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High revs not an intake leak 19 Jul 2016 11:29 #735449

  • Dwoolley
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I bought this bike about a month ago. Its a 1982 KZ440 LTD. I recently bench synced the carbs. She is idling much better than before, but she still revs high after she has warmed up for a few minutes. I sprayed some carb cleaner on the intake boots, air box boots, and a small crack on the air filter thats near one of the bolts attaching it to the air box. She didn't rev up high or low after spraying each part so I'm a bit stumped at what could be causing her to idle so high after warm up. the idle hovers between 5k and 6k after warm up and at start up she idles at about 1.5k. Any ideas as to what could be causing the high rpms at idle? I thought it would be the intake boots since they both had cracks in them, but the test proved that they are still good. Any tips on what to check next would be greatly appreciated.

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High revs not an intake leak 19 Jul 2016 13:33 #735467

  • TexasKZ
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The spray test typically only responds to really big leaks. If you can see cracks, it is time to replace the boots. The crack in the airbox is not likely to be the culprit, but should be easy enough to repair for peace of mind if nothing else.
Have you carefully inspected, lubricated and adjusted the throttle cable?
Could the seals around the throttle shaft be leaking (cv carbs)?
Do the low speed mixture adjustment screws have o-rings that are deteriorated or missing?
Have you cleaned and properly lubricated the timing advancer? If it is gummed up or malfunctioning, you could get an erratic idle.
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High revs not an intake leak 19 Jul 2016 13:40 #735470

  • Nessism
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In addition to what TexasKZ mentioned, if your 440 is one of the ever increasing number of chopped/bobbed/cafe bikes with heavily modified intake system and/or exhaust, then the carb jetting may be way off which can cause the issue as well.

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High revs not an intake leak 19 Jul 2016 13:40 #735471

  • Dwoolley
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I have lubricated and adjusted the throttle, I'll repair the air box just for peace of mind like TexasKZ said. I'll do the other fixes and get back to you. I'll also replace the boots just for safe measure too. I'm pretty new to having a bike, but I'm pretty sure that the bike hasn't had a modified exhaust or intake system installed. I'll look at the jets just to make sure .

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Last edit: by Dwoolley.

High revs not an intake leak 19 Jul 2016 14:10 #735475

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Yeah, I'm with the others - intake vacuum leaks, 75% cause of high RPM engine idle speed problems on old bikes. This usually occurs when the carbs are removed, cleaned, and reinstalled using the old parts. Disturbing the carbs seems to highlight any problem parts that were (marginally) working o.k. before.

Any cause for a leaned out mixture on a warm engine will do this, but if it starts and idles o.k. until the throttle is blipped, or if loading the engine momentarily brings the idle back down briefly, next I'm looking for vacuum leaks....

Worn CV throttle shafts on Mikuni BS34's have been tough to find / isolate for me in the past - very frustrating! I chased float levels , mixture screw O-rings, CV diaphragm installation, slide movement, throttle linkages, etc. for days before I found the throttle bores were worn out. What a pisser!

Another possibility is a torn or cracked CV diaphragm, but I've only had that happen a couple of times. Cracked or hardened (with age) carb manifolds can be very deceptive - it all looks good to go, but the problem doesn't go away until new parts are installed...
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