Modifying the OEM fuel petcock to remove vacuum function

[Gryphon Rider]

I thought I'd put this all in one post for easy reference.

Why would you modify the OEM fuel petcock to remove vacuum function?  Can you relate to these symptoms?

Someone is riding down the highway and the bike starts to sputter just like it does when you need to switch to reserve, only he hasn't ridden that far yet.  He switches to reserve and is able to go a little farther before the bike starts to slow down again.  When he pulls over and idles on the side of the highway, the engine recovers and he is good to go for a couple more miles, then the problem repeats.

If this problem started soon after doing work involving your fuel tank or the various hoses connected to it, it is possible that you have kinked the tank's vent hose.  You can test this by opening the fuel filler, then closing it with something between the tank and the cap to prevent the cap from sealing, such as a soda straw, plastic coffee stir stick, or a zip tie used for wire bundles.  Another possible cause is a problem with the vacuum hose that runs to the fuel valve, such as a disconnection or leak.

Barring those issues, the problem is caused by a small tear in the vacuum diaphragm within the fuel petcock.  Think of the vacuum operated valve as a second shut-off downstream from the manual valve (although they are actually one assembly).  The vacuum shut-off is normally held closed by a spring.  When the engine is running, engine vacuum overcomes the force of the spring and pulls open the valve, allowing fuel to flow.  If there is a small tear in the vacuum diaphragm, there is only enough vacuum to partly open the valve.  This may not be a problem when the fuel tank is full, because the weight of the gas overhead means more fuel pressure, which helps to overcome the spring, and fuel can flow.  Once the fuel is down to a lower level, there is no longer enough fuel pressure to help overcome the spring, and the valve starts to close.

Normally, this problem is solved by installing the rebuild kit, called "COVER SET,rooster" part number 16953-MBZ-B51, which goes for $28-40 at US dealerships, and was $45 at my local Canadian dealership in 2009.  If you don't want to spend the money right away, or you want to be able to ride while you are waiting for the kit, you can do the modification described below.

What the modification does is move the spring so it holds OPEN the vacuum valve so fuel is always allowed to flow.  Please note that this modification will do NOTHING to prevent hydro-lock.  In fact, if your vacuum shutoff is working properly right now, keeping it intact is BETTER than eliminating it.  If you get in the habit of manually turning your valve off all the time regardless of the function of the vacuum shutoff, you eliminate one of several possible causes of hydro-lock.  If you perform this modification, you NEED to practise this habit.

The following instructions assume that you have access to the back of the fuel valve.  When I did it I had the valve removed from the tank already, so I don't know how hard it is to do with the valve in place.  If the valve is on the bike, be sure it is in the off position.  In either case, ensure there are no sources of ignition and your work area is well-ventilated.  The four photos are from someone else's online instructions (I forget where I got them, so let me know and I'll add the link see note at bottom*) for installing the rebuild kit, but should help you picture the parts.  The hand-drawn sketches are by me.

1. Remove the vacuum hose, four screws, and the cover.
2. Remove the spring (to be used later) and the black plastic disk (#1 in photos) that separates the spring from the large diaphragm (#3).
3. Push the middle of the small diaphragm (#2) so that it is away from the valve body hole that it is designed to seal against.
4. Reassemble, being careful to have the edges of the diaphragms in their proper positions, and with the spring in position between the small diaphragm and the valve body.
5. Devise a method to plug the vacuum hose so you don't lean out your fuel mixture or, in the event of a leak in the small diaphragm (#2), risk having fuel travel through the vacuum hose to the engine, possibly causing hydro-lock.
6. The leftover black plastic disk can be used as a shim to stop something on your Harley from rattling.




Another visual of the normal assembly of a similar valve:


*After using the clue provided by NITRO, below, I have found that the photos were provided on October 2, 2008 by Cholla on the VOAI message board, linked to here: http://valkyrie-owners.com/SMFORUM/index.php?topic=7055.0

[Valkpilot]

Excellent write up, GR.  Well done!

[Disco]

This should also be added to the Shop Talk page.

Great work (again), Gryphon Rider.   

[Hook#3287]

Great write-up!  And timely too.  I'm just fixing to rebuild a petcock soon.

Thanks

[garyheskett All 49 x 3 st.louis]

A great write-up.  This should definately be in "shop talk".   It would have made my breakdown outside Durango on a late Saturday afternoon a lot easier.  We finally figured out to reverse the spring but that was about four hours later and about 4-6 times of removing the tank, taking the petcock apart, and saying "well, that didn't work either, what else". 

[PhredValk]

I'll keep this, thanks Griffin. I'll be installing a Danmark fuel valve this spring, and want to get rid of the vacuum line to #6 to remove its potential for hydro-lock.
Fred.

[Oklahoma_Valk]

Gryphon. I cant thank you enough for this write up. This mod solved more than one problem for me tonight!! There ought to be a paypal "donate" button on this post.

-Okie

[Bone]

I did the conversion in a similar way in 04/10. Waiting for the cover set to be shipped I converted mine so I could ride. Haven't installed the cover set I've rode with the manual shut-off most of my life.

[Hoser]

For 80 bux you can get a manual petcock, and remove the whole mess they call a oem petcock.  Just put a plug on the #6 carb vacumn nipple. Turn the petcock off when you park the bike and turn it on when you start it.  That's the way they all worked when I first started riding. Hoser   

[Oklahoma_Valk]

For 80 bux you can get a manual petcock, and remove the whole mess they call a oem petcock.  Just put a plug on the #6 carb vacumn nipple. Turn the petcock off when you park the bike and turn it on when you start it.  That's the way they all worked when I first started riding. Hoser   

No kidding man. The problem is they've overcomplicted things these days. I just cant believe a good simple petcock costs $80. Im a poe boy. Trying to fix it all the handy dandy cheap way. I told God that in my next life I plan to be wealthy. I've got big plans!!

Hehe

-Okie

[98valk]

http://www.valkyrieforum.com/bbs/index.php/topic,8943.msg201638.html#msg201638

why hydro loc happens per honda service bulletin, has nothing to do with the petcock. basic ventilation design of the carb vent hoses is not maintained.

http://www.valkyrieforum.com/bbs/index.php/topic,22852.0.html

mod the petcock

http://daughertymotorsports.com/howto/Tankvalve/tankvalve.html

If re-doing the oem unit, install a 1/8" npt snubber in the vacuum line to the petcock. The diaphragm will then take a very long time to wear out.
http://www.valkyrieforum.com/bbs/index.php/topic,3408.0.html

my oem petcock diaphragm went bad at 13k, I rebuilt it and installed a snubber in the vac line, now with 23k on the new diaphragm and no problems.

[Mr.BubblesVRCCDS0008]

I converted mine to a manual petcock about 5 yrs ago. We were leaving for the romp in the swamp ride the next day when mine started dripping from the weep hole on the bottom side of the valve. All the other bikes I've ridden had manual valve so why not convert this one. Anyway 5yrs and 60k miles later with no problems.

[Led]

For 80 bux you can get a manual petcock, and remove the whole mess they call a oem petcock.  Just put a plug on the #6 carb vacumn nipple. Turn the petcock off when you park the bike and turn it on when you start it.  That's the way they all worked when I first started riding. Hoser   

 

[NITRO]

The knowledgeable valk rider that created the pictures you used is Cholla from the VOAI (his handle is on the last pic).

[YoungPUP]

Is ther a honda manual petcock off of another bike that will fit ours and fix all of this???

[98valk]

Is ther a honda manual petcock off of another bike that will fit ours and fix all of this???

if there is it would have to be a pre '80-'81 street bike, one of those yrs is when the govt mandated a vacuum petcock for all street bikes, or it could be from an off road bike/atv any yr and any manufacturer.
Case in point the manual petcock that fits my suzuki DR200se is from a yamaha raptor. I don't know how the poster came up with that but it fits and works.

[Mr.BubblesVRCCDS0008]

 When I did mine I just sandwhiched some 1/8 in thick gasket material between both halves where the rubber diapharm lived (discarded it) and plugged the vaccum line from the carb. I still have on, off and res, on this set up. Very easy to do.

[Mildew]

I tried to sandwich mine the first time and the material couldn't handle the gas. It leaked after a while. I haven't had a problem with the jb weld. I ran out of gas once but it may have been my fault. I might have had it on reserve already. I've done the petcock long ago. I think I spent $5 on the fix.

[YoungPUP]

Gov't mandated Vacuum petcock?  My '02 Shadow spirit was manual.

[98valk]

Gov't mandated Vacuum petcock?  My '02 Shadow spirit was manual.

something I read in a magazine review of the '81 CB900C, they stated it was govt mandated since the '80 900 was manual and the '81 and later were vacuum operated. I never looked into it further.

[YoungPUP]

Cool.  No harm meant, was just curious.

[Oklahoma_Valk]

Gov't mandated Vacuum petcock?  My '02 Shadow spirit was manual.

I too had the spirit. 1997 spirit 1100. Twas an outstanding reliable bike. Comfy and smooth.

[98valk]

Cool.  No harm meant, was just curious.

non-taken, I'm always in learning mode also. 

[gordonv]

Gov't mandated Vacuum petcock?  My '02 Shadow spirit was manual.

I too had the spirit. 1997 spirit 1100. Twas an outstanding reliable bike. Comfy and smooth.

I believe most Shadows have an electric fuel pump. I have the 98 ACE T, and bought a spare pump which I was going to apply to the Valk IS.

[Farther]

My '08 Shadow was a manual.

[MilkyTech]

What a great post!  Thank you for this.  Saved me a couple hundred for a tow.  Exactly as the OP described, I was driving down the highway on my 98 CBR900RR when it started to sputter as if it was running out of gas then stalled before I could even switch to reserve (which wouldn't have mattered since it had plenty of gas).  My wife picked me up and drove me home where I did some research and found this post.  Went back to my bike on the highway and quickly removed the tank, took apart the petcock, modified it as posted, plugged the vacuum hose with a screw, and it started right up!  Gas was coming from the vent hole rather quickly but at least I was able to get it off the highway and home without having to get a tow on a Sunday.  Ordered a new petcock and back in business!

[George B]

Great post. I did this as well, in a similar fashion. I have been in the habit of shutting off the petcock as soon as I kill the ignition for 50 years. The OEM vacuum valve is fine if working correctly, but will eventually fail. I'm just old school.

[Valkorado]

I was looking to refresh my memory before making my new OEM petcock manual and came across this post. I remember I'd tried the R&M Belly Tank technique many years ago.  I've never owned a belly tank, but found this to be another straightforward method.  See page 5.

Edit by f6gal to remove bad link. 

[Bagger John - #3785]

My '08 Shadow was a manual.

This is a bit of a thread bump...

The CBR600 series (F1, F2, etc) use a petcock that is identical in form to the ones used on our Valkyries, but it's manual only - no vacuum diaphragm. A fuel pump is used with this setup as both a control (shutoff) valve and to pressurize the delivery stream.

It's possible to use the CBR petcock with our Valks. You must carefully remove the reserve standpipe from each petcock then put the Valkyrie's pipe into the CBR's petcock...or cut the CBR's standpipe to "Valkyrie" length as it's a good bit longer. The Valkyrie petcock filter screen and O ring are used here. Install the petcock in the tank, plug the #6 runner nipple, fit and wire up a Dan-Marc valve then reinstall tank and away you go. All the Valkyrie OEM fuel switch and trim pieces work with this petcock.

My I/S has a manual petcock. If it's not going to be run for any length of time (say, during winter storage) I drain the carbs and leave the bowl drain screws open. Also do this with my vacuum petcock equipped Tourer, though that one is getting a CBR valve when I disassemble the bike to do a front-end swap.

[Avanti]

My '08 Shadow was a manual.

This is a bit of a thread bump...

The CBR600 series (F1, F2, etc) use a petcock that is identical in form to the ones used on our Valkyries, but it's manual only - no vacuum diaphragm. A fuel pump is used with this setup as both a control (shutoff) valve and to pressurize the delivery stream.

It's possible to use the CBR petcock with our Valks. You must carefully remove the reserve standpipe from each petcock then put the Valkyrie's pipe into the CBR's petcock...or cut the CBR's standpipe to "Valkyrie" length as it's a good bit longer. The Valkyrie petcock filter screen and O ring are used here. Install the petcock in the tank, plug the #6 runner nipple, fit and wire up a Dan-Marc valve then reinstall tank and away you go. All the Valkyrie OEM fuel switch and trim pieces work with this petcock.

My I/S has a manual petcock. If it's not going to be run for any length of time (say, during winter storage) I drain the carbs and leave the bowl drain screws open. Also do this with my vacuum petcock equipped Tourer, though that one is getting a CBR valve when I disassemble the bike to do a front-end swap.

Are you using part number (16950-MBW-023) ?

[JimC]

Are you using part number (16950-MBW-023) ?

Yes, but last time I looked there were no OEM to be found. You can find knockoffs though, I bought one and can see no difference between the two.

Jim