Motorcycle Repair: 82 honda cb650sc nighthawk not running, clean fuel tank, vacuum hoses

Question
QUESTION: hi, i purchased an 82 honda nighthawk cb650sc and it ran alright. i sanded the rust off of the gas tank and resprayed it. shortly after it stopped running. the battery died and i recharged it but nothing happened. i cleaned out the carbs and it wouldnt even fire. i poured a little gas into the cylinders and it fired but then stopped. i was wondering if you knew what i could do to fix it?

thank you for your time,
keith robinson

ANSWER: Keith, if the bike sat with gas in the carbs for a few weeks, the fuel probably separated and has blocked the idle jets. Try to drain the carbs and reprime them again. If the bike has a vacuum operated fuel valve, you will need to apply vacuum to the diaphragm line (the one from the carburetor) to open the flow of fuel from the clean fuel tank supply to the carburetors. The battery will run flat trying to hold enough vacuum on the valve to get all the carbs refilled.
If the idle jets are plugged up again, you will have to pull the carbs and clear the jets again. Be sure that the choke cable adjustment is correct, so the choke goes fully on(butterflies closed) when you are trying to start the bike.
Be sure the battery is fully charged. The ignition system is transistorized, so will shut down when the battery voltage goes too low... below 10v or so.

Bill Silver

---------- FOLLOW-UP ----------

QUESTION: Hi Bill

I couldnt find the diaphragm line. The repair manual i have says its the second line on top but theres only one and it goes to the fuel tank. I was wondering how do you apply vacuum to the diaphragm line.

Thank you again

Keith Robinson

Answer
Keith,
Keith, you can't "adjust the jets" to change the engine speed. Either there is a problem with the spark advancer sticking and/or there are air leaks in the inlet side of the fuel system at manifolds or vacuum hoses. There is a master adjustment screw for idle speed adjustment. The idle mixture screws generally are capped to prevent tampering, but can be accessed and richened up a little to smooth out idle mixtures and transition from idle to part throttle speeds.

Bill Silver


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http://www.bikebandit.com/houseofmotorcycles/honda-motorcycle-cb650-1982/o/m1976

Go to automatic fuel cock section.

Line from #2 carb to diaphragm needs to be activated. You can suck on the line with your mouth to open it or find a Mity-vac or similar hand vacuum tool at auto parts stores to make it a little more sanitary. :>)

Bill Silver