This is part of a continuing series. To start at the beginning, click here.

If you’ve been keeping up with my project bike posts, you know I’ve gotten the Nighthawk back to excellent running (and looking) condition. All I needed to do was spoon on some sticky new Bridgestone rubber and I was —

Oh crap, the petcock is leaking. (That’s a fairly accurate representation of how it went when I got the text from a neighbor saying my beloved Honda had a very stinky puddle of gasoline under it.)

It wasn’t much of a surprise, as the ethanol in today’s gas destroys rubber O-rings, especially older rubber that isn’t specially treated. So I did a quick look-up on to get the original part numbers, then called up my friends at Honda North America, just an hour away in Torrance, California, to see if they had any new old stock parts for me.

They did, and sent them on their way to me in short order. The petcock was only leaking badly in the Off position, so I was able to ride it the short distance to my boss’ house (again), since he has a lift and a plethora of tools.

Honda Nighthawk 700 petcock
Fuel was dripping from the bottom of the petcock, which meant the diaphragm (at the rear, which is on the right in this photo) was the most likely culprit.

Fortunately, since we were only servicing the lower part of the petcock, it didn’t need to come all the way off the tank. We just had to access the rear of it, which meant unbolting the tank and propping it up at the back. Ultimately, we decided to just remove the tank entirely to make it easier. This required draining the remaining gas into a container first, but it only took a few extra minutes and was worth it.

Honda Nighthawk 700 petcock
First we tried just propping up the rear of the gas tank, but it soon became clear it would be much easier to just remove it from the bike entirely.
Honda Nighthawk 700 petcock
The old diaphragm (right) and an O-ring for reference. Notice how the O-ring is bent and not circular; it was hardened and no longer pliable. Hence the leakage.

Less than an hour later, the petcock was reassembled, the tank was back on and yay! No more gas leakage!

Now onto those tires….

Yes, we finally got the tires on. Read the complete review here!

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  1. I have found that sometimes you can soak the whole petcock to fix this issue. I had this happen on a Honda XR dirtbike after it sat a few years. The dealer wanted $75 but I was advised to soak it which made the gasket material swell up again making it leak free.

  2. I enjoy reading of your adventures with the Nighthawk! 👍👍👍

    The petcock on my ‘93 Nighthawk failed a few years back and I finally tracked down a repair kit.

    As you know, another function of that device is to shut down fuel flow upon loss of engine vacuum (note the line to the intake).

    Happy trails and have an excellent Thanksgiving!

  3. A few drops of olive oil on a moist clean rag does a great job restoring those greyed out swtch blocks, plastics, etc.
    For near zero cost.


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