Another backfire question

I know this is a much asked question, I did an extensive search, but am still confused. I just bought a 03 yz450f. I love it, it runs great, but it backfires on hard deceleration. I have adjusted the fuel screw, and replaced the header gasket. It has great throttle response. Is this normal or does something need to be done. It is a lot of backfiring. Also, what exactly is backfire. I'm a 2-stroke guy, so this is all new. Is it detonation or something else? I am not excited about having to rebuild a 4-stroke anytime soon. Thanks for the advice, I am really excited that I found this forum, lot of great info :)

If you have adjusted the fuel screw out to the maximum of 2 complete turns from fully closed and still get decelleration popping then you are too lean on the pilot jet and need to go up a step or two depending on OAT (Outside Air Temp/Altitude). Might want to check into JD Jetting kit for your local.

Bonzai :)

Yamakazi is on the money as he usually is! :)

Not to highjack the thread but. . .

Along the same lines, my fuel screw came from the factory at 2.75 turns out. I've got the stock 42 pilot. Should I switch to the 45 and try to get it fewer turns out on the fuel screw?

I've heard of some screws falling out when they don't have enough tension on the spring, right around 3 turns.

I've got the Zip-Ty screw.

well I checked my pilot jet, it was already a 48, but I went up to a 50 (the largest the manual lists). It was still backfiring on hard decel and the performance went to pot :).

Any other ideas?

A 50 pilot jet? :) You have some other problem, not too small of a pj.

The 450s run great right out of the box, no jetting required unless you live in a high altitude area. You've either got an airleak, exhaust leak, tight valves, too low an idle speed, no plug gap or dirty carburator. Get the jetting back to stock and go from there...

As I mentioned on another post, I disconnected my TPS & found it greatly reduced the backfiring on hard deceleration! It also made the throttle way more linear.

Give it a shot, you might like it :)

I know this is a much asked question, I did an extensive search, but am still confused. I just bought a 03 yz450f. I love it, it runs great, but it backfires on hard deceleration. I have adjusted the fuel screw, and replaced the header gasket. It has great throttle response. Is this normal or does something need to be done. It is a lot of backfiring. Also, what exactly is backfire. I'm a 2-stroke guy, so this is all new. Is it detonation or something else? I am not excited about having to rebuild a 4-stroke anytime soon. Thanks for the advice, I am really excited that I found this forum, lot of great info :D

You have a very easy problem

its an "afterfire" problem as you describe ,instead of

a "backfire" problem...backfire- is thru the intake track and "afterfire" is thru the exhaust

look at your accelerator pump circuit with all other jets in the stock position-at near sea level condition

I finally figured it out :devil: The problem was my hot start plunger. The thread had stripped and it was partially out. I replaced it, no more backfire or afterfire or whatever it was :thumbsup:

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now