Motorcycle Repair: 1980 honda cb900c, regulator rectifier, throttle cables

Question
i recently was giving a 1980 honda cb900c from a family memeber. it was her husbands and he recently past away so she had no use for it and gave it to me. well i got it it was assembled except the gas tank wasnt secure seat was off, frel lines were disconected. i took out the carbs and cleaned them and reinstalled and reran the fuel lines. i went to start it and it wouldnt. i discovered i wasnt getting vaccum to the diaphram i tried my best to seal the vaccum line up and with a little pump from the vacuum pump to fill the carbs it will start and run from 3 minutes to 9 mostly around five minutes then it dies out and then its hard to start for like 10 mins then sometimes when i start it, it kicks up like the throtle is wide open i need some help here to get it back in shape in time for a christmas present for my fiance's father. any ideas? would using a battery charger as the main power source affect it. the battery holds a little charge but i dont want to buy anything until im sure it will run thanks for any ideas

Answer
Dan, the vacuum fuel valve is often a problem for these bikes. Check to see if the diaphragm for the fuel valve isn't failing/leaking internally. IF you can hold vacuum on the line with a Mity-Vac or similar, then you can rule that item out. Otherwise, you will have to bypass the fuel valve or buy a new one.

The choke system is designed to provide a fast idle, when you pull fully out, then reduces as you push the choke cable back in. If you have the choke on and the fuel levels are low, then then engine leans out and tends to race away. Of course you could have throttle cables hanging up, too.

Running on a battery charger may damage the charging system or some of the lights/electronics, which are dependent on a constant and steady source of 12+ volts. Trying to start it with a bad battery pulls the voltage down to the spark units and the regulator rectifier. The charging systems on these bikes can be a problem, too. Check the condition of the stator plug connections to the harness. The rotors tend to fail on these models, too. Aftermarket replacements are available for around $120...

Make sure all your vacuum lines are going to the right places. You should get a vacuum signal from the carbs to supply the fuel valve diaphragm. Either you have the wrong lines hooked up or the vacuum port is blocked still.

Bill Silver