Restoring a 1973 BMW R75/5 Motorcycle

December 31, 2006


Filed under: 1973, airhead, BMW, motorcycle, R75/5, restore — Penforhire @ 9:01 pm

That title sounds like a bit of optimism doesn’t it? Or maybe a new Seattle band? It is actually the original model of fuel tank petcock fitted to R75/5’s. This is the switch that turns on and off the flow of gasoline from the tank to the carburetors. This bike has two, one on each side of the tank. They are mounted strangely, to my way of thinking, with a reverse-threaded nut forming what I call a ‘union joint’ to the gas tank threads.

Here I did get a bit of bad news good news. The bad news is Everbest petcocks are not considered rebuildable. If they fail or leak you toss them. Oh, there are some curmudgeons I read about who just don’t give up that easily (pry them open, cut some cork, mash it back together) but the repair manuals suggest aiming for the nearest trash can. The good news is my dad must have replaced them along the way with petcocks from 1974 or later, which are Karcoma’s. They happen to be rebuildable.

Good thing because both are overly stiff and one of ’em, guess which side, feels like rubbing two pieces of sandpaper together. Here’s a picture of the guts of one of them before I started cleaning up the pieces. The castings have some crudely formed spots that are a biatch to clean.


The shorter of two nylon tubes rising out of the body is the “Reserve” position. This is a very simple way to guarantee a certain amount of fuel is in the tank when you hit reserve. I like this system. Your bike starts sputtering and you reach down to flip the petcocks to reserve. A lot of modern bikes with automatic petcocks, like my FJR, just have an instrument light or indicator that blinks at you and/or starts counting miles to let you know when you hit reserve. The problem is you might not notice this until you are too far from a gas station!

After disassembly I sort of understand how this thing works. The two metal rings just right of the handle create a “detent” feel at every 90 degrees, that happens to line up with holes in the black gasket, shown left of the handle. You can see two holes in the base of the handle. The center hole drains down to the carb while the other hole rotates around to the four positions. Looking at the bent handle position — left and right are both “off” positions (dead-ends inside the petcock), up is “on” (to the longer tube), down is “reserve” (to the shorter tube).

Riddle me this, Batman. Why are there holes in the black gasket for the off position? Why not dead-end them to the gasket instead of cavities in the petcock? I honestly don’t know the answer. Room for trapped fuel expansion comes to mind but is not correct because the center hole is always open to the carb. Huh. One of the mysteries of life.

Notice the black plastic cover with white lettering on the right side? Well this is just a friction-fit and I’ll have to ask Joe how to remove it without damage. The plastic is soft and easily damaged by whatever I used to pry it off. If it makes you feel better, I did less damage to the second one I disassembled. Before final re-assembly I need to refresh the white paint on the lettering.



  1. Thanks for the article and picture. It’s my understanding, however, that the LONGER of the two tubes is the reserve position and is adjustable as such… the shorter you make it, the less reserve. The shorter of the two tubes is the unusable fuel limit and is there to act as a crap trap so that you cannot draw fuel from the very bottom of the tank.

    In other words, when the valve is in the ON position, fuel is drawn through the long tube until the fuel is drawn down to that level. Switching to RESERVE then allows fuel to flow through the short tube until fuel gets drawn to that level… and you are effectively out.

    Just wanted to mention that in case someone is trying to adjust their reserve amount… adjust the long tube.


    Comment by Samuel Longiaru — May 2, 2008 @ 11:27 am

    • glad you were trying to help, but think about what is happening in the tank. The two tubes rise up from the bottom. the longest tube (highest opening from the bottom of the tank) will be above the fuel once the fuel drops below it so at that point it can’t supply anymore fuel..that is the normal operating tube.. now switch to the other, short tube because it’s opening is way down near the bottom of the the amount of fuel in the tank between the tall tube and short tube is the amount of reserve fuel you have. You could run on the reserve tank (short tube) all the time but would get no warning.. when the fuel falls below it you’re done. In a pinch you can slosh your tank violently to one side in the hopes of getting the bit of fuel from one side over to the other (and then maybe the new total fuel amount on that side is enough to rise above the top of THAT side’s reserve tube you might get another mile or two down the road.

      Comment by Kent Lauridsen — September 5, 2017 @ 6:26 pm

  2. You are correct about the adjustment. Fuel flows through the shorter tube in reserve, switched to when you move the petcock to reserve position.

    Comment by penforhire — May 4, 2008 @ 12:37 pm

RSS feed for comments on this post. TrackBack URI

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s

Create a free website or blog at

%d bloggers like this: