the valve is in the PRI position, fuel is free to flow all the time.

Problems develop when the spring-loaded diaphragm valve doesn't seal anymore, allowing fuel flow when it should be off; when the diapgragm gets a tear or hole in it and doesn't operate properly plus allows fuel to get into the vacuum line from the intake manifold; and/or when the valve itself becomes worn so that it leaks, either externally or internally.

The cure is to rebuild or replace. This is for those of you who wish to rebuild and have it actually work.

I bought this petcock off eBay for my 650:


It needs to be rebuilt.

Chacal sent me one of the larger ones from the 700s and 900s to rebuild at the same time. You will see both styles used in this article, differences and similarities will be obvious.

First thing we need to do is disassemble it:


CLOCKWISE FROM TOP: Tank mount o-ring; In-tank filter; petcock body; diaphragm block; diaphragm (magic o-ring above it;) diaphragm spring; vacuum port/diaphragm chamber cover, with vacuum nipple and vacuum nipple check valve (tiny disc;) cover retaining screws (4;) outlet pipe flange w/screws (2) and gasket; front plate w/ screws (2;) wave washer; fuel lever; o-ring for lever; valve body seal.

The larger valve is substantially the same except its output pipe is integral:


Now that the valve is apart, we need to examine it carefully to determine serviceability. Close examination of the valve seat will tell us if the unit is rebuildable or toast.

Salvageable seat:


Although there is some minor pitting and wear, this seat can be polished out with no problem.

This one's junk:


This valve seat is beyond any polishing. Deep pitting from corrosion has taken its toll, it's only useful as an example of a bad seat. Those craters aren't coming out.

So now let's begin rebuilding our petcock by getting everything religiously clean, using carb cleaner and some of my personal favorites:



There's a vacuum port check valve (nothing more than a tiny bakelite disc) in the cover plate. If you can't unscrew the vacuum nipple, don't sweat it; just flush it out with carb cleaner from both directions, and make sure it's free in there. You can hear it rattle about, and test by blowing in the port.

Once we have everything clean, let's begin the refurbishment by polishing the valve seat:


I use my Dremel, but you could use a "button" in an electric drill, or do it by hand using an industrial-sized wooden stalk Q-tip.



Once our seats are polished and the parts all flushed clean, we need to clean up all the mating surfaces. That includes the surfaces for the diaphragm block and cover; the outlet plate mating surface on the body (if so equipped;) the flat side of the fuel selector lever and the tank mounting flange.

This simply won't seal well if reassembled as is:



This needs to be "dressed" as well, even though it looks "OK."

Use anything from 800 to 1200 grit wet-or-dry sandpaper on a flat surface. You can tape a hunk to a small piece of glass; I use the little surface plate on the back of my bench vise:



Here's a "before and after" of dressed versus untouched mating surfaces; the parts that have been cleaned up will seal much better than the unsanded ones.


See how nicely the valve I'm holding in the pic a couple pics above turned out?

Now we need to polish the o-ring seating surface to remove any "wear rings" that might be present, and give the new o-ring a nice smooth seat:


Once again, I used my Dremel but I also did one by hand. Either way was equally effective, doing it by hand just took longer is all.

Now that we have our valve seats and o-ring sealing surfaces polished and our mating surfaces dressed, flush everything thoroughly with carb cleaner, scrub with an old toothbrush, Scotchbrite, or whatever you want to get everything sparkling clean.


Now we're gonna need some PARTS: I know this seems like a shameless plug, but chacal is the best source out there for this stuff; a lot of the other kits on the market simply don't work.


The big-body petcock kit is on the left (NOTE it DOES NOT include the diaphragm block, you will need to re-use the original) and the "standard" kit on the right (its diaphragm includes the block.)

REASSEMBLY TIP: When you put the vacuum check valve and vacuum nipple back in the diaphragm cover (if you took it apart,) put a dab of RTV on the threads of the nipple just in case. A TINY DAB and just on the threads we don't want to glue the little valve flap in place.

Now let's start by giving all the rubber parts a sparing wipe with silicone grease, and let's get the diaphragm ready to go on.


The "business" (valve) side of the diaphragm assembly:


The backside (spring side) of the diaphragm:


Note this passage on the valve side of the block:


You can see it in the pic of the big-body diaphragm block above; both styles have it.

The passage goes pointing down and toward the rear when the petcock is on the bike: The orientation is the same on both styles:


A word about diaphragm springs: Both styles of replacement springs are slightly longer than the originals:


In both pairs, the replacement springs are on the left. You can also use a washer as shown to increase spring tension and improve sealing; but if your seat is in good shape it should not be required.

Set the spring in place; and go ahead and install the cover.


Snug down the screws; then gently poke the stem of one of your q-tips in from the front and lift the valve off the seat and let it return a couple of times; "orbit" the cover/diaphragm assembly around a little bit so it all seats nicely.


Then tighten up your screws. Be sure you have the passage in the diaphragm block positioned correctly. (See above and below.)

Continued in next post

Ride safe--- Fitz
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For the petcocks so equipped, fit a new output pipe flange gasket,


And install the output pipe flange;


Some styles of petcock have a non-removable outlet pipe; some of the "standard" petcocks have a drain screw rather than a removable pipe; a new gasket here would be a good idea (and a new screw if corroded.)


Now let's install the valve body seal be sure to smear it sparingly with silicone grease too:


The bigger petcock is the same; be sure you install the new o-ring on the valve lever flange;


And go ahead and install the lever (be sure you lubed the o-ring) by rotating it into place;


Don't forget the wave washer on the front; then go ahead and install the face plate as well.

Now we need to install the in-tank filter standpipe. The base flange of this part will need to be filed or ground down slightly to be able to fit flush. Once again, I enlisted the trusty Dremel:


This can be accomplished with a sharp fine flat file as well.

The filter pipe MUST fit flush or even slightly below the surface of the petcock body, to avoid interfering with the installation of the petcock on the tank.


Place the petcock body on your bench and press the (now recontoured) standpipe down into the petcock by pressing on both sides of the oval; it will "seat" fully and fit as pictured.

COMPLETED. Be sure to use new sealing washers (they are a special steel-mesh/composite) when refitting the petcock to the tank.


It's a good idea to fit an in-line fuel filter ASAP, the in-tank unit you just replaced is only effective down to a certain size particle.


Chacal offers these fuel filters and clips; seeing your choices always helps decide what will fit your bike the best.


Different color fuel and vacuum lines are also available if you want to personalize your bike.

ALTERNATIVES TO REBUILDING: You can replace the petcock with a modern, vacuum operated unit (these are the same replacements offered by Yamaha) or eliminate it altogether by installing a simple on-off fuel valve:


L-R, BACK ROW: Stock valve (we just rebuilt;) "short" Aftermarket/Yamaha unit HCP625; tall modern unit HCP626.
FRONT ROW: On/off replacement fuel valve kit contents; "tall" stock "drain plug style" unit for comparison.

I have the HCP625 on my Black '83 since I was not all that concerned about stock appearance but wanted to retain the vacuum-operated feature.

Side by side comparisons of the (non vacuum) simple ON/OFF replacement valve and the stock valve:



I'm not advocating replacing the vacuum operated petcock that came on your bike; I just wanted you to see the alternatives that are available.

AS ALWAYS, THIS POST IS A "WORK IN PROGRESS." Suggestions and comments are welcome; if there is anything you would like me to add or expand on please say so.



  • Petcock Rebuild.pdf
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Excellent write up. I have found over the years, sometimes a slightly bigger o-ring on the pintle can help stop dribble leaks out of the outlet.
Think while assembling the gray wafer and the diaphragm to the petcock and make sure that the pintle is straight and receives full spring force into the seat. My first attempt leaked and my second didn't, using the same parts. I don't think the gray wafer is in any of the kits so be careful with it. Also, his petcock was in bad shape. You might not need all that polishing, just cleaning. You don't need all that disassembly either, e.g. taking the stainer out.
I bought my 79 a few years back decided to build a lil bobber, from the get the petcocks leaked. So I just replaced them with the mikes replacement non vacuum replacements. Then mikes came out with a rebuild kit for the 79. Decided I liked the style of the originals, so picked up a few pair of rebuild kits. So then I jumped into the rebuild, cleaned the shit out of them, resurfaced the gaskets faces and everything *but for some reason I can not get the right side to stop leaking. I have taken It apart a few times now "carefully" just to try to resettle everything. Still leaks I did notice that some of the leak was coming from the mounting screws I had also replaced those with the new washer and gasket but didn't have a good seal so I switched those with some old originals I had and it seemed to help until I went for a ride then the leak came back. Just wanted to see if anyone had any suggestions on what I should do. Or if I should just go back to the non vacuum ones if I go back to those should I get new carb mounts? Or if it's just the p.o.s. aftermarket rebuild kit. Thanks Brandon.*
I am sold on the Idea of rebuilding and reusing the stock parts for most things when possible.
Many of them are great and many times the replacement part is not fixing the problem for me.
That Rebuild is on my list as the mikes non-vacuum replacement is working, but seeps fuel.

Thanks for spending the time to put that together!
bentlyb it can be tough figuring out just WHERE the leak is coming from. I like to carefully remove the paint from the bottom of the tank where the rubber gasket seals.I also tend to remove the standpipe and check the top of the petcock for flatness by sanding it on a sheet of sandpaper on a flat surface. Good luck and please report back if you get the leak stopped! I have several petcocks on the bench right now and have tried some new techniques gleaned from bigfitz' how to but have not put any back in service yet. I think buffing the inside perimeter of the body where the handle seals might be a big improvement in sealing reliability for me. I hate the rough feel when the handle won't turn smoothly.
Outstanding stuff! Got one question: Is it wise to use the silicone based lubricant in the valve? We only use petroleum jelly on this kind of stuff in the aircraft business, because it is fuel soluble, and does not clog screens and filters. Just wondering what the consensus is. I suppose if it's used sparingly, that won't be and issue, and maybe being non-soluble, it would stay in place and doe the work?
Good question, too bad, but I "think" there is nothing that will "stay there" I have been using a small amount of synthetic silicone grease as an assembly lube. :shrug:
Didn't know if the silicone grease would flush out or not. Guess ya just gotta ride it often enough to exercise that rubber! Part of the reason I keep Marvel Mystery oil in my gas is to try to help out with the rubber VS alcohol thing. So far, so good.
One thing to add to make it more useful for the xs650 vacuum petcock would be pictures of how the fuel outlet pipe is mounted in the petcock body. A good rebuild would include re-doing that area as well. If anybody has had that apart. Mine has a slight seepage there.
There are at least 4 different exits used on XS650 petcocks. I bet the one you have is the one with a die cast 90 that is "kind of" located by two pins.

Last one of those I did I removed the 90 cleaned inside and out and smeared it with epoxy before putting it back together. I think that's about how it was done originally. there is a groove in the 90 to hold some glue. If anyone knows differently let us know?
My take on mounting area clean up.
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^ Yep, that's the one. I wonder if JB Weld would be a better alternative to epoxy. I've never used it and I don't know if it starts out thin enough.
There are at least 4 different exits used on XS650 petcocks. I bet the one you have is the one with a die cast 90 that is "kind of" located by two pins.

Last one of those I did I removed the 90 cleaned inside and out and smeared it with epoxy before putting it back together. I think that's about how it was done originally. there is a groove in the 90 to hold some glue. If anyone knows differently let us know?

I have a xj550 with that type of exit at the petcock, I always wondered how it was originally sealed. I think you are right about the glue being used in the small groove originally.

I use about three wraps of teflon tape there. It seals and snugs it up but still lets you adjust it some while installing. Never had it leak or vibrate out with the teflon tape.

Scott, first post!!!
^Not sure if teflon tape is fuel proof. Not sure all epoxies are either. Would be well to have fer sure methods in a tutorial. I vote for jb weld, if it's thin enough to coat the outside of the pipe. But I'm just assuming it's fuel proof since it's so popular with mechanics.