AboutBill Silver Expertise 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.
Experience 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.
Organizations VJMC (Vintage Japanese Motorcycle Club) of North America
Publications VJMC newsletter, as editor for two years and as contributing editor currently.
Expert: Bill Silver Date: 5/12/2008 Subject: carb adjustment
Question My 83 honda nighthawk cb650sc is sputtering/loosing power when I give it about half throttle. in fifth gear i cant go any faster than 55 mph before it sputters. I drove it without the air filter and it ran perfect, no hesitation. I have been told the carb needs adjusting. when i bought it i was told that the carb was totally overhauled. I took the carb apart and it looks clean. floats look good (have not measured to check with specs. yet) #1 Question-Could it be that the accelerator pump needs adjustment? My manual says see table 1 for correct clearance, but in that table 1 I do not see anything about the accelerator pump clearance. do you know where to find this measurement? #2 Q. could the pilot screw adjustment have anything to do with this loss of power.
Answer Rafael, go here to look at exploded parts illustrations:
The carbs have idle circuits, needle circuits (for transition from idle to main jets) and main jets. In addition you have a choke enrichening (starter valve set) circuit.
The fuel tank has a vacuum operated petcock, with diaphragms which can fail. Diaphragm failures in the petcock generally cause a lean condition due to the vacuum leak in the line to the carburetor fitting, plus it will either lean out or shut down the fuel flow to the carbs. This is the normal set of issues on these bikes, however you say that taking the air filter element out causes the bike to run well, indicating that the mixtures are too rich, rather than too lean. If the carbs were overhauled with an aftermarket kit set, using new needles, the needles may not be calibrated to the same standard as the OEM parts.
The accelerator pump is used to also overcome the lean transition on the needle circuit, due to EPA restrictions. The illustrations shown above do not indicate an accelerator pump in the design. If your bike is actually a 82 SOHC CB650SC, then we have a different circumstance. It is a completely different bike. It helps to be very specific about your model year and version. The 82 bike does have accelerator pump circuits. There is generally a small 1.0mm or so gap between the throttle linkage and the end of the pump shaft. You want the throttle to just begin to open before the pump squirt kicks in. Adjusting the accelerator pump only affects the transition between idle and midrange, not high speed. If your bike only goes 55mph, then there are other problems than the accelerator pump adjustment.
The choke system on this bike uses manual butterflies on each carburetor. Be sure that the choke cable is adjusted properly, so the butterflies are wide open when the cable is pushed all the way in. These carbs have a fuel control system : http://www.cmsnl.com/honda-cb650sc-82-nighthawk-650-us_model475/partslist/E++19....
which also has a diaphragm. If you swap the intake and fuel lines, it may be possible that you are sucking fuel directly into one of the carburetor intake ports. Pull the plugs and see if they all look the same or if one is darker than the others, indicating a problem on just one cylinder... perhaps the one with the fuel control valve attached.
Finally, you could be experiencing a problem with a spark module that is giving weak spark under load/accelerating. Be sure that the battery is fully charged and the charging system is functioning properly. These bikes are know to have charging system issues and the ignition is battery powered, so low volts = no sparks.
