YZ450F Let Off Throttle and Backfires

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YZ450F Let Off Throttle and Backfires

Post by 4Strokes » Wed Jun 01, 2016 3:58 pm

Topic: 2005 yz450f backfiring. Carb adjustment questions?
Author: amg1127
Posted: 07/05/2009 8:25:07 PM

Hey guys I just purchased an 05 yz450f and it backfires pretty crazy when I let off the throttle. I just changed the spark plug. The previous owner said he had it set up to run in the mountains of KY and all I needed to do was change the needle. I have the service manual but I have absolutely no clue how to do this and was wondering if anyone could help walk me through the process. I've heard it is really easy but I'm not sure where to start.

What size needle do I change it to? Can I purchase at my local dealer or do I have to order online? This is my first racing 4 stroke and I'm a pretty novice mechanic. I have no experience at all adjusting carburetors so any other tips on how to get my fuel mixture correct would be appreciated.

I ride upstate NY, which is about 230 meters (750ft) above sea level, when now the temp is 75-85F with moderate to high humidity. Thanks!

Reply by GlennF on 07/06/2009 06:06:52 AM
Your bike, being jetted for higher altitudes probably has leaner pilot and main jets. Backfiring is due to lean pilot jet or fuel screw setting. You can start by turning the fuel screw out to richen the idle mixture. If the popping doesn't stop after you are out 3 turns from bottomed out, you should go to a richer pilot jet. Check plug readings to make sure your main jet is not too lean, as it can cause engine problems.

Reply by 250f canuck on 07/06/2009 2:20:30 PM
When it comes to jetting I have had excellent results with JD Jetting kits. They are a little pricey, ~$70-$80, but are totally worth it. They come with 2 custom needles and an assortment of main jets usually, as well as a set of instructions on how to set the carb up properly for your conditions. Their customer service is great from what I hear too.

Reply by amg1127 on 07/06/2009 5:09:33 PM
Thanks a lot! I'm going to try playing with the fuel screw and see if that helps. If not I'll look into that JD kit and let you know how it goes.

Reply by amg1127 on 07/07/2009 1:46:52 PM
I ordered the JD kit it should be in by Thursday. I will let you guys know how the installation goes.

Reply by 250f canuck on 07/07/2009 7:17:44 PM
;)

Reply by amg1127 on 07/10/2009 3:31:37 PM
Just put the JD kit on and the backfiring has stopped. The bike sounds awesome! I ran the red needle with the 170. It was super easy to install and trust me I am a very novice mechanic. Thanks for your help and the recommendation... the JD kit was well worth the $$$. FYI they are on sale for $59 right now at Cycle Gear.

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