Questionhey falcon, ive got quite the question for you. im an engineering major who was itching for a bike, so after a long process of looking around i bought my friend's 1978 kz650sr that had been sitting around for a year or so. i test rode it several times to make sure that i wasnt going to be buying someone else's headache, and it seemed to run great.
well, it took me 40 of 50 miles home, where it ran out of gas. evidently, the little fuel light doesnt work. so, i fill it up, and it takes me another five miles before one of the battery contacts shakes loose and it dies again. then it take me four of the last five home, and at a stoplight i notice it idle really low and dies. the electric starter brings it back to live, and i coax it home by keeping it at high rpms the whole time. so i get home and rest for a few hours.
it hasnt moved since.
i tried to start it a bunch, and coaxed it to life a few times with "pri" for fuel and a full choke, but it would idle very low, sound like it was firing only a couple pistons or so, and promptly die. many tries later and the electric starter gives me a tasty grinding noise now. when i try to kickstart it, it doesnt even try to start, except maybe 1/15 tries, where it tries for one or two combustions. when i kick it without the key on "ignition" and when i kick it with the key on ignition, it sounds the same. the lights come on, however, when i turn the key.
the guy who sold me it said that i should replace the spark plugs. so i went and learned about spark plugs, took mine out, and discovered they were totally black on top, indicating a too rich mixture, i guess. i replaced them, and it still doesnt fire.
i have to split this message in two. sorry. next half to follow. im sorry!
AnswerJeff, let't try to do this all in one message instead...
First of all, you didn't specify the year of the bike, so there could be two paths to take as far as the ignition system is concerned:
Points and condensers, which need maintenance and electronic ignition which generally doesn't require maintenance.
First, you have to start with a baseline of information about your bike:
Compression readings. If low and even, cam timing is suspect. If low and uneven, probably engine wear and damage. If even ( and above about 150psi), then you have something to work with.
Second, Verify that you have a good steady spark at all the spark plugs when turning the engine over. USE FRESH plugs to check. Fouled plugs will spark down the sides of the electrodes instead of across the gaps. Weak and uneven sparks could come from dirty, worn point contacts and/or a loose/failed condenser... or bad spark plug caps (usually about 5k ohms) or bad coils. Isolate spark problems to either an individual wire/plug or to a coil set 1-4 or 2-3.
If you have points/condenser setup, the points need to be checked for proper gap after they are cleaned up so the faces are shiny and dry with no big pits and free of grit between the faces. Usual gap is set to about .014" when they are at the maximum opening location. Then the points are set to open at the F firing marks for each coil pair. I would also check the mechanical spark advancer behind the point plate to see if it is sticking during its advance/retard function. Lubricate the advancer cam so point rubbing blocks don't wear prematurely.
If you have electronic ignition, it is more of a pass-fail system. You have good sparks or not. If you don't have them on one set of coils the trigger on the plate could be dead or the wiring could be defective or the module has died.
The fuel tank petcock has a diaphragm that may have failed causing a vacuum leak and restricted fuel flow to the engine. There is also a filter screen built into the petcock, up inside the tank, requiring tank drain and parts removal for inspection. You can check the low fuel light sender while you are in there too.
The carburetors have rubber diaphragms that should be checked for any pinholes or tears. Any damage will stop the carb function. Drain the float bowls and see if there are signs of water or debris that could be causing metering problems.
If the bike is all stock (air filters/exhaust), then hopefully, the carb jetting is still as it was from the factory. If those items were changed, the carbs may have been rejetted by someone who may or may not know what they are doing. Remove the jets and note the sizes to compare with stock settings.
The carbs have a fuel enrichment system instead of choke butterfly valves, so those should be checked for any kind of wear or maladjustment. They all need to be closed fully when the choke lever is turned to OFF position. The carbs have a lot of small O-rings that need replacing after 25-30 years, too. Check Ebay or m/c shops for carb kits.
Go to: http://kzrider.com/ for more ideas and http://www.powersportspro.com/partsfish/login.asp to register and then locate your particular machine's microfiche illustrations. Buy a shop manual and plenty of good quality tools for your maintenance needs.
Bill Silver aka MrHonda.
