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Motorcycle Repair/1981 cb750 custom


I am having surging problems at 6000 rpm.i have cleaned the carbs,gas tank,lines filters,replaced ignition wires and plugs,removed the fuel shut off.if I barely open the throttle it will rev up to 7000 fine but if I open the throttle open around 4500 rpm quickly in any gear the motor will bog out.can you give me any ideas where to go with this? The idle and midrange are perfect.

Ken, what kind of exhaust and air intake systems do you have on the bike now? If OEM, then be sure that your charging system is keeping the voltage up to the ignition system. The spark units are known issues. You may be going onto just one set of coils under load. Or the coils may be breaking down, as well. I don't have specs handy for DOHC coils with transistor ignitions, but usually they are in the .5 to 1.5 ohm range, read on the primary winding (+ and -) terminals.

When you whack the throttle open, spark voltage demand is high for a second and if the secondary wiring is damaged, then you don't get a strong spark that will fire the spark plugs under load. Check spark plug caps to be sure that they are around 5k ohms each.

You might want to check the function of the spark advancer unit, too, but it runs in engine oil so seldom has a problem.

Make sure that you have a fully-charged, load-tested battery in the bike and that the charging system is keeping the voltage above 13v off idle. Defective rotors on these bikes will shut down charging system voltage and/or spike power to the regulator/rectifier and fry it.

If your bike has altered air filter/exhaust system, then it probably needs a larger main jet installed to richen up the mid-top end range. You can also put very thin shims under the needle clips to raise the needles slightly, which will richen up the fuel delivery.

Make sure the gas cap is venting properly, too!

Bill Silver

Motorcycle Repair

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Bill Silver


Need help with vintage Hondas from the 1960s? I am an expert with 250-305cc bikes in particular and most all of the other pre-91 models, in general. I do NOT claim to have a great deal of experience on Gold Wings, Cruisers, ATC/ATVs and dirt bikes.


I have owned/ridden/maintained Honda motorcycles for 35 years. I have written five books on Honda repairs and collecting. I was a service manager for two Honda shops back in the 1980s.

VJMC (Vintage Japanese Motorcycle Club) of North America

VJMC newsletter, as editor for two years and as contributing editor currently.

3 years auto shop in high school, teacher's aide in Automotive Technology in Jr. College, Diesel mechanic course in college, self-taught mechanic and automotive writer.

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