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CB750 K1 jetting Help

Bhitt_us

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Hi, 1971 CB 750, runs well, carbs synced. With gas off, slowing down I get allot of sputtering as I down shift. I have check plugs and moved needle to lean out mixture with no improvement. I am guessing it is running rich. All jets are stock and I have stock pipes & stock air cleaner. Top end rebuild recently, Carb completely rebuilt. Any help?
 
Your always going to get a little bit of "sputter" on a 50 year old bike with 50 year old carb technology. If the throttle is closed which I assume it is as you said the gas is off....the only fuel is coming from the idle jet and idle air mixture screw not the needle. The needle affects slightly off idle through the rest of the throttle opening but doesnt affect actual idle mixture. Try doing some adjustment to the mixture screw see if you can get it closer to what you would like. The carbs on these are very basic so they are not nearly as clean and efficient running as a new fuel injected bike
 
I would suspect it is running too lean if it sputters when you back off the throttle. One way to help diagnose it is to remove the air cleaner and take it for a ride. If the problem gets worse or stays about the same, it is too lean.
 
Thanks, I will try removing the air cleaner and see if gets worse. I have been trying to adjust the air screws with the carb sync gauges on. I need to try a couple of different settings to see what is best. I last time tried to get the highest idle on each carb while on carb sync gauges. Not sure if that is the best to try and eliminate popping while down shifting with throttle 100% closed
 
Also since I am .50 over on the top end should I look at starting with different main or slow jet from stock 120 & 40 or does that not change much?This is new bike for me, I just finished this project after a 2 year build finding 100% OEM parts.and all are new old stock. The engine only had/has 6,150 miles.
 
I just finished restoring a 1971 CB750 and had sputtering when I gave it full throttle. I removed the air cleaner to troubleshoot and the problem did not occur. So that told me it was running too rich at full throttle. At full throttle, the main jet is the one to change, so I went from a 120 to a 112.5 and it now runs great throughout the rpm range. Once you decide if it is too rich or too lean, you need to figure which jet needs to be changed. If it occurs at all ranges, I would lower or raise the needle jet as appropriate. If you need to make it richer, raise the needle and if you need to make it leaner, lower the needle. Take a look at the plugs and that can help troubleshoot too. I don't think your .5 over on the pistons will make any difference with the jetting.
 
Easy jetting......with everything on and as it will be go ride it. Hold throttle at different throttle settings....start closing choke at each throttle setting you need to fix.....if it starts to run better as you start closing the choke then it is lean, if it runs the same or starts bogging down then it is already a little on the rich side. Easy way to verify jetting running issues.
 
Easy jetting......with everything on and as it will be go ride it. Hold throttle at different throttle settings....start closing choke at each throttle setting you need to fix.....if it starts to run better as you start closing the choke then it is lean, if it runs the same or starts bogging down then it is already a little on the rich side. Easy way to verify jetting running issues.
That a very interesting procedure i will have to remember that as i have my carbs torn apart on my 1970
 
I just rebuilt the carbs on my '70. FWIW... Yamiya recommended going to the 110 main jet instead of the 120 with the clip on the middle position of the needle. Also I drilled out the top holes (A) on the emulsion tubes to 1mm to allow a little more air to the fuel/air mix. Also note the slow jets that came with the Yamiya kit. The jets that were in my bike have much larger holes than the new jets that came in the kit. They are both marked 40. Bike runs great now. It was way rich before. Plugs would get sooty in 200 miles. I also bought the OEM floats, float valves and seats from Yamiya.
 

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