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hi im new to the site and love it so far can someone help me my crf450r backfires and throwing flames out the exhaust. what can it be? is it a lean or rich mixture jets or what? please help

Welcome to the site !!

First , we need a bit more info ,

What year is the bike ??

What is your elevation ?? (where you ride it the most)

Have you checked the Valve Lash to make sure they are in spec ??

Has the bike sat for any length of time ?? (more than 1 month with gas left in it ??)


Does the Hot Start Lever work freely ?? , and does it make a difference using it when the bike is hot ??

Has the carb been taken apart or cleaned recently ??

Was the Fuel Screw removed for any reason and possibly incorrectly adjusted or damaged ??


Was the bike running fine , then started doing this ?? , or did this just happen and the bike was running fine then suddenly started popping ??

All this info is needed to figure out why its backfiring .....usually the main culprits are:

Dirty Pilot Jet

Old Gas

Intake Valves have tightened up and need shimmed and/or replaced

Fuel Screw O-Ring is missing or is damaged

Fuel Screw has been mis-adjusted

Hot Start Lever stuck

Hi sorry about that its a 2007

its about 300ft it can be less be its close to that.

yes i did before start up and they where but exact to the workshop manual.

no was empty and has new fuel lines installed

yes hotstart works freely didnt at first but fixed it haven't tested it when the bike is hot yet will test it tomorrow (didn't need it, started easy when hot)

yes that was the first thing i did and it was kinda dirty. i blow all the jets clean with air.

there was no fuel screw at first (the guy i bought it by seem to have lost it) but then i got a extended fuel screw i have set it in the way they explain but i can get the idle nice but the revving still has the flames and backfire

no the bike was running like that from the start but i thought it can be because of the fuel screw that was missing.

i will recheck the valves and check the hot start and redo the fuel setting.

thanks alot for getting back so quickly

OK , first thing to check is (you can twist the carb by loosening the front and rear clamps , so you can get it somewhat sideways so you can see better)to remove the fuel screw and make sure that none of the old parts were left in there , the O-Ring can sometimes stick up in there and then when you install the "new" screw/Washer/O-Ring , it will not seal properly , if you have a thin poker (or even a Zip Tie might work)that will fit into the center of a O-Ring it will remove it , if it was left behind , if you have a magnifying glass and a small pen-light you should be able to see up inside the hole real well , its possible that the O-Ring and the washer could still be up there and that is the issue , , after verifying that the channel is clear , then reinstall the fuel screw , with (from the handle end) is the Washer , then the O-Ring (closest to the tip) , i always slightly oil or grease the O-Ring so it stays pliable , then snug it down "gently" , just lightly seated , then back it out 1-1/4 to 2 turns , if you have to go more than 2 turns , then you need the next larger Pilot Jet (this is directly from the Manual)

The jetting recommendations for your bike per elevation is:


Jetting Guide for Main Jet:(pilot 45) CRF450R/X

Sea level....165
2000 ft.......162
4000 ft.......160
6000 ft.......158
8000 ft.......155


Temperature/Elevation:
One main jet size (up/down) for every 2,000 feet or 25 degrees in temp.

I would also recommend just replacing the Pilot jet as opposed to blowing it out with air , sometimes you can't get the minute particles and they can cause this issue , the Pilot Jets are around $6 or so , so its not that much to just replace it and be certain its going to get the proper amount of fuel

That should get you going