QuestionHi Bill, I have a question about the carb adjustment on a Honda '71 CB450K4. I'm kind of a newbie at the bike maintenance, but I have some experience with old auto carbs. The Haynes book is rather vague on the adjustment process and I have always just let the bike shop adjust it. I would now like to do it myself. I am used to the auto method of a specified rpm drop to get to the proper adjustment. How do I know if I've done it right on this bike? I know I am going to need to take the carbs apart and give them a good cleaning because I can't keep a proper idle. When it is warmed up, the idle jumps way up to about 2500rpm and sticks there for awhile. I feel comfortable with all of the carb disassembly instructions otherwise. The bike has 4,800 miles on it and is otherwise in excellent shape. (It was my dad's bike.) Thanks in advance for taking such a basic question.
AnswerFrank, That is a nice low-miles machine! Generally fast idle issues are related to either throttle cables that have degraded internally or at the junction, causing one or both carbs to hang up off idle.... OR
Air leaks at the carburetor mounts...OR
Incorrect ignition timing, which is usually attributed to a sticky mechanical spark advancer, located on the end of the camshaft, under the points plate. Remove the point plate and check the advancer for a free movement of the weights to swing out and return. The original lubrication between the cam and the base of the unit dries out and the point cam tends to hang in one position.
Do move the handlebars back and forth to full lock, at idle, to see if the throttle cable is misrouted and pulling the throttles open at idle.
No doubt the carbs could use a good cleaning, but the vacuum slides seldom are the culprit unless a lot of varnish has built up between them and the carb body. The butterflies are spring loaded and seldom give problems unless loaded up with deposits. There was a bulletin about the choke crossover link hanging up on the wire harness, breather tube or fuel lines on some models. Check for clearance when the choke is fully closed and open again.
Once the valves are adjusted, the ignition timing checked and functioning properly, set the idle mix screws to between 3/4 and 1 turn out. You will have to listen to the exhaust note as you set the idle speed for each side, then adjust the cable synchronization last, ensuring the both throttle arms are lifting at the same time. This is pretty much a visual adjustment as there are no vacuum ports to access for using carb synch tools, like mercury sticks or vacuum gauges.
Check Ebay for shop and parts manuals, which are invaluable for learning about the correct way to adjust and tune your machine. Also: www.motorcyclememories.com and www.helminc.com for manual sources. Check out the www.vjmc.org site, too!
Bill Silver
