Honda 450 K7 Losing Idle

| 10/15/2014 8:00:00 AM

Motorcycle Classics tech expert Keith Fellenstein

Editor’s note: If you’re having trouble with that old Suzuki, BSA or BMW, Keith Fellenstein is your guy. From motorcycle tuning tips to detailed motorcycle engine repair, he can draw from a wealth of experience to help guide you to success. Send questions to: Keith’s Garage, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as the subject.

Losing idle

Q: My 1973 Honda 450 K7 doesn’t idle down properly after it gets good and hot. According to info I’m finding online, this might be the result of an air leak around the carbs or a weak advance return spring. I have done the “spray with carb cleaner” around the carbs with no noticeable change. How can I determine for sure that my return spring is truly the problem? — Lynn A. Metzger/Lawrence, Kansas

A: This can be tested using a timing light. Hook up a timing light and shine it on the timing marks on the alternator rotor. Rev the engine up and watch the timing marks move to the fully advanced mark. Release the throttle and they should settle back to the initial timing mark pretty quickly. This is not fool proof though. If there is a reason why the idle isn’t coming back down, such as too much fuel for some reason feeding the engine, the timing will remain advanced as long as the engine speed remains high. MC

3/21/2016 4:30:33 PM

Your problem may be that there is not enough free play in throttle cable(s). Adjustment of the throttle plates for proper idle speed should never be made at the cable adjustment, but at the throttle screws. A simple way to determine if this is your problem is to merely loosen your cable adjustment. Use your idle speed and mixture screws to set idle with the cable fully relaxed. Adjust the cable to take up only the excessive free-play in the throttle. Dave

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