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Old 11-16-2012, 08:12 PM   #1
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Default Honda vacuum petcock questions

On the Nighthawk that followed me home recently, about the only issue left is cold starting. It barely runs for 3-5 minutes like it is starved for fuel, then it suddenly runs well on choke,and warms up soon after that. I cannot be sure if it actually fuel starved or is just cold natured.
So the thing is-- the Petcock has only On, Off, Res Even though it has a vacuum operated petcock,there is no PRIme position.
Maybe some other Honda owners have the answer to this. I have not had the bike long enough to get this sorted.
Thanks in advance
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Old 11-18-2012, 09:56 AM   #2
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Have you tested for fuel flow in the Res position?
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Old 11-18-2012, 10:07 AM   #3
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Quote:
Originally Posted by WintrSol View Post
Have you tested for fuel flow in the Res position?
No. The only test I have done is apply vacuum to the small line from the engine, and the fuel does flow. Also,the accelerator pump is functional,and that does supply a little extra on the cold morning start if twist the throttle before applying the choke. I'll get back to you on the RES thing.
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Old 11-19-2012, 04:46 PM   #4
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What year?
My Ascot (83) had the same positions (ON/OFF/RES) and it would not flow in ON or RES if there was no vacuum. I ended up converting mine to not need vacuum (was having other issues with it).

You can (probably, depends on construction) temporarily bypass the vacuum part by disassembling the unit and installing the spring on the opposite side of the diaphragm. After doing this, vacuum is no longer required in the ON/RES positions.
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Old 11-20-2012, 09:44 AM   #5
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No position will prime the carbs. I researched this on a nighthawk specific forum and they agree. It is really not a big deal anyway. Just pull of the vac line from the engine and apply a light vacuum to the petcock diaphragm, to fill completely empty carbs that have been off the bike for cleaning, for example.
To test for function,pull the fuel line from the carbs and crank the bike. I use a clear fuel filter, so I can also observe whether the fuel is present as well
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Old 11-20-2012, 05:18 PM   #6
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Or, you could follow Squisher's advice, and move the diaphragm spring. Vacuum-controlled petcocks are for riders who don't remember (or don't want to) turn the petcock to off, and develop a leaking float valve. Neither of my bikes have one, and I don't miss it.
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Old 12-15-2012, 07:39 AM   #7
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Default Figured it out

Sometimes an issue declares itself in a false way. It turned out that since I had put a pretty high concentration of Sta-Bil in the fuel and then started the engine cold a few times without riding the bike to get it up to full operating temperature---That caused a little black film to form on the spark plugs, and then the spark became weak.
These bikes are pretty lean running in the idle circuit to meet EPA,so you crank the choke on to make them start. All kinds of moisture comes out of the pipes, and a few of these starts to show your buddies your new bike and the plugs are fouled. I also suspect that the Sta-bil might contribute to plug fouling at higher concentrations....
Out comes the cleaner and a little sandpaper and the little beastie fires up again. So actually there is no issue with the petcock at this time, so I figure that I can still use the petcock turn OFF feature without disabling the vacuum feature.
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Old 12-15-2012, 11:03 AM   #8
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Sta-bil made my CB450 run poorly in the spring, and I had used the recommended dosage for storage. CBs are noted for fairly weak spark, and the Sta-bil apparently pointed this out. I now use Seafoam, and never have any issues.
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Old 12-15-2012, 02:21 PM   #9
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I haven't had problems using Sta-bil, but I don't go over the recommended amount.

The only think I notice is a thicker exhaust smoke at startup and it doesn't smell as good as untreated gas burning. There is almost a burning plastic-like smell to burned Sta-bil.
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Old 12-21-2012, 12:25 PM   #10
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i would try seafoam....its been very helpful to me...pour the whole can into the tank.
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