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Motorcycle Repair/1968 Honda CB450K1


I bought this bike new in Lusaka, Zambia where I was working as geologist in the late 60s-early 70s. I spent a year on this motorcycle touring east Africa, India, Nepal, Asia minor, Europe and across the USA to California.  I replaced the motor in the 1980s with a 72 450 motor. Over the past 15 years or so the bike has rarely been ridden. Now in Colorado.
I have recently restored the bike --no work done on the engine except for a few gaskets and rebuilt the carbs--that's the problem.  
The bike runs fine but consistently will not idle down when throttle is closed--runs at about 3000 rpm. I have to put in gear and slightly release the clutch to slow the engine down, and then it idles fine.
I have boiled out the carbs, rebuilt them carefully and cleanly  using high pressure air to clean jet passageways. It is not the linkage--the carbs sit at their idle stops, but run at 3000 rpm, till I slow it down with the clutch.  I am baffled?

Jim, unless you have vacuum leaks at the intake manifold rubbers/o-rings, then the most likely problem is with the spark advancer. Oil cakes up and turns to varnish on the advancer shaft, causing the point cam to get hung up slowing down the advance/retard function. The other issue with high miles advancers is that the return springs can stretch out allowing the spark advance to swing out too early in the rpm range.

Spark timing is always critical on these engines as the CV carbs can be very responsive to changes in engine vacuum signals. Spark timing changes the vacuum signals to the carburetor metering ports, which causes the engine to run rich/lean depending upon which circuits are affected and how far the timing is off from stock.

If the engine has good compression and initial spark timing is set at the F mark, then the carburetor shaft arms should just be about 1/8 to 1/4 turn from fully closed. Back out the idle speed screws on each carburetor arm until you see daylight, then turn them back in until you feel/see them touch down on the stop and add just a slight bit more equally on both sides.

Be aware that these carbs can be affected by wear on the throttle shafts where they rotate in the carburetor bodies. If you can detect much in the way of side-play in the shafts, that's enough to create a vacuum leak.

Start the engine and then spray around the carbs/manifolds/shafts with brake cleaner or something that will affect engine idle if ingested. Repair any vacuum leaks, check the spark advancer for proper function, set the ignition timing correctly and it should all settle down again.

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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