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Back Fire On Lift Off?

Guest tigercatkid

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Guest tigercatkid



Just stripped and cleaned my ZX-9R carbs and replaced the float valves. All floats to correct height.


Now I get a lot of back-fire when I lift-off the throttle - and the engine idles fairly lumpy - I assume this is a fuelling issue, but what could be causing it ?


Fuel mixture screws all at 3 turns each, 160 main jets, needles raised with two M2 washers each, idling at 850 rpm when warmed up.


Is it too rich or too lean... how would I check and how would I adjust ?


Do I need bigger jets ?


Thanks for any suggestions !

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(Assuming you mean backfire out of the exhaust, not the inlets)... people often think this means "too rich" but I believe that's not necessarily the case - the simple, factual answer is that fuel is still burning after the mixture has been exhausted from the combustion chamber, but a lean mixture can also result in this because it burns slowly, incompletely, and then continues after the exhaust cycle. So, your fuelling/timing is certainly off, but I read somewhere you shouldn't assume it's definitely rich. No point measuring fuel in the exhaust as that will only tell you what you already know.


As for how to tweak on ZX9R bike carbs, pffft, no idea, I'll duck out of this conversation at that point! Presumably something around the idle adjustment?

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Let me assume your engine was running fine before you cleaned up the carbs and put in new floats. And you haven't changed to bigger jets or anything like that so no reason to assume that anything has gone wrong with them. 160 jets seems to be about right for a 2litre. Are you sure you have put your carbs back together properly (no air leaks or gaps or badly seated gaskets) and your carbs are sealed to the manifold runners (no air leaks there?) Are the rubber joiners in good condition? Vacuum hose to brake servo all good? Just about the only thing that you can reasonably adjust is the idle mixture. Try a quarter turn in (or out) on each idle needle and see if it gets better or worse. You won't end up with a finely tuned engine but you might make it drivable enough to get it to a rolling road. Carb Cleaner doesn't always agreed with rubber "O" rings etc on carbs. Sometimes when cleaning them its easy to overlook the "O" ring on any needle valve assemblies or seats. Sure its a pain to think about stripping carbs to check it so maybe that's a last resort. Do the simple checks first


Colourtune is another option that might let you "see" if the mixture is giving a nice clean blue burn.


Just a few thoughts



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All the above point raised are good pointers I would suggest a few more


you mention all the floats are at the correct height now, have they changed from previously? why did you change the needle jets? a leaking valve would have the effect of raising the float level previously.


You mention idle is "lumpy", did you balance the carbs after re fitting, bike carbs go off balance through normal use, let alone a strip down, even if you haven't touched them, agree with possible air leaks mentioned above. You have set mixture screws to a base setting have you moved them from base.


Balance first, then set tick-over mixture by turning all screws in and out 1/8 turn TOGETHER until you achieve best tick-over, you will probably have to adjust tick-over at times doing this, adjust the mixtures again when you do, usually, depending on use, you then lean them by a 1/4 turn from that point, but if the primary jet has not been increased it may flat spot when you blip the throttle. The mixture screw on bike carbs also has an effect over a larger throttle range than a normal carb, so increasing fuelling may get around a poor primary jet. Note, some mixture screws are air bypass, and work in reverse, out to lean.


As above this is only tweaks to get it to a rolling road, All fixed jets and needle jets need setting on RR, including primary and fuel compensation, to be right.

Edited by knights_templar
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one of the awkward things i found with the zx carbs was getting the diaphrams to seal properly whilst screwing the caps back on in the end i used silicone grease in the recess and this held the diaphram in place whilst i reassembled

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