The why of auto fuel valves

Tips and Recommendations from Guru Mike Nixon

Moderator: Whiskerfish

Post Reply
User avatar
mikenixon
Early 'Wing Guru
Early 'Wing Guru
Posts: 984
Joined: Thu Dec 10, 2009 1:51 am
Location: Prescott, AZ
Contact:

The why of auto fuel valves

#1

Post by mikenixon »

Image

On the left in this picture is a Suzuki GSXR connecting rod and on the right a Honda CBX. Both suffered a situation commonly called cylinder hydrolock. This is the reason most powersports vehicles have a vacuum-operated fuel valve, or as Honda calls it, auto valve. On bikes lacking this valve, it is good idea to remember to always shut off the manual fuel valve. The CBX rod I replaced just a few months ago on a bike the customer bought not knowing it had this issue. The CBXs that do this often break the rod and throw it through the engine cases. More reasons to be habitual about your manual fuel valve is 1) it prevents your garage floor from be covered in dangerously-flammable fuel which worse, produces a hotbox of explosive vapor in your garage, and 2) on bikes lacking an overflow system (neaning all Gold Wings, for example), it prevents the engine's crankcase filling up with fuel and leading to main and rod bearing wear. Food for thought?
User avatar
Whiskerfish
President
President
Posts: 35164
Joined: Thu Feb 16, 2006 9:34 pm
My Album: http://www.ngwclub.com/gallery/v/wingmans/whiskerfish/
Location: Norfolk Va

Re: The why of auto fuel valves

#2

Post by Whiskerfish »

Should be required reading for any new owner of a old bike!!
"Agreement is not a requirement for Respect" CDR Michael Smith USN (Ret) 2017
"The book is wrong, this whole Conclusion is Fallacious" River Tam
2008 GL1800 IIIA "TH3DOG"
1975/6/7/8/9 Arthur Fulmer Dressed Road bike
1975 Naked Noisy and Nasty in town bike
and a whole garage full of possibilities!!

Psst. oh and by the way CHANGE YOUR BELTS!!!!
User avatar
Track T 2411
Honored Life Member
Honored Life Member
Posts: 8372
Joined: Thu May 29, 2008 9:37 pm
My Album: http://www.ngwclub.com/gallery/v/wingmans/Track+T+2411/
Location: Prairie du Sac Wisconsin

Re: The why of auto fuel valves

#3

Post by Track T 2411 »

My '79 Yamaha had vacuum petcocks, and worked great, until they didn't. Fuel leaked past the diaphragm through the vacuum lines right into the cylinders over winter storage. Fortunately I'm in the habit of checking the oil level first thing after any long layup.
I'm much more likely to forget to turn the petcock back on after a stop, lol!
ob1quixote
Titanium Member
Titanium Member
Posts: 460
Joined: Thu Oct 09, 2008 8:15 am
Location: Charleston SC

Re: The why of auto fuel valves

#4

Post by ob1quixote »

I can add that a gasoline/oil mixture in a crankcase can ruin the stator by attacking the epoxy that insulates the coil.

Was a whole lot easier to fix on a DRZ400!
User avatar
mikenixon
Early 'Wing Guru
Early 'Wing Guru
Posts: 984
Joined: Thu Dec 10, 2009 1:51 am
Location: Prescott, AZ
Contact:

Re: The why of auto fuel valves

#5

Post by mikenixon »

Track T 2411 wrote: Tue Mar 23, 2021 11:32 am My '79 Yamaha had vacuum petcocks, and worked great, until they didn't. Fuel leaked past the diaphragm through the vacuum lines right into the cylinders over winter storage. Fortunately I'm in the habit of checking the oil level first thing after any long layup.
I'm much more likely to forget to turn the petcock back on after a stop, lol!
That little problem did-in a lot of roller bearing crankshaft Yamahas...
User avatar
pidjones
True Blue Steel Biker
True Blue Steel Biker
Posts: 2541
Joined: Wed May 22, 2013 4:06 pm
Location: East TN

Re: The why of auto fuel valves

#6

Post by pidjones »

My problem has never been forgetting to turn the petcock OFF. Mile or so down the road I've been reminded to turn it ON, though!
ob1quixote
Titanium Member
Titanium Member
Posts: 460
Joined: Thu Oct 09, 2008 8:15 am
Location: Charleston SC

Re: The why of auto fuel valves

#7

Post by ob1quixote »

Also, as far as manually shutting the valve off, it did not effect the gas flowing through the valve and into the vacuum line on the DRZ400. I replaced mine with a suitable non-vac petcock.

But not before replacing 2 stators.
User avatar
delling3
Gold Member
Gold Member
Posts: 1167
Joined: Mon Sep 17, 2012 8:14 am
Location: Sterling Heights, Michigan

Re: The why of auto fuel valves

#8

Post by delling3 »

Yup, I owned/restored a '79 Yamaha XS-750, which was the first bike I owned with vaccumm operated fuel petcock. I remember that "everyone" scrambled to replace them with a manually operated valve, due to the propensity for diaphram failures (maybe exacerbated by ethanol fuels?). I am convinced that nothing is truly fool-proof (at least for this particular fool), I do think the simplicity of a manual fuel valve is nice - assuming that you actually use it!
XS 750F.JPG
You do not have the required permissions to view the files attached to this post.
User avatar
Sidecar Bob
Honored Life Member
Honored Life Member
Posts: 6541
Joined: Sun Jun 11, 2006 10:14 pm
Location: Kawartha Lakes, Ontario

Re: The why of auto fuel valves

#9

Post by Sidecar Bob »

Early vacuum petcocks (like the one on my '78 GS400) had single diaphragms with the vacuum line on one side and fuel on the other so when (not if) the diaphragm started leaking raw fuel would be drawn directly into the cylinder, causing all sorts of running issues. Unless it ran down the vacuum line while the engine was stopped and caused the problems mentioned above (this could also cause hydrolock).

Later vac petcocks (as on the CX/GL500/650 models) have a double diaphragm with the vacuum on one side of one diaphragm, fuel on the other side of the other one and the 2 connected together. A drain/vent hose is connected to the space between them so that any fuel that gets past the diaphragm will exit through it instead of into the cylinder.

Unfortunately, the later style (at least as used on Hondas) do not have a Prime position so if the float bowls are empty (or the fuel in them has evaporated below the level needed for starting as is common with CX/GL500/650 bikes after sitting for 4 or 5 days). The problem is that the vacuum petcock doesn't let any fuel into the carbs unless the engine is turning fast enough to produce enough vacuum to open the valve so you have to crank the starter long enough for the engine to supply enough vacuum to the petcock to open the valve for a long enough time for the float bowls to re-fill and it is pretty easy to run the battery down doing that.

The re are a few ways to deal with that:
- If it has been sitting for a few days and you expect that it will be hard to start, before you touch the Start Button, vigorously whack the throttle open several times to operate the accelerator pump and squirt raw fuel into the carb throats.
- If it has been sitting for a week or more and whacking the throttle doesn't help there are 3 ways to fill the bowls:
1) Disconnect the fuel line from the petcock, connect a funnel and pour 90cc of fuel in directly.
2) Disconnect the vacuum line from the petcock (this is the small barb farthest from the petcock's main body), connect a short piece of clean tubing in its place and apply vacuum (with a vacuum pump or just suck with your mouth) for about 10-15 seconds.
3) Disconnect the drain/vent line from the petcock (this is the small barb between the vacuum line and the main body), connect a short piece of clean tubing in its place and blow gently into it for 15-20 seconds.

Or you can replace the vacuum petcock with a non-vacuum one and eliminate the problem entirely.
Post Reply

Return to “Mike Nixon's Spot”