Hi everyone! I have a 1978 cb750k I picked up from a guy who said it had been sitting in a barn for the last 20 years. 6 months of tinkering in my shop and I have it on the road. Problem is it stumbles from 4k on. If I easily give it throttle it will continue to pick up rpms slowly but if I hold it wide open it acts like it just doesn't have any more to give. I have cleaned the carbs an sync'ed them along with new pionts and condensers. Valves adjusted and cam chain adjusted. New air filter in stock airbox. It has the stock 4 pipes but I cut the ends off for the sound. Opening up the exhaust helped out a little. A buddy said he seen a puff of bluish white smoke out of my #2 exhaust when I let off the throttle. He also said he could smell fuel. The problem is a lot more evident in 4th and 5th, but it sounds like crap even in first when I get to 4k. Hope someone has some ideas!
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motogtp
CB750 Addict
I have seen others comment on cracked emulsifier tubes in the carbs. Usually very small and hard to see if you aren't looking for them. I don't know if that is the problem but it appears that too much gas is getting in.
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Float heights set correct and even? Verify what size main jets, may have been changed in the past. Have the needles been shimmed? What do the plugs show? Compression? Hold it wide open and start to close the choke...if it gets worse it is rich, if it starts to get a little better you are lean.
Sorry it took so long to get back on here, life took over for a while. I tried the choke trick, made it worse. I cleaned the carb parts thoroughly, but they are a little eroded. Never seen that before. I set all of the floats with a dial caliper as per spec. I will admit I didn't check the needles to see if they have been shimmed. I couldn't read any numbers on the jets. They were rough like they had been sandblasted. I wonder if the seat is failing in the number 2 carb causing it to flood. That still doesn't explain why it would do it at mid rpm and not at low rpm.
Alright, update time. Pulled the carbs to check float highth and needle seat. Pulled the bowl off #2 cause I knew it was running rich from the plug. Ya know they say not to over tighten the jets? Well don't under tighten them either! Main jet had fallen out. Gonna check the rest of them and try her out!
Glad you got it figured out.
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