I have an 82 Honda 450 Nighthawk motorcycle that I'm rebuilding and I need to tune/balance the carbs. I have them bench-balanced and reset the idle screw on both. Now it runs a lot better, but every 10-15 seconds the idle raises and it starts sputtering out the carbs.

I assume this is just a lean mix, but could it also be vacuum or something? I don't want to tune the carbs just to realize that I need to fix something else and tune them all over again.

  • Nope, that question is more about why a cold engine has to run faster than a warm one. Jun 27 '15 at 23:06
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    @EricFossum - Do you still have this issue? If not and you have solved it, could you please populate the answer to the problem, it would be good for the site. If you are, can you share any additional troubleshooting measures? Regarding a solution, Could you please respond with make/model/year so a more defined solution can be shared with you once we figure it out? Thanks Dec 12 '15 at 3:07
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    The easiest way to tell is by trying this. Ride the bike with the throttle raised for about 100 meters, then, hold the clutch in and close the throttle so as to let the bike coast. If the the engine speed does not begin to drop immediately, you have a lean mixture. This is a definite give away of a lean engine but the lack of this sign does not eliminate the possibility of a lean engine.
    – krthkskmr
    Feb 22 '16 at 8:13
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    How are the carb boots? Any cracks that could periodically let air in and lean out the mixture?
    – Mysterfxit
    Mar 1 '16 at 5:07

That is probably a air leak n the intake after the carb. Probably the gasket. Airs sucked in and the idle goes up. Then the mixture causes the engine to start to die and the idle goes down. Repeats over and over. Check gaskets, hoses, head warpage.anything that would allow air sucked in after the carb.

Sputtering out the carbs. The fuels not being regulated as it enters the carbs. Check the power needle, float level adjustment, regulator.


In addition to the carb gaskets, double check the rubber intake boots for cracks that can open and close as the engine vacuum changes, causing occasional lean running, making the RPM increase.

A cheap diagnostic is to warm up the engine, then when the idle is hunting like that run a garden hose or other smooth stream of water over each gasket and rubber intake boot, one at a time. When the water goes to the crack it will prevent it from sucking air and you will either hear it sputter, see it start blowing steam (if the leak is bad) or stop hunting (if it's just a crack the water fills it in for a few seconds). In either case, that's your culprit.

Another test: If you take off the rubber boots, go in a dark room and put a bright flashlight in each, flex the rubber and see if any light comes through the cracks.

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