Motorcycle Repair: cb700sc stalling, vacuum leak, trickle charger

Question
Bill,

I just bought a 1985 Nighthawk cb700sc. It is in good shape but I believe it had been sitting for awhile with the prior owner. I noticed it is a bit cold blooded when starting up. I have filled the tank with fresh gas and there does not appear to be any rust in the tank. I've been putting a few miles on it nightly for the last week or so with no problems other than it being a bit cold blooded at first. Last night it started fine and I ran it about 1/4 mile from my house and it sputtered, lost power and stalled. I tried starting with the choke on for awhile then off for awhile. Finally with the choke on it started but was idling rough and would not seem to build up rpm's much beyond 2k. When I tried to gently apply throttle it would go from idle up to about 2k but any more throttle would decrease the rpm's back toward stalling. It then stalled again and stayed stalled until I ran most of the juice out of my battery. I had to push it home. I've got it hooked up to a trickle charger now but have not tried to restart it yet. Any helpful advice on where to go from here would be appreciated.

Thanks!

Answer
Brett, the "sitting up for awhile" is the first possible clue towards why you are having problems. Any fuels that have been sitting in fuel tanks for more than about a month will deteriorate badly and often the residues will plug up the idle circuits of the carburetors.

The fuel tank has a gas valve (petcock) that can be the source of problems on two fronts. There is a nylon filter screen up inside the fuel tank that could be plugged up. You have to remove the petcock to remove and check it, meaning drain the fuel tank first.

Secondary issues are with the vacuum diaphragm on the petcock which, when defective, can create a vacuum leak, as well as virtually no fuel flow. Two problems for the price of one!

I would drain the float bowls, using the drain screws at the bottom of the bowls. If you can catch the fuel coming out, look to see what kinds of debris, water or whatever comes out.

Fully charge the battery at about 1 to 2 amps, MAX. If you put a car charger on the battery at 4-10 amps, the battery will be fried. They only last about 2 years anyway, so you may want to get a fresh one.

I believe that the ignition system is battery powered, so if the charging system is weak/not working and/or the battery is tired/dead cells, the engine won't run, especially if the voltage gets down around 10v at the battery terminals. Once you get a hot battery in the bike, check the charging system, once you get it fired up again. REMEMBER the fuel flows through the petcock only when the engine is turning over or running! If you drain the carbs, the engine is going to have a hard time getting fuel back to the carbs, until it  has been spinning over for awhile. Get a spare piece of vacuum hose and put it on the diaphragm fitting and suck on it to get the fuel going into the carbs before you try to light it up again. If you have a vacuum pump device, like a MIGHTY VAC, then you can spare yourself the line sucking experience.

Well, there goes your weekend!

Bill Silver