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Discussion Starter · #1 ·
Hello Everyone,
I got the new motor in the sprint about a month ago. It starts up easily, warms up quickly, but once the bike is hot, it idles poorly. It holds a pretty steady idle until the engine gets hot, and then when I come to a stop, it tries to die. I also have a slight backfire while decelerating, but it sounds like only one cylinder is backfiring. I am thinking that the air/fuel mixture needs to be adjusted, but I don't even see adjustment screws on the carbs. Where are they located?
 

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Could be a pilot circuit setting. If the carbs are ones you had before, you'd think the setting should be OK. But if not, they could be out.

Worth a check for air leaks around the carbs/airbox joints. It could be a carb problem but could just be a leak. It could also be an iffy float needle or float height setting, slightly flooding one carb. Popping on the overrun can be unburnt fuel making it's way through the exhaust valve and then banging on it's way out the exhaust.
 

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Discussion Starter · #3 ·
Thanks for the input. You were right on the money. I inspected the airbox and connections, and sure enough, the airbox has a crack in it, which is letting air in unrestricted.I ordered a new airbox and in the meantime, if I leave the choke on ever so slightly, it richens up the mixture just enough, and it idles like a dream. I clocked over 100 miles today, and it ran great. Thanks again.
Could be a pilot circuit setting. If the carbs are ones you had before, you'd think the setting should be OK. But if not, they could be out.

Worth a check for air leaks around the carbs/airbox joints. It could be a carb problem but could just be a leak. It could also be an iffy float needle or float height setting, slightly flooding one carb. Popping on the overrun can be unburnt fuel making it's way through the exhaust valve and then banging on it's way out the exhaust.
 

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Thanks for the input. You were right on the money. I inspected the airbox and connections, and sure enough, the airbox has a crack in it, which is letting air in unrestricted.I ordered a new airbox and in the meantime, if I leave the choke on ever so slightly, it richens up the mixture just enough, and it idles like a dream. I clocked over 100 miles today, and it ran great. Thanks again.
The crack in the airbox will make no difference at idle, as the restriction here will be the closed butterflies and not the airbox. Is it also difficult to start when hot? If so I would recommend checking the valve clearances. Some of the intakes might be too tight. After you've done that, try syncing the carbs and adjusting the pilot screws. If this makes no difference, replace the pilot jets. Check and adjust the float heights while the bowls are off. Resync and readjust the pilot screws when you are done.

Regards,

Derek
 
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