Tony wrote on 12/31/69 at 16:00:10:OK, I know I have the same old question again but:
For some reason I cannot seem to get my carb tuned correctly with the K&N air filter. I have already done the spacer mod and installed a 155 main jet. Backfires are constant at a throttle down, including shifting. I reinstalled the stock filter and the bike runs great with little or no backfire. I searched the old topics and it apears i have done everything. Do I need a bigger jet or a different pilot jet?
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Tony, you have provided the information needed for the fix in what you have already stated..."backfires with the K&N .....does not backfire much with the stock air filter".
What this tells you is that you are running LEAN with the K&N, because the K&N flows much more air which leans out the fuel/air ratio. When you put the stock filter back on, it does not flow as much air which makes for a richer fuel/air mixture.
So, in order to use the K&N (which is preferrable) you need to compensate for the extra air flow, and the way to do that is to provide more fuel...in the pilot circuit...so you need to go up a size on the pilot jet.
This does assume of course that there are no air leaks (as stated before) and that you are correctly tuning the pilot jet adjustment screw.
Keep in mind that a main jet change affects 3/4 -full throttle.
A white spacer or clip position change on the needle affects 1/4-3/4 throttle.
A pilot jet change and screw adjustment affects idle-1/4 throttle...AND THIS IS WHERE THE BACKFIRING ON DECELERATION ISSUE COMES FROM.
***Note that I said "throttle position"...not speed or rpm. Throttle position is what changes where the slide is in the carb. You will find that almost all backfiring on deceleration occurs when the throttle is at or near the idle position, and this is when the carb is operating in the pilot circuit...regardless of the mph or rpm.
The problem is caused by an incorrectly jetted/tuned pilot circuit...and the FIX must be within the same pilot circuit.