5.02.1 Vac Petcock Conversion

From 650wiki

Jump to: navigation, search

< Back to 5. Fuel System

This method allows you to convert the two vacuum operated petcocks to manually controlled petcocks. No parts or special tools are needed to complete this conversion. The conversion assumes that all the seals and gaskets are working properly and do not leak or bypass. Note that the right petcock has an R cast into the valve body, an R stamped into the faceplate and an R cast into the selector switch. The Left petcock is blank. This is how you tell which is which.

The petcock (valve)has two major systems. The manual selector switch on the front to select Prime, On or Reserve is the one you use. The second system is the vacuum operated valve that the engine uses to turn the fuel supply on and off. Since there is no OFF position on the selector, a bad seal on the vacuum operated valve can cause fuel to leak or flow into the carbs when the engine is at rest. If the floats in the float bowls are not working or if one of the check valves in the float bowls leaks, the gas will pass thru the carbs, into the cylinders, past the rings and down into the sump, dilluting your oil and filling up your crankcase. If you want to assure yourself that you will not suffer this fate, or if you have modified your engine to use differernt carbs or intake manifolds and would like to eliminate the vacuum operated system, here is how you do it.

The petcocks do not have to be removed for the first part of this modification, but it is highly recommended. If not, then the tank must be empty or lying such that gasoline cannot flow thru the petcocks while you are working on them. Remove the two screws holding the selector position indicator plate (faceplate) and remove it. Under it is a wavy spring washer. Remove it. Now carefully wiggle the selector switch until it lifts from the valve body. You may need to spray a little WD-40 on it to make it easier. Set these three components aside and repeat the process on the other petcock.

With both apart, this is a good time to inspect the rubber port seal inside the valve body. It has four holes that line up with the ports or blank ports in the valve body. If it looks worn, torn, or cracked, replace it.

Now,take the selector switch and faceplate from the left petcock and install it in the right petcock. Don't forget to put a little WD-40 on the O-ring round the outside diameter of the selector switch. Also, don't forget to put the wavy washer between the selector switch and the faceplate. Do the same with the remain parts on the left petcock.

If you want to be assured that your vacuum operated valve will not leak, you can remove the four screws from the back of the petcock to remove the vacuum valve assembly. Be careful and peel the rubber diaphragm/gaskets from the flanges. Slip the valve assembly from the plastic spacer plate and then reinstall the valve with the backing plate only and no spacer plate. This will put added spring pressure on the valve seal to assure no gas will leak by.

You are now done. The faceplate markings work as follows:

PRI which was PRIME is now PRIMARY for Primary fuel supply.

ON is now OFF.

RES is still RESERVE fuel supply.

Don't forget to plug the vacuum ports on your intake manifolds if they are still there. There is no need to plug the vacuum port on the back of the petcocks.

Easy, huh? Plus, you now have a simple anti-theft device. With the fuel selector in the ON position, it is really off and a thief won't get very far if he manages to start your bike and drive off. See the following images of how the petcock works in the new manual configuration]]Image:Petcock Parts.jpg Image:Petcock On.jpgImage:Petcock Off.jpgImage:Petcock Resv.jpg

Lee's Version

My own version of this vacuum to manual conversion works as follows:

Remove and discard the piston bellows gasket and spacer plate. Reinstall the piston with a good o-ring and a much stiffer backing spring or spacer of some sort (bolt, etc.) that forces the piston into the fully seated position as you bolt the petcock halves back together with a new gasket that you've made.

PRI is now ON. ON is now OFF.

RESERVE is still RESERVE, but you will have to do two things to actually have access to the RESERVE fuel and its full flow with this version of the conversion:

Flip the filter tower from its original orientation (requires some filing and fitting), and file away the stop tang from the chrome face plate so that the lever can be pulled up to the 11:00 position (left side). No filing or shortening of the lever itself is required.

Now is also a good time to lap the back surface of the lever plate smooth of any corrosion or oxidation for smoother operation.

Don't forget to stub off the intake side of the petcocks with short screws of proper length and pitch. And hang onto the vacuum barbs for future use. They're a perfect fit for the vacuum ports when balancing your carbs.

Lee Callicutt

Plugs for the vacuum ports

An easy & safe way to cap the vacuum ports of the intake is to use a 3/4 inch length of the original rubber tube and the ball of a ball bearing to keep it airtight.


Personal tools