The problem is only at idle it will pop out the airbox and die. Stock air box/filter, stock pipe, no mods to ing. The valves were adjusted and are good, carb was cleaned, good fuel flow, new spark plug, good fuel level in float bowl, bypassed parking brake, timing is perfect, ex is cleared, lifted the needle and it ran worse, good performance above idle, 170 psi comp., strong spark, mixture screw is unopened from factory, gas is good. Still the problem wins. Any ideas????

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71 CB 750 K1
94 PC 800
76 CB 550 K
75 TS 185
76 TC 185
63 YSA-1
06 YZ 450F
84 XL 350R
02 KX 85 with 75 XR 75 motor XR 200 sus.

Most likely if you did not remove the idle mixture screw for cleaning then the idle passages are not cleared. Before you remove it turn it in counting the turns till it stops so you can put it in the same position when you are done. Make sure the tank is clean as well. Once you get it right keep sea foam in your gas and it should prevent it from happening again.

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1984 Honda Magna 700, 1987 CBR1000F, 1991 Nighthawk 750, 1994 FZR600, and for the dirt 1995 KTM 300 EXC and finally a 1986 KX500

Thanks for the thought. Pulled the carb and removed the mixture screw. The idle circut is cleared and it did not fix the problem. Went first two turns out the three turns out. It adjusted the idle, but did not get rid of the backfire.

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71 CB 750 K1
94 PC 800
76 CB 550 K
75 TS 185
76 TC 185
63 YSA-1
06 YZ 450F
84 XL 350R
02 KX 85 with 75 XR 75 motor XR 200 sus.

Who did your valves? Sounds to me like they might be a tooth or two off. Not hard to do, especially with that motor.

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Have owned - 2001 Suzuki Volusia
Current bike - 2005 Kawasaki Z750S
MMI Graduation date January 9th, 2009. Factory Certifications in Suzuki and Yamaha

It sure sounds like a valve issue then since you have ruled out a lean condition. Maybe a burnt one from bad clearance before or an improper adjustment. Another big issue on air cooled engines with bad fuel is it can build up gunk on the valve stems and stick them or make them sticky or slow. I see this a lot on briggs ect.. lawn equipment that sits. The owner thinks it fine to use the caramel colored fuel because it seems to run fine........ then engine damage.

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1984 Honda Magna 700, 1987 CBR1000F, 1991 Nighthawk 750, 1994 FZR600, and for the dirt 1995 KTM 300 EXC and finally a 1986 KX500