Fuel is likely leaking into one or more cylinders as it sits and cools. Then when you start it up, all that pooled fuel ignites and fires out the pipe. The "compression" you think the starter motor is fighting is a precursor to hydraulic lock and can actually cause it to bend a connecting rod. Your bump it and wait starting method is clearing out the full cylinder so it doesn't backfire. There is pressure in the system when the motor is shut down after a ride and it's blowing all the fuel in the line into the leaky carb. Fix it before you ride it anymore.

 

+1 right on!

 

ah man, i had a 2000 zx7 with a 94-97 9r motor swap and that thing would kick my ass trying to start and sometimes same thing. it would always scare the shit outta me cause i didn't know when it was going to backfire, but i got use to it after a while. but not sure why its doing that.

 

So whats the fix then? Do the carbs need to be rebuilt? i cleaned them over the winter and it ran fine for alittle bit and then got progressively worse upon start up. it feels like it lost power, not so much up top, but down low and mid-range is suffering

 

I have the same issue with my 2000 9R Im hoping to get in there and fix this and the horrible low rpm stumble in one shot. Im anxious to see how yours works out. keep us posted.

 

if my 2000 e1 zx9 sits for a couple of days in the sun then as soon as the starter is pressed BANG i suppose it must be fuel vapour laying in the exhaust, sure as hell sounds loud at 6.30 am.

 

I start it now by blipping the starter till it doesnt spin so it doesnt backfire then hold the starter for about 5 seconds till it spins again

WTF??

How the hell do you "'blip" a starter??

 

:squint

 

:headscratch

 

:dunno

 

Appart from what HH wise accurated said: How is your valves guides and stems, in regard to carbon buildup? Some of them could remain open for a few seconds when starting by stuck in place, and causing those backfires.