Author Topic: Fuel Shutoff Repair ( ST1100 ) *  (Read 5714 times)

Offline KoTAOW

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Fuel Shutoff Repair ( ST1100 ) *
« on: February 02, 2008, 06:28:21 PM »
Taken from Mike Martin's web page: [URL updated]


Fuel Shutoff Repair

As you know, the ST1100 has an automatic fuel shutoff valve which is opened by engine vacuum. Sometimes this valve fails. One mode of failure is a cracked diaphragm, which causes fuel starvation. Another failure mode is a small fuel leak. This is usually detected by the odor of gasoline. Careful inspection of the valve will show traces of fuel residue at its bottom.

Fortunately, parts are available to repair the valve. Tim Shevlin has written up a procedure for installing the kit:


After being informed that the K & L diaphragm repair kit, p/n 18 4356 is the only one available, (from CA Sport Touring and others) I set out to develop an installation procedure that works every time. Based on just-completed installation of 3 kits, there seems to be a lot of inconsistency in the size of the large replacement diaphragm, with 2 of the 3 being noticeably 'too small', requiring repeated attempts to achieve a perfect vacuum seal. (Sealing of the smaller fuel shutoff diaphragm in the kit is not a problem, although, ironically, it is the one that causes gasoline leaks in original valves). The following is a workable procedure that takes 10 minutes with luck, but much longer if you happen to get a kit with a severely shrunken diaphragm.

Before teardown, obtain for testing purposes 2 ft. of plastic hobby tubing or whatever fits the valve vacuum port. Remove and set aside the valve mounting bracket until the valve is reassembled and tested. Use a magic marker to mark the three valve sections in such a way that the parts can be reassembled in proper alignment.

Complete valve disassembly, noting the shorter screws on the non-bracket side. Use finger pressure to pry apart the plastic diaphragm stiffener and two diaphragms from the small metal anchor fitting.

Assembly-- Install the stiffener (spring recess out) and two diaphragms in the anchor fitting, with the thicker groove on the large (vacuum) diaphragm facing inwards towards the fitting. Fill the large diaphragm retainer groove in the center casting with a suitably sticky grease, such as fiber-type wheel bearing grease, then pass the small diaphragm through the center casting hole, shaping it to fit its mating groove. Press the large diaphragm edge into its groove, discarding extruded grease. If the diaphragm appears to be hopelessly too small, center it the best you can and proceed:

Key Step: Lay the center casting with its installed diaphragms on a flat surface, large diaphragm up with the spring sitting in its recess. Properly orient the vacuum port on the metal cover and with one hand lower the cover onto the spring, pushing straight down until the two castings touch firmly together. While continuing to hold the two castings tightly together (against spring pressure) with thumb and fingers, pick up the assembly and with your free hand, connect the test tubing and cycle just enough vacuum to observe if the diaphragms move. If OK, insert the assembly into the fuel shutoff casting, making sure all parts are maintained in as perfect alignment as you are capable of. install and tighten the 4 retainer screws a in progressive sequence. You must keep the spring-loaded parts tightly together by hand until the screws are tight.

Testing: Using a Mityvac or test tubing and tongue, pull and hold a vacuum at the valve, checking for no bleed down by peering into the fuel outlet to observe action of the rubber valve. If aligned correctly, the valve will repeatedly pull straight away from its seat when vacuum is alternately applied and released. Next, block the fuel inlet and pressurize the outlet, checking for no audible air leakage. Finally, pressurize the inlet, with outlet open and no vacuum. A small amount of air leakage across the closed valve is normal, but any seepage should disappear some time after the valve is filled with gasoline.

« Last Edit: June 06, 2017, 10:38:19 AM by John OoSTerhuis »