Motorcycle Repair/starting problems
QUESTION: I have a 1982 Honda CB900C with approximately 65000 km. The bike was starting well when I pulled the bike into the garage to fix a short in the instrument lighting. Now the motor will turn over, and very rarely the bike tries to fire, then nothing.
I've been through the electrical, it's getting fuel, I've got spark from all four wires and the sequence is correct. According to my repair manual the next possible culprit is the spark advancer. I have so far been unable to get the cover off, but am sceptical that this is the problem as the bike was working well before.
Any suggestions as to other possible causes?
The spark advancer is seldom a problem, but still within the realm of possibilities. The spark units get their timing signals from the pulse generators on the mounting plate, but there isn't much to go wrong in there generally.
You will probably have to use an impact driver to remove those screws if they have never been out before. It is worth having a look just for peace of mind, though. The advancer has mechanical weights and springs like the old points/condenser systems which can wear out over time.
If the automatic fuel valve diaphragm is leaking you will get little fuel flow and an air leak all at the same time. Check your vacuum lines from the #2 carb over to the diaphragm and see if you have any air leaks. Apply vacuum to the diaphragm to see if there are leaks.
Try draining all 4 carbs into a little cup to verify that all four are getting enough fuel to run off the idle circuits. To refill them apply vacuum to the fuel valve diaphragm either with a vacuum tool or just suck on the line for a minute (assuming the diaphragm is holding a vacuum signal). All four carbs should be refilled again.
Are you seeing unburnt fuel on the plugs after you crank it for 30 seconds or so? If the plugs are coming up dry then you have fuel flow issues. If overly wet, they could be fuel fouled now. Try a fresh set of plugs.
When all else fails start at the beginning... check compression first ... should be about 170 psi all the way across. Adjust the camchains for the intake and exhaust cams.
Make sure that the spark units are still okay, back by the battery. The insulation gets all gummy when they get hot and/or old and become unreliable.
Always start out with a fully-charged, load-tested battery. With the headlight sucking juice off the battery when you turn the ignition to ON, the battery voltage starts heading south right away. If voltage gets down below 11 volts the electronics start to shut down. Will assume that your wiring repairs are not interfering with some other function of the bike. When you start pulling wiring connectors apart, sometimes similar looking wire get plugged into the wrong colored circuits.
The charging systems on these bikes have been an issue for many years... bad rotors, worn out brushes, overheated stator connectors and burnt out reg/rectifiers are all common problems, so insure that the charging system components are all up to specs.
The intake manifold rubbers tend to harden and die over time. Not a bad idea to replace them, if you have to pull the carb rack.
---------- FOLLOW-UP ----------
QUESTION: I tried an impact driver to remove the screws - I broke 3 bits so far. I'll have to try heating them first.
I'm suspecting a possible air leak because when I drained the carbs for the second time, I noted a lot of bubbles coming out and less fuel than usual.
As far as he plugs go, you read my mind. that was actually the next thing on my list to do.
I will look at adjusting the cam chains. Hadn't thought of that as a possibility. The spark units are good - already checked them thanks, and I actually bought a new battery as the old one had 2 dead cells.
The repair to the wiring shouldn't be the culprit. The instrument lights weren't working. Found a broken common wire and soldered a new piece in. Working great now.
I will re-check the charging system. Replaced it all four years ago. I don't understand how that should affect starting the bike if I have a fully charged battery though?
Incidentally, I did have the carbs off. I had them cleaned this spring prior to riding season. The boots were actually in pretty good shape!
Thanks for your input. It tells me I was on the right track and gave me a few other ideas. Here's hoping I crack this mystery before riding season is over. I've attatched a picture of my girl from when she was behaving.
The trick to getting the Phillips screws out is to hit the heads with the impact driver bit a few times to help seat the bit and to jar the screw threads loose a little. Usually works for me.
The float bowls can't really retain "bubbles" in the gasoline, so I am not sure what you are seeing there. Do check to see that the bowl vent tubing isn't pinched off somewhere. That will cause some flooding issues.
Camchain adjust is just an advisory reminder. Often overlooked until it is too late and something breaks.
Verify that the auto fuel valve is delivering fuel to all carbs with good flow and no vacuum leaks.
Bike is looking good for its age and mileage.