Topic: Missing at cruising speed

[jdamman]

Hi all,
I finally have my (my dad's? my grandpa's?) CB750 K1 on the road thanks to the help from this board.  Last night, I rode it about 40 miles to get it to my place (I had been working on it at my grandpa's farm) and it ran well except for two minor problems.  The first thing I noticed was from an idle (about 1200-1500 rpm?-waiting on tach cable) when I blip the throttle to start moving, sometimes it will fall on it's face a little, so I have to back off and try again.   Sort of a stumble.  Oh, here's the list of what's been done:

-full disassemble clean and rebuild of carbs (hadn't run for 6-7 years, off the road since early 80's) and bench sync (needle clips middle position) (new jets #120 mains and #40 slow-stock)
-new points and condensors
-new spark plugs
-entire tune up (timing a little advanced, doesn't quite get to the "F" mark when full advance is just past the seccond mark, dwell 22-24 deg.)
-aftermarket 4-2 exhaust (uncle blew off two mufflers when grandpa first bought the bike in '73)

I haven't done plug chops since I've ridden it this much, I tried when I put the new plugs in, but I don't think I rode it enough, because they looked good after a few miles, but when I pulled #1 out after the 40 mi. ride, it was real black, but it had idled for a while.  The other problem I'm having is when I'm under partial load (cruising, not accelating or going up a hill, but not coasting) I can hear the right bank (3 & or 4) cutting out.  I've tried to adjust the idle mixture screws best I can, but my warm idle may be too fast since there is very little audible difference when I try to adjust them, except when I completely close them.  The bike idles fairly smooth, a little miss here and there, and it runs like a b*tch under acceleration and full throttle.  Is this as good as I can get the "grandfather" to run without a carb-sync tool?  Oh, and the 2-3 points have some variation in the timing, but 1-4 is ok, I think my pivot point is a little sloppy in the hole.  I would appreciate any and all suggestions especially if anyone else has cured these particular symptoms.
Thanks,
Joel
Ames, IA

*If my parent's scanner would have worked, I would have posted a picure of me at about 3.5 yrs old sitting on this bike in front of my dad, reaching for the handle bars with my grandpa standing behind us in my grandpa's farm yard- oh well, maybe next time)

[KB02]

First of all, way to go on pulling it out and getting her to go!!! Congrats!!

As for your descepancy on the 2-3 point vs 1-4, the 2-3 point can also be adjusted independant of the 1-4 point. I had to do this on my bike as well (saving pennies for a Dyna). That might help the the stumble as well. Worth a shot anyway.

[jdamman]

Thanks, already detailed that, I actually had to try a couple of times because on both 1-4 and 2-3 the timing and point gap both varied when I adjusted the other, so I had both the dwell meter as well as the timing light hooked up simutaniously.  I think I'm to the point of splitting hairs on that, except for the timing crawl of a few degrees at full advance on 2-3.  Appreciate the suggestion though.
Joel

[Ceej]

i have this same problem with my bike in a certian rpm range.  Its stumbles a little while cruising between about 2300-2800.  If im cruising above or below this its perfect.  I also get a little bit of a stuble when i slowly roll on the throttle.  Under moderately fast acceleration it runs great.

I know this doesnt help you, but at least youre not alone.....Let me know if you figure it out and I'll do the same for you

[jdamman]

I'd be glad to, gotta take my class "M" test tomorrow, then I think I'll have some time to work on it, gonna see if I can figure out what cyl. is cutting out and why, starting with med speed plug chops (that works, right? before I thought you needed to ride the snot outta it, and cut off the engine at a higher speed, but I figure ride it under missing conditions, shut it down and read plugs...)   I'm not quite sure what rpms it happens at, (busted tach cable) but it only is a problem under a light load.