QuestionHi Bill-
Thanks for all the great work you do here. I recently acquired a 1979 cb400ti, and I've got two related questions.
there is a peculiar thing going on with the bike's idle. first off, the bike runs great, starts on the first or second kick even in the cold (with choke naturally). even though it sounded pretty good to me, out of curiosity, I decided to check the bike's idle speed. I hooked up an inductive tach clamp hooked up to a multimeter and got a reading off of both cylinders. each was ~3500rpm. this seemed kind of ridiculous given the sound, so I did some research, and concluded that maybe the reading was double because of a wasted spark ignition system, so
question 1: does the 79 CB400ti have a wasted spark ignition system? I can't seem to find this answer anywhere, including the shop manual.
so assuming I'm right about that, the bike was still idling over 1700rpm, which is a solid 500 over the factory recommended 1200rpm (+/- 100). So I set to trying to get the idle down. Having changed the oil, replaced the air filter and run some seafoam through, the first thing I did was try to adjust using the throttle stop screw. here is the first strange thing...no matter how far in I turn the screw, I couldn't stall it out. the bike just kept on humming. I can speed up the idle by turning it out (so I know it's actually moving the throttle slide), but I can't slow it past the ~1700rpm mark.
after that, I discovered that the bike was running very rich the hard way, I ran out of gas, probably only got ~90 miles off a full tank. I figured maybe that could be the explanation, maybe the pilot screw was way out and throwing so much gas in that it would idle at any throttle stop opening. so I took to the the pilot screws, being careful to set them lightly, and then turning them out to the factory setting (1.5 turns I believe). Having done that, I STILL couldn't kill the idle with the throttle stop, although it had gone down to about 1500rpm.
So I figured I'd try leaning out the idle mixture completely by lightly setting the pilot screws. even then, even with the throttle stop screw bottomed out, it STILL idled, although now pretty close to the factory idle of 1200rpm.
Although i guess I had succeeded in getting the idle down, I didn't like the idea of running the bike with the pilots screws all the way in, it just seems to be asking for some horrible lean mixture problem. so
question 2: what do you all make of this? any idea why I can't seem to get the throttle stop screw to kill the idle, no matter where the pilot screws are? I'm considering going in for a thorough carb clean, but it seems to me that gummed up jets would have the OPPOSITE effect, i.e. make the bike more prone to stall out at small throttle stop openings...no? any opinions about running the bike with the pilot screws (ostensibly) set to a very very lean mixture?
I'm sorry about the super long question..thanks again,
ben
AnswerBen, Because the engine only has a single coil, it has to have a wasted spark, doesn't it? :>)
The motor is 360 degree crankshaft, so both pistons go up and down together, like the pairs of a 4 cylinder bike that also have a single, dual lead coil for each cylinder pair set. One piston is on compression and the other one on overlap exhaust stroke, thus wasted spark. Next revolution, the piston strokes swap sides.
After 30 years, the carbs should come off for a good cleaning and inspection. Replace soft parts, like gaskets, and o-rings from a kit. Be ware of aftermarket carb kit jets and needles, though. Re-use the original ones if at all possible.
Uncontrolled fast idles are generally related to issues like:
*air leaks in the intake tract.. intake manifolds, O-rings
*in rare cases, the carburetor throttle shaft screws can work loose and the throttle plates shift out of center, preventing closure of the throttle
*failure of the air cut valve diaphragm
*ignition spark advance constantly advanced
http://www.cmsnl.com/honda-cb400t-ii-1979-general-export-mph_model15775/partslis... for parts list with numbers.
Bill Silver
