ZX9R popping through exhaust on cold start

[stoinkythepig]

My 01 9R (carbureted) has been popping through the exhaust on cold starts for a couple of years now. Seems to happen mostly between 40 and 65 degrees, after it's been sitting for many hours. If it's really cold or warm-to-hot outside, it does not do it.

The bike is completely stock except the idle mixture screws have been slightly adjusted out to reduce a harsh transition from off throttle to on throttle (common issue on this vintage 9R). The mixture has been adjusted since 2002 and the popping started around 2009-2010. I doubt it's the cause, but it might be related.

The bike has 95,000 miles and might just be worn, but it runs normally after it starts and has plenty of power and smoothness. I have not done a compression or leak down test.

My starting routine: I apply the choke fully, leave the throttle untouched and engage the starter. It cranks for a couple of seconds, pops one or two times out the exhaust, starts and runs roughly for a second or two and smooths out to a steady high idle.

I've tried not engaging the choke fully, but it does not start that way.

Bike runs perfectly otherwise.

Plugs are fresh and make no difference.

Valve adjust makes no difference.

Mileage is unaffected.

There's no mouse nest in the bike and a clean air filter makes no difference.

It's not a big deal, just looking for ideas.

[DaveZX6r]

When was the last time you pulled your carbs and cleaned/inspected them? When I got my bike it had a similar problem. It turned out two of the float needles were worn/ varnished and were leaking fuel past. When it was running it wasn't as noticeable because whatever leaked in just got burned off. But when it sat the gas would slowly leak into the cylinders. Then when I turned it over bang. If you have a pyrometer hit your headers and see if they are all the same temp and that they warm up similarly.

[stoinkythepig]

I haven't had the carburetors apart yet, that I can recall. My carbs have electric fuel cut-off valves to prevent fuel flow from the bowls to the jets when the ignition is off. I suspect I'd have to have a failed float valve and a failed cut-off valve on the same carb for gas to leak into a cylinder when the bike is sitting. I suppose that's possible. I'll pull the bowls off the next time I take off the carbs. Thanks.

[hondarider102]

Wirelessly posted

I'd say it's carb related as well

[MikeyZ]

I have a ZX9 as well and know the exact problem your having.

If you do not shut the fuel off before you turn the engine off then some extra fuel leaks down into the cylinders- when you go to start it again, the extra fuel creates the noise.

To avoid it- turn off your fuel before you park the bike. It is a very common problem for these 9's.

Edit: Just read your above post- Probably worn needles?