Got the carbs back on and the bike running. It has a pronounced stumble off idle and really doesn't run crisp at all. I can tune around it by really opening up the idle bypass (and increasing the idle) but then I remembered that when I rebuilt the carbs, I didn't change anything, but the guy who owned the bike for it's entire lifetime was a mechanic and he lived at around 3500 feet so I have to assume he tuned the carbs for that altitude. I'm at sea level which isn't a huge difference but it wont be the same. I cant say what the jets are or the needle position. They are what they were. The engine/carbs/airbox are 100% OEM K2 stock items. Any ideas about what the proper main jet, idle air bypass and needle height should be for sea level operation?
I will open it back up tonight and see what I have.
This can be caused by 2 areas:
1. Carbs, not quite up-to-snuff.
2. Spark advancer, either stuck (points cam won't turn, grease got gooey and stuck) or the advancer's shaft (on the end of the crankshaft) is bent more than 0.006", and wobbles. The spark (especially the 2-3 side) will jump erratically if this is the case.
A side note: do NOT turn the engine CCW direction (backward) with the big nut by the points. This WILL bend the spark advancer's shaft, and then you must straighten it to less than 0.006" runout (0.002" was OEM) to make the spark timing meet the marks correctly again.
The jetting: 3500 feet is not significant. Above 6000 feet they might need a slight tweak, but not much. The mainjets should be #110 if the bike was made prior to 3/72 (with #657A carbs), or #105 after that (#657B carbs), and with today's gas #105 will be fine in either carb type. The needles in the slides should be clipped in the 4th notch down from the top (blunt end) unless you have the HM300 pipes for some reason, then use the middle (#3) notch. The idle air screws should be set between 3/4 and 1.0 turn out from closed (gently, please...) position. The float height on the #657A carbs (brass floats, originally) will be 25mm on one side and 26mm on the other (you will find they are staggered, which is correct, not both at one height). In the #657B carbs they (mostly) have plastic floats and the float level is 26mm, although if you have replaced the float valves with the modern ones that have stiffer springs, you might want to go to 25mm to make sure the float bowl level stays deep enough.
Also: start your testing with regular-grade gas today. It burns at about the same speed as 1970s premium, and using higher grade fuels now will tend to quickly foul the plugs. Also, use either #D7EA or X22ES-U plugs for testing, and only use today's D8EA plugs for touring. The original plugs were D8ES-L type, not made today, which is halfway between D7E and D8E heatranges. Today's D8EA plugs are simply too cold for these engines unless you are touring all day with them.
