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Discussion Starter · #1 · (Edited)
Hi all - first post (after introduction anyway).

Mine is a CB450k5 from Michigan, imported via a nice ebayer last year. I'm getting it on the road after it has been sitting for quite some time. It has a few little issues which need sorting. I'm used to the inline 4s so a lot of the stuff is fairly familiar. So far I've replaced the chain, throttle and clutch cables and throttle grip, changed the oil, cleaned the filter, changed the air filters, overhauled the front brake etc

Reason for post though - I rebuilt the carbs and put all the bits twice through an ultrasonic cleaner. After a few false starts (literally) I have the bike running and pulling well on a test ride. I get the following symptoms though and I've run out of diagnostic ideas:

After a short while from warm start running with idle at 1000 or so the idle begins to fluctuate between 700 and 1200. Heading into town, at stops the engine dies unless I give it a little throttle.
If I increase the idle speed to compensate it runs at 1800 or so which is too high. I've chased leaks (I replaced the intake boots) with much wd-40 and I don't see an obvious difference in performance wherever I spray it.

Here is a video
of the problem any tips on diagnosis would be very welcome!
 

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Hello,

Check your Synch.
Had more or less the same thing on mine. Bad synch cause more or less the same symptoms as an air leak, high idle after a little throttle, and a lot of strange idle issues
 

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Also, check the seals on the throttle shaft. Spraying around them may show the felt is worn.
 

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The biggest cause of the dreaded fluctuating warm idle is worn torsion bar valve actuators inside the cylinder head. I've worked on a ton of cb/cl 450 Honda's, from the late 60's, through 70's. Nine times out of ten the worn actuators were the culprit. Unfortunately, the head has to be removed to replace them. As the other poster's suggested, make sure you have 0 intake leaks, and that the sync of carbs is correct. The 450 was the only model that used those dreaded cam torsioners. The 350,500,750 Hondas never had idle issues. Most of my customers just put up with it, as it was pricey to fix. The motor has to be removed to do this repair.
 
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