Topic: Cb750 K2 Petcock blockage

[jgary]

Hi.  I'm a long-time lurker, first time poster.  My neighbor gave me a 1972 CB750 recently, and I'm trying to get it running again.  The bike sat for 3 or 4 years outside, but covered.  It has two fuel lines coming out of the petcock, but only one delivers fuel.  I disassembled the petcock but for the life of me (at 10:30 last night) couldn't figure out the path that the fuel takes to get from the big tube that goes into the tank through the petcock and back out through the fuel lines.  Anyone have an exploded picture of the flow?  Thanks.  Oh, if you're really bored you can check out my verbose log of work on this bike and another CB750, have a look at www.4buster.org.

John.

[Bob Wessner]

John,

Re: your web site, very nice. I like your reference to the carbs being an ongoing saga, boy, can I identify with that! Judging from the number of posts related to carbs on Greenspun site that we are trying to move over, I think others can also.

[Jonesy]

The pipe going into the petcock is what the bike feeds off of while the lever is in the "on" position. There is a hole in the top of the petcock where is mounts to the tank. This feeds the bike while running on "res(erve)". Both have passages to the part where the rubber washer is sandwiched between the petcock and the selector lever. The fuel continues past the selector into the filter/sediment bowl, and then out the hose nipples to the carbs. The simplest way to determine the porting is to blow a bit of compressed air through the pipe and see where it blows out. If the petcock is plugged up, take it apart and soak it in carb cleaner. Put it back together with a petcock rebuild kit and new mounting hardware and you're back in business!

[jgary]

Here's a similar description from the VJMC Mailing list:


John,
I don't have a digram, but the flow is relatively simple to understand.

The tall pipe at the top of the petcock is the main fuel supply. Behind
the switch lever you'll find 3 holes. One is the tall pipe (main fuel)
the other is the reserve supply and the 3rd leads to the center hole
on the bottom of the petcock in the sediment trap. The fuel is pushed
out through the screen and then to the lines that lead to carbs. The
fuel selection is performed by the cutout in the petcock lever
'joining' one of the delivery pipes in the tank to the center hole
in the sedement trap.

Ellis
 

[jgary]

The only compressed air I have is internally generated, so I ran a pipe cleaner down one outlet to the other, then back again.  I had the petcock bowl on at the time, and when I took it back off there was a bunch of crud in it that I knocked out with the pipe cleaner.  Reassembled the petcock incorrectly, mounted the tank, put in some gas, and it ran out of the petcock in a stream.  Not a dribble, a stream.  Once the tank was drained I took the selector switch off and put the spring washer on top of, rather than behind, the selector switch.  Reassembled, no leaks, and a few minutes later the bike that hadn't run in 4 years was started!

John.

[Jonesy]

Good deal! Another victory for our side!!!!

[Bob Wessner]

After reading these posts re: the petcock, I'm now wondering if I assembled mine correctly last year. I get the overall idea of the flow and change between on and reserve, but now that I think back, I'm not sure I paid any particular attention to the orientation of the rubber gasket with the holes. The petcock works now, but now I'm a little concerned I may not really have a reserve. Can the rubber gasket with the holes be installed incorrectly so fuel flows, but maybe no reserve??

[Jonesy]

If you used the rubber seal with 4 holes, you can't install it wrong. Of course, now I'm wondering... did the K0 use a slightly different petcock? I think the unused hole in the seal goes over a small projection in the petcock body to keep it from spinning when you move the selection lever. I don't think there is a way to put the lever on wrong, either... Of course, you can quickly ckeck buy pulling off a fuel line and moving the lever to the ON and RES positions. If you get fuel flow out of both, you're OK.

[Bob Wessner]

Thanks Dan. I wasn't sure the flow test would be valid unless the fuel level was actually below the tube top in the tank, but I guess it makes sense that it would be.

[Jonesy]

With the reserve inlet at the bottom of the tank, you can run through the whole tank on reserve... and then walk the bike to the nearest gas station. This happened to my buddy on his GoldWing after he forgot to move the lever back to ON after a fill-up. It concked out in a rough part of town, too!