Petcock modification

Is there a way to modify the petcock so that I can keep the reserve but eliminate the vacuum line

 

Phoenix, what is the end result you are looking for?

The vacuum line is only to allow the petcock to work normally in the on and reserve position. The PRI will bypass the vacuum part of the petcock and let fuel in to fill the carbs after cleaning them. There are aftermarket petcocks out there that do not use the vacuum method that is OEM on our bikes.

I have had so many of the petcocks fail I change mine out for one that has "On, Off and Res" so that I can turn them off when not is use.

 

Buy a Pingel petcock and be done with it. The best I've ever used, simple, and very high quality.
Cheers, 50gary

 

I’ve been thinking about the same question. Spent a lot of time looking at the passages in the stock petcock and believe a slight modification to the back of the lever (removing a bit of metal with a Dremel tool) and disconnecting the vacuum line will turn the petcock from reserve-on-prime to off-on-reserve. If I’m right, it shouldn’t even require removing the petcock from the tank.

Haven’t done the mod yet because the tank is too full at the moment (oh boy, excuse to go riding!). I have a spare so I will try the experiment in the next day or two and post results and pics.

Obligatory model/year note: my XJ is a 1980 Maxim 650; I am not sure all models used the same petcock.

 

As promised, I did the conversion from vacuum to manual today. Took the bike for a nice long ride, long enough to verify that off, on and reserve functions work as intended.

So, if you want to do this conversion yourself, here's how to do it.

First, of course, drain the fuel tank. That's obvious, but my lawyer insists...

Second, remove the tank from the bike. Disconnect the vacuum hose from the petcock, plug it and stick it somewhere out of the way (I looped it down between cylinders 3 and 4 for now; eventually I will remove it entirely and put a cap on the cylinder 3 vacuum fitting). The spring behind the diaphragm will keep the vacuum valve in the petcock closed.

Third, remove the two screws that hold the face plate, wave washer spring and petcock lever/valve in place. You don't have to take the petcock off the tank to do this. You're now looking at the petcock body and its fuel passages.



The center hole goes to the main pickup in the fuel tank. The hole passes through the petcock body and into the vacuum valve chamber on the other side. The hole on the right goes to the reserve pickup in the tank. The hole at the bottom is the "priming" bypass, going around the vacuum valve and straight to the outlet.

There are also two passages cast into the inside of the lever:



In "normal" operation, the petcock works like this: in the "on" position (lever facing down), gas flows down from the main pickup into that center hole, then out the vacuum valve on the other side of the petcock body to the outlet. In the "reserve" position (lever facing forward), the straight passage in the lever connects the reserve inlet to the center hole, and gas flows by that path through the vacuum valve to the engine. In the "prime" position (lever facing back), the curved passage connects the reserve inlet to the bypass outlet, and gas flows around the (closed) vacuum valve to the engine.

We will now pervert this arrangement. Disconnecting the vacuum line leaves the vacuum valve closed all the time. The only way for gas to get out of the petcock is through the bypass outlet at the bottom. So, with the handle pointing back, the former "prime" position is now "reserve." Now we need to connect the center hole (the main inlet) to the bypass outlet. We do this by cutting away the metal in between the two passages:



Now, when the lever is pointing down, gas will flow in the center hole (the main inlet) and out the bypass to the engine.

When the lever's pointing forward, the solid metal blocks the bypass and the petcock is closed. Got that?

I cut the passage using my (Harbor Freight imitation) Dremel tool:



Clamped the handle in a vise and worked with the ball-end cutter bit. This was kinda slow work, took maybe fifteen to twenty minutes. It's important not to scuff up the face of the valve, because it needs to seal well against the gasket in the petcock body.

That's it. Put everything back together and you now have a manual petcock. Forward is OFF (which I believe is now a federal standard). Down is ON, and back is RES. I took the bike for a 70-some mile ride today, ran it onto reserve and verified that reserve works properly. So, if you want to go old-school and have a fully manual petcock on your XJ, now you can.

 

Very nice!

 

Very nice, but does this do anything to alleviate the problem of a leaky petcock? Have I not followed this correctly?

 

Mini, the original poster wanted to know how to convert the stock automatic petcock to a manually-operated petcock; nothing about a leaking petcock........

 

But why?

 

Some things are unknown, and perhaps unknowable.........

 

Maybe because sometimes the seats are pitted making the petcock trash; or you could mod it to be a manual and be on your way

 

In my case, just a matter of personal preference. I have had thirteen bikes with conventional petcocks and three with the vacuum-operated style (and one of them was a KLR, whose vacuum petcocks have a reputation for unreliability, so manual conversion is a pretty common modification). Fully manual is what I'm used to, and what I prefer.

 

Mmm, pitted seats would make it leak but not fail to open? This mod would still let it leak...