1972 cb750 four petcock outlet problem

[chen4745]

hello!! every one i got a 1972 cb750 but my petcock got some problem with it !!
these two outlet float rate are different so two of my carbs can't get enough fuel...
i've disassemble and clean up petcock and put it back again but same thing happen again?????
i just wonder is that a design problem or i've to buy a new petcock or any why to fix this problem???
Thanks for Helping....

[shultz_1978]

Are you for sure it is in the petcock and not the carbs? Have you checked the fuel filters?

You can check the fuel flow through the carbs with the drain screws on the bottom of the carbs. Use a gas can or something to put your drain hoses into. Then open the screws  all up and look to see they are all flowing gas.

[TOOLS1]

I have never had one of those old style petcocks apart, so I have no idea of how they are set up. I wonder if it might just be the way they are designed, so that both outlets flow from the same chamber, and that one outlet would let the gas flow before the other.
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[chen4745]

hello thanks for help!!! but in these case i've to explain about the carbs
i've use a container with fuel let the carbs to get fuel from it and test ride for few kilo but it ran normaly and smoothly it;s a good bike,but with tank it become different anyway at last i found out the problem come from tank or petcock...
than i remove my tank and fill up fuel let petcock useing naturly gravity to check out that two outlet floating condition but it came out different so just can't figer out what happen?????

[MarkPBG]

I have a 72, and i have had my petcock completely apart. The insides are just a hollow chamber where there's a small brass screen as a filter, then below that the bowl and the two nipples. Theres nothing in there that regulates the flow rate, the flow regulation is all done at the carb when it meters the fuel. One of your nipples must have some debris clogging it. Most likely the forward nipple since the path of fuel flows by the rear nipple to the front nipple. They're both fed by one hole inside the bowl. A whole new petcock is around $30 plus shipping if you can't get the debris dislodged with a blast of compressed air. If you're careful, you can disassemble it to clean it and put it together with the original seals, but I would have a new seal and screen handy in case the old one rips.

The screen is Honda part number 16957-268-010, and the o-ring is Honda part number 16997-273-006
Oh, and the book says to use 1/4" fuel line, but that's a bit loose and can leak, 3/16" fits, but is REALLY tight, and will fail when the hose degrades to the point of cracking. If you can find 6mm fuel line, I think that is really the correct size for this application.

Here's the fuel path in the petcock, as you can see, there's nothing in there but the o ring and screen.

[Lucky 1]

One of those fuel inlet holes you have shown is reserve.
I should not flow fuel until selected. It may flow less if the tank is nearly empty and sloshing.