\"chuff\" back thru carbs, lite pop on decel

81 kz750E with stock Keihin carbs, ~21,000 miles, V&H 4in1, stock airbox with foam washable filter, dynojet jet kit installed as "stage one", finally replaced the original plug wires! :whistle:

Not sure I should complain because essentially the bike runs great but I was wondering what might be causing a what I call a “chuff” (no bang) back through the carbs when lightly opening the throttle :huh:. At times it seems like if the stock air box wasn’t there, it might even knock the carbs off. It does this regularly when cold and occasionally when warm at light throttle under light load. I found it reduced when warm by lowering the idle speed to about 900 rpm (with the adjuster knob behind the carbs). It's still annoying at least (and hopefully not a problem).

When I got the pipe I also got a Dynojet kit (yeah, I know, but I'm lazy - false economy?) and drilled the slides, installed the kit needles and have the “Stage 1” main jets installed (130). I had tried the “Stage 3” main jets (136) but it seems to be too rich (“breaking up” at WOT). I have also raised the needles from the recommended fourth slot to the fifth slot and the mid range seems a bit stronger (just to see if a bit richer mid-range would run better - I think the light popping on decel was reduced a bit). Idle mixture screws are 3 ½ turns out as recommended in the kit instructions.

Ideas? Thanks much

When you modify your airbox, especially using a foam element as you did, jetting becomes a crap shoot. You obviously are flowing more air than with a stock set up based on your symptoms. The DJ kit includes the jet needles and main jet with the bogus numbers. OK... here is the problem... you didn't rejet the pilot circuit. Unfortunately, there are no replacment #38 pilot (Keihin calls them slow jets) that I know of. To richen things a bit, you raised your jet needles and got SOME relief... but not enough because the problem is in the pilot circuit. The jet needles only contribute SOME of the gas in the mid-range. On the CV34, there are two sets of other jets; the secondary main jets (which are really the main jets) and the MAIN jets. The main jets are offset from the center of the bowl and on your bike are #62. These are really what make a difference in the mid-range. If you can find some #68 which is the STOCK SIZE for later versions of the CV34 (used in 84/85? on the ZN700 and ZN750 models I think), and install those jets instead of the 62s, this would clean up your backfiring issue I think. I would also go back to the stock jet needle at that time as the Dynajet jet needles are generic and not suited for your needle jet which is on the smallish side. You might also check for air leaks... the vacuum caps might be cracked and you may also have a hole in one of your hoses connected to the vacuum fittings. You may also have a bad seal in either the carb holder or exhaust gasket... the exhaust gaskets are crush type and should only be used once otherwise they will leak and produce the symptoms you describe.

Thanks wiredgeorge! I'm with you I think - I'll be on the prowl for the slightly bigger (real) main jets and check the impact of changing needles back to stock. I'll also check for vacuum leaks (still some original rubber there) and exhaust - I noticed the fit on 1 and 2 for the V&H was off a bit and I saw some leakage when cold there the first time I fired it up after that, but nothing since. I'll get some new gaskets and reinstall. Time for some 'sperimentin' (half the fun is taking it apart and putting it back together )