petcock problem 81 GS850

[DaveDanger]

I completed reassembling the petcock on my 81 GS850 this afternoon, with a rebuild kit obtained from my local 'Zuki dealer. Basically a new diaphragm, gasket for the diaphragm plate, an o-ring for the revolving port shaft, and an o-ring for the base of the petcock to the tank. (My existing diaphragm was so stiff that vacuum couldn't flex it).

At any rate, it now seals and works fine in the "ON" & "RESERVE" position, but I couldn't make it flow fuel in the "PRIME" position. Had to prime the carbs with my external tank that I use while adjusting carbs, then reconnected the bike's tank and lines and cranked and drove the bike around a bit. I had a tiny spring (resembling the one in the tail end of a ballpoint pen) left over that I couldn't figure out the location for. It occurred to me as I sat and read the replies on someone else's petcock post, that that spring in some fashion mechanically pushes the diaphragm open to allow the fuel to flow in the "PRIME" position.

Way to go guys! you've solved my initial problem and I never even posted the question!!

Of course, I still have to figure exactly where that danged spring goes. I played with it for the better part of an hour to begin with, and it never "fell magically into place". Anyone have a fresh memory of the order of assembly internally?? (Where does that little spring go?!?!?)

Thanks!

[cloudbreakmd]

In the middle of the diaphragm behind the small metal piston (if your kit came with one). There should be a "hill" in the middle of the diaphragm that the spring fits behind.

[cloudbreakmd]

Spring is 92081 in picture.

[DaveDanger]

Good picture there cloudbreakmd... but it doesn't perfectly match mine The spring you indicate is there, and it's correct. That one I have, and it's mounted correctly. That one pushes the diaphragm and piston into the closed position. I have a second spring, smaller in diameter but stiffer, that was in my storage container when I reassembled my petcock. I suspect it is supposed to be on the other side of the petcock... it seems like it should be inside or under the rotating port valve (11009 in the diagram, although mine is a tapered cone and longer). Seems like it should somehow or other push open the diaphragm piston when the lever is rotated to the "Prime" position, and since my spring isn't there, Prime does nothing. *Sigh*

Thanks for the help, and I'll continue to search.

Where by the way, did you locate that diagram?

[cloudbreakmd]

http://www.morrowmarsh.ca/concours/techpages/petcockkit.htm

Wait a minute... Read the words below the picture in the link... I dunno, maybe helpful...

[DaveDanger]

I followed your link, and it took me to a diagram I had just departed. I did check the words below the picture, and they do point correctly to a spring that I do have, and is installed correctly. But I have a second spring that is not illustrated in that photo.

That diagram is about 95% identical to my petcock, but is a tiny bit different.

[Boondocks]

Re: petcock problem 81 GS850

...At any rate, it now seals and works fine in the "ON" & "RESERVE" position, but I couldn't make it flow fuel in the "PRIME" position. Had to prime the carbs with my external tank that I use while adjusting carbs, then reconnected the bike's tank and lines and cranked and drove the bike around a bit. I had a tiny spring (resembling the one in the tail end of a ballpoint pen) left over that I couldn't figure out the location for. It occurred to me as I sat and read the replies on someone else's petcock post, that that spring in some fashion mechanically pushes the diaphragm open to allow the fuel to flow in the "PRIME" position....

My bike is a 1980 GS1100ET and has no reserve (fuel gauge instead), but On and Prime settings only. Nevertheless, the function of the petcock may be similar to your bike. In that case, I am quoting the Suzuki Service Manual as to the petcock function. Note that the Prime function does not activate a spring to close the diaphragm, but causes the air return orifice to close. This creates a negative pressure independent of the engine and causes the diaphragm to remain open. In this case, there is only one spring, that which closes the diaphragm when negative pressure is not present. It could be that this air return orifice is clogged.

When the engine is not running and the valve in
the ON position, the fuel valve is kept in the closed
position by applying pressure utilizing a spring so
that no fuel will flow to the carburetors.
When the engine is engaged, a negative pressure
is generated in the diaphragm chamber "C"
through the vacuum (negative pressure) pipe
which is connected to the carburetors, and builds PRI
up a negative pressure which is higher than the
spring pressure so that the diaphragm is forced to
open the fuel valve and thus allow the fuel to flow
to the carburetors.

On the other hand, setting the valve in the ON
position keeps the air return orifice open.
Negative pressure does not accumulate on the
diaphragm at the time of engine stopping, and
then the spring pressure actuates the diaphragm
to move back to its original position and closes the
fuel valve.

However, setting the valve in PRI position with a
screwdriver causes the air return orifice to close,
resulting in negative pressure in the chamber "C"
under the diaphragm. This negative pressure
doesn't allow the fuel valve to close and therefore
the fuel keeps flowing to the carburetors.

The Clymer manual had no details of the petcock. If you would like a PDF copy of the Suzuki manual page with parts illustrations as well as the quoted text, PM me your email address and I'll send you an attachment. Looking at the numbered illustrations should make it easier to understand the petcock operation.