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74 that runs rough

M Saunders

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greenfield IN
I have a stock 74 cb 750k that i am having problems getting it to fire properly on all 4 cylinders. I rebuilt the carbs and bench synced them and set proper float height. I Installed new plugs, plug boots and points I gaped then dweld the points then set timing. cylinders 3 an 4 seam to run good and 1 and 2 dont cyl 2 pops in the exhaust and its exhaust pipe runs 30*f cooler than the rest but when i pull the plug it looks good not fouled. Cyl 1 runs same exhaust temp as 3 and 4 and dose not pop in exhaust but it seams to miss out at around 2500 and fouls the plug really bad dry and black carbon. I did a cold compression check on all 4 cyl they were about the same i think a little low around 90 psi. I will do a hot test later today. the motor has 12000 mils and has set for 20 yrs. I would greatly appreciate any input or info that could help me.

when i checked the compression i forgot to have the carbs all the way open it is really exactly 140 psi on every cylender I did this test with a cold motor and did not add any oil to them
 
Last edited:
I have a stock 74 cb 750k that i am having problems getting it to fire properly on all 4 cylinders. I rebuilt the carbs and bench synced them and set proper float height. I Installed new plugs, plug boots and points I gaped then dweld the points then set timing. cylinders 3 an 4 seam to run good and 1 and 2 dont cyl 2 pops in the exhaust and its exhaust pipe runs 30*f cooler than the rest but when i pull the plug it looks good not fouled. Cyl 1 runs same exhaust temp as 3 and 4 and dose not pop in exhaust but it seams to miss out at around 2500 and fouls the plug really bad dry and black carbon. I did a cold compression check on all 4 cyl they were about the same i think a little low around 90 psi. I will do a hot test later today. the motor has 12000 mils and has set for 20 yrs. I would greatly appreciate any input or info that could help me.

when i checked the compression i forgot to have the carbs all the way open it is really exactly 140 psi on every cylender I did this test with a cold motor and did not add any oil to them

Hi, I have the same problem on my 78, cylinders 2-4. Did you ever sort yours out?
Mike
 
Start out by adjusting the air screws on the side of the carbs see if that helps it out any if I,m not loosing My mind in is richer out is leaner, Check after all those years the main jets are all the same I believe I,m running 120,s now, I had one carb that the needle seat was worn I had to lower the needle clip one notch helped out greatly. Good luck to Ya both.
 
I had similar problems with my 74 until I bought new stock coils and a pamco electronic ignition.
 
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