'91 - '03 750 NH petcock; mod to make manual?

[slmjim n Z1BEBE]

Petcocks are vacuum operated. Petcock diaphragm is opened to allow flow by vacuum pulses from #2 cyl. No 'prime' position that bypasses the need for vacuum as on many other vacuum petcocks.

When the engine is running, vacuum pulses are frequent enough to essentially hold the diaphragm open continuously.

If carbs are drained, much cranking can be necessary to allow enough fuel to pass with the petcock in the ON or RES position because flow is only briefly intermittent during each vacuum pulse. This can be overcome at dry startup by applying vacuum to a hose attached to the vacuum port on the petcock continuously, either by mouth (think sucking very thick milkshake through a straw) or, using a vacuum pump. Doesn't take much; ~ 3 in./Hg or less will do it. Only takes 60 ~ 90 sec. of continuous vacuum to fill the float bowls.

Some OEM petcocks also have a port on the petcock diaphragm casting that is vented to atmosphere. The same effect can be realized by applying gentle pressure to that port, which opens the diaphragm. Again, doesn't take much pressure or time. Think blowing something stuck in a straw out of it.

Let's assume a '91 - '03 petcock is otherwise operational. Has anyone tried modifying the needle on the diaphragm to allow fuel to flow when the petcock is in the ON or RES position, yet stop flow when OFF? In other words, make the petcock manual instead of vacuum?

Good Ridin'
slmjim & Z1BEBE

 

[Biebs]

I tried to get a manual petcock because I am old school - turn off the gas when parked. Mine petcock leaked. Could not find a manual to replace the vacuum petcock. Ordered and tried a rebuild kit - still leaked. Converting the stock petcock to manual may be an option.

Ok - solution

eBay has petcocks for reasonable prices 2 years and no leaks.

https://www.ebay.com/itm/146820019430?fits=Submodel:CB750|Year:1992|Model:Nighthawk+750|Make:Honda&_skw=petcock&epid=14024186844&itmmeta=01KT4MJGJ6V2BM7FMFRRWKN70K&hash=item222f27b4e6:g:KvQAAOSwMKdjtF9Q&itmprp=enc:AQALAAABAGfYFPkwiKCW4ZNSs2u11xD7TN3Ozi38KV1+TPt73jWgVgD8Ygh4RC6rl7+z+SgAX4IPT0rzA2gEG5+RTMwY53+ywQp9XhCYgWbgqqEpvcA7F72ECugoXVaL0t7ndDLw+m1j72h0aHeQRMd4leTwG--oxRYLeazJjdr/WARmQ4NuF85t9zk7W0pVH+53dIbyqoSLya2q4py1ZuaXSBx8q0IWDGj+wLO3XfPLNMmF+dWf+RItQI4/C/rrKfa2QXX4SIv318rhQORIbVGIeZm5Oqbdh+eO+9ntPQSGg3fEFfoIldiribsqT6pa2vHUmXw+UBrg6pKaNhRuf+D6GdJA1Ik=|tkp:Bk9SR6aJypTRZw

 

[slmjim n Z1BEBE]

^^^ Doesn't answer the mod-to-manual-operation question ^^^

We might spring for a cheap one from fleaBay to test the mod.

Good Ridin'
slmjim & Z1BEBE

 

[Dewe]

I have an 01 Honda Nighthawk,and I ALWAYS turn the petcock to "off" about 30-45 seconds before I end up shutting off the bike after each ride! By doing so,the engine drinks just enough fuel out of the float-bowls so their not completely empty,but aren't completely full either (which can lead to badly worn rubber-tips on your carburetor's float-valve needles and will cause them to leak over time.)

Beside that fact though.............if my bike has sat long enough,and the bowls need quick filling......You can either do it through the vacuum hose hooked up to Cyl. #2 OR,I choose to hook up my brake-bleeder gun to the vacuum outlet on the petcock and pump it a few times while I watch the fuel flow quickly through my see-through Fuel-filter,on it's way down to the float-bowls! It's handy and takes very little time to do!