Testing petcock

[Guest]

I have a 79' 850. Most of the advice I have seen thus far around petcocks is if in doubt, replace. However it looks like the part for the 79' is a bit hard to find and expensive to boot. I know a handful of used petcocks came with my bike in the ol' box of parts that come with every project. This leads me to believe that it's likely that the petcock on my bike has been replaced, renewed or at least swapped. Is there any good tests to see if a petcock is working properly? I imagine attempting to actuate the switch by pulling a manual vacuum may work as a test. Anyone played with this?

As always with the depth of knowledge and volume of previous posts, I could have easily missed finding the thread that has all the answers to my questions. If so please do just post a link.

[tom203]

Get some clear vinyl tubing that fits on vacuum nipple on petcock. Mount the petcock to be tested on tank. Add gas to tank.
1: Put petcock in " PR" spot, does fuel come out ? It should
2: switch petcock to "ON" spot does fuel come out? It shouldn't
3: put your vinyl tubing on petcock's vacuum port and suck. Does fuel come out? It should
3a: stop sucking, fuel should stop flowing and there should be no sign/taste of fuel in the vinyl tubing.

Put little faith in 35 year old rubber stuff in petcock- once they see gas, they could vanish. What are the centerline dimensions of your petcock mounting bolts?

[tkent02]

3: put your vinyl tubing on petcock's vacuum port and suck. Does fuel come out? It should
3a: stop sucking, fuel should stop flowing and there should be no sign/taste of fuel in the vinyl tubing.

A better test, put the hose from the petcock into a container with a lid, fill the hose and then fill the container partway up by putting it on Prime for a little bit or by sucking on the vacuum line. The idea is to get the liquid in the container over the end of the hose, then mark how high the level is. Wait a while, like overnight. If the petcock seals tightly there won't be any more fuel in the container even if you wait a week. If the fuel in the container rises it is leaking and should be replaced or repaired.

The pre 1980 petcocks can be turned into manual petcocks with an Off, Prime, On and Res positions if you are interested. Actually Res and Prime will be identical. This also allows them to be used after the vacuum portion is no longer working. Easy to do. I wrote something up about it several years ago, but I can't find the thread now. I might do one today and show how it's done.

[robinelwood]

I also have a '79 850. First I rebuilt the original with a kit. Then I rebuilt the spare with another kit. Both worked sometimes, and sometimes the diaghpram just didn't pop closed and it leaked. Then I added a manual fuel shutoff. Eventually I spent the damn $98 dollars to buy an new OEM replacement through Ebay. There are a couple on there now for that price.... everrett powerspots, ocean county powersports are two... Money well spent, in my opinion.

[tom203]

That's a nice looking bike!

[robinelwood]

Keep meaning to make the picture smaller... but yes. Love that bike. Fixed a few things since then, too.