Been working on 1982 CB900F project for few months. Bike was running poorly when purchased, 13k miles. Removed carbs, they looked in good shape but cleaned passages, replaced acceleration pump, gaskets etc. Cleaned but kept stock jet sizes. Did not break carb apart, so did not check air cutoff valve except on carb #1 which was in good shape. Replaced carb and air box boots. Shimmed all valves to high side of spec about .13. Bench synced carbs so that all butterflys barely covered front pin holes. Compression is 170 to 165 across all cylinders. Took bike for ride today. Starts right up. Idles rough a runs rough below 2500rpm. Above that bike pulls like a mule and accellerates almost pulling your arms off and runs smooth. I'll bet I need to pull the carbs off again and do a break apart cleaning. Short of that, any suggestions?
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1982 CB900F, Request advice
- Thread starter Rcb1000
- Start date
amc49
CB750 Guru
Pay close attention to idle and off-idle (primary jet), people have fits with them. The pilot tends to clog but lucky, that one is not pressed in. The idle and off-idle have a strange seesaw effect on each other, the idle gets plugged and lean idle but the plug richens up primary a whopping amount and it drives people nuts. Super lean then super rich as soon as the throttle gets moved at all. As well, the passages leading up to idle discharge point have to be clean too and the mixing bowl that supplies the holes behind butterfly. Most end up cleaning carbs way more than once to get all that opened back up. The pilot feed is around .013" and you use the smallest string on a guitar, the 'E' string to clean it out. Set idle mix to over 2 turns, 2 1/2 being common.
http://www.cb750c.com/publicdocs/Sea...anual_revG.pdf
http://www.cb750c.com/publicdocs/Sea...anual_revG.pdf
Thanks for the advice. You have been very helpful in the past. One other thing popped up today. Number 3 cylinder quit firing at all rpms. I suspect fouled plug, ran out of time, will check plug tomorrow. Plan to pull the carbs anyway and break the rack to do thorough cleaning.
OK back again, if AMC49 is out there. Broke carbs apart and went through them using MacGregor instructions. The carbs are clean and all orifices open. I bypassed the air cuts as recommended. Put all back together. New dr8ea plugs (could not get dr8es). Pilot screw 3 turns out as recommended to account for air cuts bypass. Bike ran great. After 3rd ride, bike bucked at low rpm, ran great at high rpm. Pulled plugs, coated with black carbon like soot. Figured running rich. Cleaned plugs, reinstalled, set pilot screws 1 1/2 out (leaner). Bike ran better at low rpm longer before acting up. Pulled plugs, still slightly sooty black. Will I hurt anything turning pilot screws in further? Should I go with hotter plug? Would you suspect bad coils or igniters (they look original)? Any advice would be appreciate.
user8086
CB750 Enthusiast
What are you jetting?
user8086
CB750 Enthusiast
If it's a kehein v43 ..#68-#102 is a good start .I see you said stock
