QuestionI just bought a 1978 Honda Hawk CB400t. This is my first bike. I had it about a day when the problem started happening. I was driving about 40 in 3rd (4 speed i think), when I pulled on the clutch put it in neutral while braking. The engine went past idle to dead. With the choke on it runs, but rpm's are high. It stays on if i shift fast enough to the next gear. Sometimes it stays on if I don't go too high with Rpm's. Any thoughts?
AnswerJonathan, buying a 30 year old bike is a bit of a challenge for a first machine.
http://houseofmotorcycles.bikebandit.com/parts/Honda/CB400TI/1978/2330625
Looking at this illustration, you can see a sideways cover and rubber diaphragm assy. The diaphram is part of an air cut (anti-backfire) valve and when the rubber tears or gets a pinhole, you get an air leak that will cause stumbling and lean mixture on that carb. A check online should find some aftermarket diaphragms that are a lot cheaper than what is shown on this site (Honda pricing).
http://houseofmotorcycles.bikebandit.com/parts/Honda/CB400TI/1978/2330640
This illustration shows a plastic sleeve filter, which is located up inside the fuel tank. Removing the petcock (fuel valve) will allow you to gently remove the filter sleeve for inspection. If there is signs of rust or corrosion inside the fuel tank, the filter will catch most of it, but if overloaded, will block fuel flow to the carbs.
Before you go chasing your tail, do a tune-up on it, which includes checking valve clearances, replacing the spark plugs and checking the spark plug caps for resistance values around 5k ohms. You can use an inexpensive auto parts store VOM to check ohms values, as well as voltage to the battery, etc.
NOTE: This engine has a CDI ignition that is self-powered. If you can get the motor to spin over, it should make its own sparks and run the engine, even with a dead battery. DO NOT run the bike with a dead battery, as it needs to be fully charged to dampen out the output from the charging system. Otherwise, you can get overvoltage problems to the lighting assys. These engines were known to have issues with the pulser coils, which are part of the charging system stators.... hard to find and very expensive!
Check all the electrical connectors on the bike and make sure all are clean and tight. The whole fuel system must be clean and have fresh fuel in it. Check the gas cap for venting holes that could be blocked. You should see something around the edge of the gasket area.
Using the choke to keep it going tells us that there is a lean condition and/or fuel flow restriction somewhere, but do the basics first, as there needs to be a balance and integration of all systems, so the bike can perform properly.
Make sure it has plenty of clean oil at all times.
BTW-You should have about 5 speeds in that transmission.
Bill Silver
