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I have a frustrating issue with my 1970 CB350. Runs excellent up to 6,000 RPM's above there starts sputtering and missing, hope there is something I am overlooking because I feel I have been all around this issue.

Top end just rebuilt ( New pistons, rings )
Valves ground
New XS Coils and wires
Pamco ignition with electronic advance
New plugs
Adjusted valves to spec.
Adjusted chain tensioner
Using UNI filters
Harley Sportster Mufflers
Carbs have been cleaned
 

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WIcb3540,

1. Check your battery voltage and voltage drop across the kill switch and ignition switch.
2. Compare the voltage at the coils to the battery voltage.
3. As the engine speed increases, any drop in voltage to the coils becomes more apparent because it affects the ability of the coils to "charge up" in the time that the coils are on, which becomes shorter and shorter with increased RPM's.
4. New coils and wires and caps are great, but are they making contact at the cap and coil? Check for 12 Volts at the caps with the ignition on but before you attempt to start.
5. Clean carbs and pod filters could mean a lean mixture which would be more of a problem at higher RPM's. Try one up on the main jet.
6. New plugs are great also, but if the mixture is rich (opposite of #5) they will foul quickly, so pull em and check em.
7. Not sure you should be winding her up to 6,000 with those new pistons and rings.
8. Could have excessive oil getting past the new rings, fouling the plugs. See #6
 

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rule out electrical issues first like pete suggests.

then:
what's compression?
verify you set your cam to crank timing properly: "LT" on crank alignes when pin on cam it exactly at 12:00 o'clock and 6:00 o'clock
what do your plugs look like?
try running without the air filters....did that change anything?

are you getting good fuel supply?...clean petcock & fuel lines, functioning gas cap vent?
 
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