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.

  • Related? mechanics.stackexchange.com/q/17953/6841 Commented Jun 25, 2015 at 18:29
  • Nope, that question is more about why a cold engine has to run faster than a warm one. Commented Jun 27, 2015 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 Commented Dec 12, 2015 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
    Commented Feb 22, 2016 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
    Commented Mar 1, 2016 at 5:07

3 Answers 3


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.


The sputtering/popping out the carbs is a lean condition. But, the idle, if it had a constant vacuum leak(still to be detirmined) would be bouncing a little more regularly. Start the bike. Get it idling without choke. Have a can of carb cleaner with a straw. Sparingly spray around intake boots/manifolds. Careful that over spray isn’t going in the air intake or the test will give false positives. If the bike dies or idle drops considerably, you’ve found a leak. (Do not use water to find leaks). The idle bouncing every ten to 15 seconds is a symptom of inconsistent float level or fuel restrictions. But you’re gonna need fresh fuel as high octane as you can get from the pump. 91/92 octane. No boosters. A couple gallons in a clean can. You’ll feed your bottle from this. A few pints at a time. Not sure if the bike has a vacuum actuated fuel petcock either. But until consistent flow can be determined we’ll keep the variables to a minimum. Also fresh spark plugs, a recent valve adjustment, and the integrity of the ignition components should be verified. A compression test couldn’t hurt either. Maybe good time to inspect gas tank and petcock for debris and functionality.

Anyway, you don’t have to throw money at it to diagnose. I wouldn’t recommend removing carb manifolds until you know they’re an issue. Bikes of this era start disintegration on disassembly. Diagnostics are how we determine what needs repair. Plug vacuum leaks (with duct tape if need be) while you search for more. Throwing parts at projects until issues are resolved, we’ll reserve for factory backed service departments.

Im concerned that you said “bench balanced” and “rebuilt” carbs. By “rebuilt”, “I’d” mean replaced all gaskets, orings, seats/needles, set float heights to spec, snaked out all circuits, and bench tested for fuel leaks and air tested slide speed???? Since Idk what has been performed I’ll start with fuel. You’ll have to have for fashion an I.v. Bottle for the balancing that has to take place on the bike. Make or purchase a manometer as well. Take the tank off and air filters. Make sure area undertank and above/around carbs is reasonably clean from dust, dirt, debris, and hardware. We don’t want garbage sucked into the engine. If you leave your project for any amount of time cover the carb intakes. I duct tape mine like I’m gagging a victim. (Rag in the mouth tape over rag)

Once you have eliminated the variables and your manometer and fuel bottle are set up you can balance your carbs. Do not attempt to set the fuel/air ratio (beyond what’s needed to keep from overheating during balancing). Once your carbs are balanced, replace the tank, air filters, and hoses. Carful that the tank position doesn’t pinch or ground anything out. Take the bike for a spin to get it good and warmed up. Then set you air fuel ratio. Keep in mind as the weather gets cooler ratio’s need fine tuning. A change of more than ten degrees before even thinking about it.

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