QuestionI have a 1985 Honda Nighthawk CB450SC. When I first start the bike with the choke fully on, it doesn't necessarily want to stay running without some additional throttle. But when it does run without throttle it seems to slightly accelerate up and down at random. Also, when shifting into first gear there seems to be some hesitation. And when cruising at normal speeds, in any gear, at random it accelerates in short bursts, but not consistantly. I feel it has something to do with the carborator but I am not sure what it would be. Any insight you may have would be helpful.
John Lorenz
AnswerJohn, your 23 year old bike probably needs a good tuneup, carb clean and thorough going-over. SEE:
http://www.cmsnl.com/honda-cb450sc-nighthawk-450-85-us_model457/partslist/
for details about the parts involved.
It is important to determine what you have to work with, in the beginning. Compression test should read out in the 175psi range on both cylinders, The air filter system should be inspected and serviced/replaced. New spark plugs should be installed. The fuel tank has a screen on the petcock, which should be checked for blockage. Some models have a diaphragm in the petcock itself, which shuts off fuel flow when the engine is not running. The diaphragms will fail, causing air leak into the engine and lack of fuel flow to the carbs.
The carburetor section shows an "air cut" valve, which is used for emission control assistance and these often fail, causing a large air leak into the engine on that carburetor. The replacement part is available through NAPCO or K&L motorcycle parts dealers, at a much reduced price over the OEM Honda part, if still available.
Additionally, there are vacuum hoses going to carbon canisters and other components which should be checked for leaks or issues with the components that they are attached to.
Hope that helps get you headed in the right direction.
Bill Silver
