How to convert your petcocks from vacuum-operated to manual (with Off / Reserve / On functions. See below for Later Type Taps Conversion

XS750-2D and XS750E models

XS750E vacuum-operated fuel taps

The vacuum-operated fuel tap (aka ‘petcocks’) on these bikes have two main failings.

The first type of failure is when the diaphragm on the back perishes, the fuel in the tank leaks into the vacuum lines, down into the cylinders and eventually into the oil.

The second type of failure happens when the vacuum plunger o-ring perishes or gets obstructed with debris from the tank, allowing fuel to pass. If the carburettor float valves don’t seal perfectly, the carburettors overfill with fuel, then the airbox fills with fuel, and it can also pour into the cylinders (and the oil). There’s a theme here!.

Both result in a dead engine from very rapid wear and tear.
Here’s my copy of the famous manual fix with Reserve / Off / On functions.

Inside a modified tap (750E version)

First remove the taps from the tank. Tip the tank forward on its nose to get any excess fuel out the way. Take the taps to the bench and label them left and right side.

Take the front plate off each tap and swap them over so the plate marked with an “R” is on the left side tap and vice versa. Take the opportunity to check the holey washers are perfect, as they will now be the only thing holding back the fuel from the carbs.

Remove the vac outlet at the back, and the vac diaphragm spacer plate as shown. Set those aside.

Drill the centre orifice to 7mm in a vertical pillar drill or very carefully in the vice with a hand drill. The aluminium is very soft and the drill bit will draw through very quickly so drill at slow speed.

Use a standard 8mm plug tap through the hole in the centre.

Get an 8mm Allen headed bolt in the drill (hold the drill in a vice and use a file to machine it down) or lathe and turn the head down till it’s less than 10mm round. Make sure you put a taper on the inside edge as shown.

Cut the bolt to length and clean up the end of the thread. Make sure once fitted that it doesn’t protrude from the centre hole.

Using a fuel resistant gasket sealer or threadlock, coat the threads in the tapped hole and on the bolt and secure it from the read of the tap as shown with a 6mm Allen key.

Make a plate to fit the back by copying the sizes from the fibre spacer plate only drilling the 4 securing holes, and also make a gasket for it the same shape from a sheet of fuel-resistant gasket paper (the rubberised type is ideal). Fit the plate.

Take the tap handle and drill/mill the surface to look like the photo at the top. I used a drill with a 7mm bit.

Take the flat spring washer from the tap handle and using your thumbs, reshape it to be more wavy, effectively tightening the spring.

Grind or file off the “PRI” on the front plate and use metal teeps or a scriber to change this to “OFF”

Refit the handle, spring and front plate

Clean the gasket face on the tank and refit to tank.

Off will be the old “PRI” position on the plate (Reserve on this tap previously), “ON” will be on, and Reserve will be Reserve (Previously “PRI” on this tap prior to changing plates)


XS750F, XS850 and Later Models

Later taps will look like this photo below.

In order to convert this tap to fully operational manual with reserve there a few variations. The plate on the back and the removal of the vacuum diaphragm is still required as above. The modification to the tap handle is the same as shown above (even although the tap handles are at a slightly different angle, see below).

The bolt that is being fitted through the centre has less material to grip to because of an internal drilling in the body of the fuel tap. Great care must be taken when tapping this out. Also drilling it will not be necessary as the hole is already 7mm. Perfect.

You will also need to drill an extra hole into the body of the tap. See photo below. Drill the top right blank with a 3-4mm drill bit. Drill gently till the hole is clear (Approx 5mm deep)

Before modification
After modification

Make sure the drill bit is biased to the left, unlike mine! If you drill the hole too large, or too close to the right, it comes through the casing side. The material is very thin there.

Fit the centre Allen headed bolt from the rear as before with fuel tight sealant, fit the new back plate, seal it use a gasket, as in the write up above, and change the ID plates from right to left petcocks.

With the labels changed fromthe other petcock,all you need do is score off or re-label PRI as OFF. All the other labels needn’t change. Remember to change the ID plate from right petcock to left, not the handles. They remain with the original side.

Don’t forget also that the reserve filter tube should show the blocked part to the left on the right side petcock, and that it needs a blob of sealant to hold it in, and that the left petcock will be the opposite/ mirror image of this one.

This is the right side petcock prior to modification with the PRI shown on the left. Once the plates are changed over, it will be on the right side and will require scored off/ changed to OFF.

That’s it!