XJ Petcock Rebuild - PICS

For an XJ bike a working OEM petcock / fuel valve is an absolute necessity. Unless someone knows differently Yamaha uses the same petcock design for all XJ bikes; mine is an XJ550.

Other than dirt and gum from old gas the most common malady is a stuck vacuum operated Diaphragm / O-ring. Second most common fault is a failed diaphragm that will not hold vacuum.

Fine matter, oil and varnish can enter the fuel system and cause the Plunger / O-ring to remain stuck - unseated above the body recess preventing proper operation. Mine failed due to oil contamination of the diaphragm.

I let the parts soak in a good quality water-soluble automotive carb cleaner and a can of computer air to clear passages.

Here's the disassembly process in photos.














The key is to get every part nice and clean, and to rinse well. Use grease around the tap handle O-ring and on the 4-hole rubber valve grommet. Be sure to sandwich install the new diaphragm on both sides of the plastic block and tighten the 4 screws carefully.

Double check the spring is between the diaphragm's steel recess and the cover.

I use a thin layer of Motoseal on the tank mating surface O-ring and two screw faces. For best results let the assembled tank & petcock sit overnight before filling up with gas. The use of an inline fuel filter downstream is a great preventative against future carb troubles. Keep the shiny side up.

 

Great job! I'll be doing this in the coming months.

 

I just recently am a proud owner of a 1981 yamaha xj650. When I purchased the bike several months ago, it would not start. My uncle helped me clean the carbs and got it working. Then we found gas in the oil tank so cleaned that all up. A few weeks ago, I noticed my tank was low again and found gas in the oil again. I took apart my petcock and found out that I am missing the spring. Apparently when I took the petcock apart, the spring popped out and is missing. That is why it leaked again. I am finding out through others that vaccumed fuel taps are a pain. Do you have any suggestions? I think buying another petcock would be a waste of $ but if need be to keep it from doing so, I will have to. Any help to just find a correct spring would be greatly appreciated as well.. I can't seem to find any rebuild kits containing the spring either. Great pics btw, exact replica of my fuel tap

 

I believe Parts N More online sells an Petcock Adapter kit that will fit many Yamahas; XS and XJ models. You get a leak proof conventional Harley style fuel tap instead of dripping fuel. Its around $30 with S/H.

Georgefix on eBay sells a K&L XJ550 /650/750 Petcock Rebuild kit for $28. That's what I used in this rebuild.

If you find gas contamination in the bike crankcase, drain it and change it for new oil immediately. Do NOT start the engine if this is the case. That stuff is deadly to bike engines.

 

I've already contacted Georgefix...the kit does not include the spring which is all I really need. I get the feelin I should just by another fuel tap.

 

Any XJ petcock will have that spring inside. Plenty available on eBay ... try a local cycle repair shop or re-cycle yard.

 

Great pics

 

One addition. You need to make sure the metal surface where the diaphragm's seal seats is free from scratches and gouges. This is a common leakage point.

Great pics - thanks!