I've thought about the issues you mention in your text. I think the problem was caused/started by my trying for a low low idle. Not good for the tappets, I know, but I just wasn't thinking about that. I noted that L.A.B. (if I understood correctly) thought a very very slightly increased throttle slide opening would lean out the enrichment mixture a bit. And since the throttle slide is never "completely" closed for a particular idle setting (you can always lower the idle by closing the throttle slide a bit more) conversely, you should be able to richen the enrichment mixture by closing down the throttle slide for a lower idle. As L.A.B. pointed out, some have had the problem I was describing and the thought was the enrichment jet might be a little too big. That jet would be very hard to replace or modify since it's a press fit and no one lists replacements for it. Being too big could be just a variation in the manufacturing process. Or the enrichment jet (and carb too for that matter) could be for a different motor. My having gas wet plugs (as suggested, I checked for smell and blue smoke on starting) now makes sense with my insistance on a low idle, meaning a slightly reduced throttle slide opening. With the enrichment circuit in it was too rich. And, kicking away, I would switch the enrichment circuit off, whereupon it would be too lean, then on again in frustration. Then, finally cleaning the wet plugs, kick with the circuit off and, sometimes, it would then start. Pretty frustrating. I think that for a particular ambient condition and the enrichment circuit on, there is one perfect throttle/idle setting. The Mikuni does not compensate for different temperatures. A carburated automobile, for people in, say, Minnesota, who must drive year round, has an added sophistication of a bimetallic spring driven choke to account for temperature extremes. The Mikuni does not, and neither does it compensate for temperature extremes in some other way. The old prehistoric automobiles opened the choke automatically. We, with our sophisticated Mikunis have to reach down and flip a lever. Somehow, with all that trouble, we motor on. I believe the settings and enrichment circuit jetting was by careful consideration of the temperature range we'd most likely putt around in and the idle we would set (so as not to harm the tappets) for our Nortons. Go outside that range in either temperature and/or idle setting and you might find starting gets harder. The circuit seems very sensitive to idle setting outside a limited range. I think if I'd set my idle originally to 1050 or 1100, a couple of kicks and off I'd go. That is the way it is now. Set the idle to 1400 for starting in the warm temperatures we're having and it's one kick every cold start. Just one. When the weather cools down the optimum idle setting for one kick starting (not optimal for a hot engine of course) will go down. Until in freezing weather I might have to lower the throttle to 900 idle or below (hot engine). I checked the throttle slide movement for these different settings and it is very slight. Doing this with the hand on the twist throttle with any consistancy would be impossible.
The starter jet appears to be a press-fit in the bowl so not easily changed although I seem to remember somebody mentioned modifying it somehow?
Previous Mikuni hard to cold start thread
Hard to Cold Start (2011)
Having real trouble cold starting my bike. Hot starting is one kick, cold starting is 20-40 :( '73 Mk2 850 Single Mikuni Trispark ignition Dyna twin outlet coil NGK suppressed leads Iridium plugs K&N filter Carb is brand new, supplied for the bike by reputable company. Everything else listed...www.accessnorton.com
www.accessnorton.com
Joe said:
The reason for using the throttle adjust screw is it is far more repeatable. It is very precise. If you check how much the slide is raised by one complete turn of the screw you'll notice how slight the movement is. Now try that, and try to duplicate that, with the twist grip. It will probably work anyway, but I was looking to correlate the number of turns from a given idle starting point with one kick starting. Kind of anal, I know, but I enjoy figuring that stuff out. Very cold and you might have to even lower the slide. If you've already got a high idle, maybe just a half to one turn in on a hot day initial start. But if you've got hard starting with just fliping on the enrichment circuit, before it starts, check the plugs. That will tell you what to do with the slide screw.
Craig, I will respond this time and that's it. I did follow suggestions -- check plugs for gas, check for oil burning, twist in the throttle slide screw (that worked so well I followed up on that), L.A.B.'s information that others had the same problem in the past and his description of the enrichment jet and it's press fit. I also recalled him saying that a slight opening of the throttle would have the effect of leaning out the enrichment mixture. Looking at the setup in the diagram L.A.B. provided I saw there was NO compensation for the effect of the slide idle position. Too high leans out the enriched mixture and too low furthers the enrichment. Mine was way too low an idle. And the plugs said it was too rich. Now there is nothing I can do if you don't like my prose and if I have not followed every suggestion from everybody. I followed the thread that worked well for me. Frankly an 1100 rpm idle probably would do pretty well all around (till the snow may fly) though it may not always be a one kick solution. Again apologies for the lack of more complete follow ups. It starts easy now. Thread is done.
