My Bike Backfires during high RPM Deceleration

Hey

Im hoping that somebody out there could help me out. I have a 1981 Kawasaki KZ750 4cyl. The Bike has a Dyna 2000 Ignition system, Vance & Hines 4 into 1 Exhaust, K&N Intake pods, New intake manifolds DynoJet carb rebuild kit as per Vance & Hines recommendations. New Spark Plugs, and Plug Wires. New Tank, Fresh Fuel All NEW Parts but when I make a high speed pass or throw a High rev it back fires on the deceleration end of the rev and at night you can see it spit fire out of the tail pipe. If I hold a steady high RPM it runs fine but it back fires when I let off the gas. The bike starts right up and idles perfect. Any suggestions on what could be causing the back fire would be a great help

Thanks

Have u done a plug chop at full throttle yet?

it sounds like your main jets are probably to lean by a bit try going up a size or two.

Mark, when doing a plug chop how long to you need to run it at a certain RPM? If testing full throttle, how long do you need to run it like that? Same for idle, and crusing, how long?

put fresh plugs in and give your self a good distance of run and shut down area i usally do it at a drag strip. make your run then pull the clutch and kill the ignition at same time keep the clutch pulled in and stop with either the brakes or coast to a stop. pull the plugs and check them out.

Steve, Backfires on de-cel can have many causes, But the most common on otherwise good running bikes, are the coupled effects of lean fuel/air mix when slides drop and close needle/nozzles, and most of main jet path. this and a less restrictive exhaust, cause a mix too lean to either ignite or fully burn before it enters exhaust, where surrounded by other hot gasses,it does ignite. a situation I see regularly on cruiser twins w/loud pipes. Raising needles/ increasing mains usually help.Plug chops are good, tho mastery of reading is almost a forgotten art. others will correct me If I"m wrong, but a couple of farts on decel are no cause for alarm, tho a machine gun chatter should be addressed if for no other reason than wear &tear on exhust/baffle (if it exists) G

Easy one ,,,
You have AIR getting into your exhaust ,
probably right by the head.
No air no backfire ,,
It just can't burn without fresh air.
Doesn't matter if your carbs are off, if they are.
Fuel CAN'T burn without air.

What is a Plug Chop?

the same symptoms will occur if the main's are a bit lean. a plug chop is done to verify jetting changes if you guys want to know more about them go to ngk's website.

I Fixed it. Since I am running individual air pods and a Dyna 2000 Digital ignition, there is no air box (no smog control) or mechanical vacuume advance. there are 2 rubber air injection lines that go into the top of the cam cover on the top front engine for the US model. These 2 air injection lines are for smog controll, but when you get rid of all the smog controll, it will suck cold air in through the canister under the tank. I blocked off the inlet for the smog canister and the bike runs WAY better and has no backfire what so ever. My manual shows that the UK model of this same bike dosent even have the injection ports on the cam cover. Thanks for all of the advice. I hope this will help some one who is having the same issue I was.

canadian models don't have it either so it must have something to do with california emissions.

Yeah, in the haynes service book it claims that it is on US modles only.