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Discussion Starter · #1 ·
im working on a buddys 04 raptor660 with billet adapter and k&n filter with outerwear NO airbox lid, white brothers R4 full exhaust

currently jetted at 148/150mains...22.5 pilots..4th groove on the needles not sure on the mixture screw...im in california SEA LEVEL


i cleaned out the carbs, installed new pilots...replaced the float valve seats and the o-rings that go around the adapter that the main jet screws into

it runs pretty good but backfires when i let off the throttle and when i give it some throttle it pulls fine

any ideas what could be wrong? the bike sat for over a year....
 

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I am unique
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im working on a buddys 04 raptor660 with billet adapter and k&n filter with outerwear NO airbox lid, white brothers R4 full exhaust

currently jetted at 148/150mains...22.5 pilots..4th groove on the needles not sure on the mixture screw...im in california SEA LEVEL


i cleaned out the carbs, installed new pilots...replaced the float valve seats and the o-rings that go around the adapter that the main jet screws into

it runs pretty good but backfires when i let off the throttle and when i give it some throttle it pulls fine

any ideas what could be wrong? the bike sat for over a year....
With an open air box you will need #25 pilots, aftermarket needles with the clips at half way and 160 to 165 mikuni mains...........
 

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Back firing when you let of the throttle is a Lean pilot / idle circuit. Put #25 pilots in with the fuel screws around 2-2.5 turns out.

Think of it this way. Changing the pilot jets is a coars adjustment. The fuel screws are the fine adjustment. If you have the fuel screws under 1 turn out then you need to step down 1 pilot jet size and reset the fuel screws to 2 turns out. If your more than 3 turns out your lean and need to install the next size larger pilot jet. Then reset the fuel screws to 2 turns out and fine tune from there.

By looking at your main jet size your running a dynojet kit.

Your mains are fine. Dyno Jet 148/150 is around the same size as Mikuni 165.


Since it has set for a year make sure the carbs are spotless. It doesnt take a very big pice of junk to plug up a pilot jet.
 

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Master of the Electron
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Sounds like you have DJ mains - and 148L/150R will be fine. YOu should have 25 pilots, but try opening up your fuel screws a half turn each and see if the backfiring gets better with the 22.5's.
 

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Discussion Starter · #5 ·
i started it up today again and went up and down the street real quick and didnt backfire...yesterday the engine might of got warmer and today i just went for a quick spin

if it needed a valve adjustment would it backfire? everything seemed fine other then the backfiring it was doing yesterday when i let off the throttle

jetting has been the same, i didnt change anything other then just installing new pilots to avoid risking the old ones being clogged even after cleaning them
 

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It should be more apt to backfiring when cold because it would be leaner at a cooler temp. Idealy you should have a slight soft poping or woof sound when letting off the throttle. If you getting loud, hard, sharp cracking like a fire cracker then it is lean.

As for the valves I dont know I havent ever ran mine out of spec. I usualy check and adjust them before it gets to that point.
 

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Master of the Electron
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Backfiring on deceleration is simply due to being a bit too lean on the idle (pilot) circuit. Turn your fuel screws out 1/4 to 1/2 turn each... if this doesn't help, or you're already at or beyond 3 1/2 turns out, then you need to up to 25 pilot jets and start the screws at around 2 turns out.
 
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