2011 WR450 Shooting flames out the exhaust

Hey All!

Did a search through some threads in relation to this problem but thought I'd post it here anyway to see if I get any different responses smile.gif

I've got a 2011 WRF 450 which is stock standard, no mods at all yet (Had the throttle stop etc removed) - only 150KMs on it and just had it's first service.

I purchased a new FMF Powercore 4 slip on muffler and put it on - bike started cold with no choke pretty easy - normally I need to put the choke on and takes a few kicks. The bike does not start if I have the choke on either..

Bike sounds good!

Once it warms up after about 2 minutes and I crack the throttle it backfires so damn loud - it also shoots flames out of the exhaust that reach up to a foot long. Bike idles fine with no flames - mid and high revs it backfires when I release the throttle.

Here's a video of the flames - although it didn't seem to backfire when I recorded it. I turned the lights out so the flame could be seen.

I've ordered a JD Jet kit which has just arrived so I'm going to mess with that later in the week. I recall there being a thread on here once where everyone posted there mods and jetting specs? is that still around? I couldn't find it.

I'm going to go with the problem being the jetting but would like your opinion too please smile.gif

Thanks all! action-smiley-035.gif

Edited by xsmilinx

Running lean now with exhaust opened up. Better fatten up the jetting before you do damage

You cannot damage a 4S by being a little lean. Total BS.

To the OP. You should always have to use the choke on a cold start. Did you reset the fuel screw? Did you mess with the carb, ie, float height?

Backfires with an open pipe like you have is normal, the price you pay for an open race pipe. Race engines are not just reved up. They are WOT then fully on the gas. You can however, confirm there are no exhaust leaks (hold a wet towel over the end of the muffler) and also confirm the ACV (automatic coast valve) is working.

Hey William, Thanks for the reply. Nah I didn't touch anything at all yet. Just slipped the exhaust on. :thumbsup:

Ive seen header pipes glow red. The exhaust valves were probably ok though

Thin wall headers glow when the bike is idling and sitting still because they are trying to shed heat that is normally absorbed by passing air. Perfectly normal.

With your pipe on, at worse you will have a weak top end. You do need to go up a size or two on the main though you need to test to see what works best. The JD kit has terrific simple instructions. Follow they EXACTLY. Do not second guess them.

JD does not explain this part in detail (I think) so follow this:

Fuel Screw/Pilot Jet

Fuel screw settings in the 'book' are recommended starting points. Every bike is different, as is the temp and altitude. Set the screw according to this method. Do it with the bike fully heated up.

Gently turn the screw all the way in. Now back it out two turns. Start the bike and fully warm it up, go for a 10 minute ride. Set the idle to speed to 1,500~1,800 RPM as best you can (I know, without a tach this is tough, just set it to were it idles relatively smoothly). Once warmed, slow the idle to the lowest possible speed.

*** When turning the fuel screw, keep an accurate 'count' of the amount you are turning it and record it in case you have to reset it for some reason. Makes life easier when you can just set it from notes Vs. going through the procedure again.***

Turn the screw in until the idle becomes rough or the bike stalls.

if it stalled, open the screw about 1/4 more turn. Restart it and slowly screw it in till you can just perceive a change.

If the screw can be turned all the way in and the bike still idles perfectly and does not stall, then you need to go down a size in pilot jet.

Now very slowly, open the fuel screw till the idle is smooth. Blip the throttle, let the bike return to an idle, wait say ten seconds. Confirm it is the same smooth idle.

If the screw has to be opened more than 3 turns to get a smooth idle, you need to go up a size in pilot jet.

If you find it does not stall with the larger jet but has to be open more than three turns with the smaller pilot jet, put the larger one in and set the fuel screw at 1/2 turn.

If the idle speed increased, adjust the idle speed knob to return the bike to a real slow idle speed. You must then re-visit the fuel screw. Keep doing this till the fuel screw is opened just enough to provide a nice steady idle at the lowest possible RPM. Once this is done, increase the idle speed to the normal one for your bike, typically about 1,850 rpm, but go by the spec in your manual.

Wow - Thanks for the detailed instructions William. I'll let you know how I go!!

hey William,

Just a small update :blink:

I've replaced my Main jet from a 162 to a 168 as per the JD Kit instructions.

Also changed the pilot from a 45 to a 48

No more flames :thumbsup: very responsive on the throttle too.

I now need to play with the fuel screw and idle as you mentioned above.. (I replaced with a zipty fuel screw while i was at it!)

The bike does not run without the choke off - even when warm.. when the choke is on it idles like it should when the choke is off.

I have the fuel screw 2 turns out.

Will let you know next week how it goes!

Thanks again

Something is not right. You should be fine with the 45 pilot. It could be a bad fuel screw. Do the test and confirm.

ah.. the JD instructions said if my fuel screw is 2 turns out then to replace my 45 with a 48. Ill play with the fuel screw to see if its working, if it is Ill put the 45 back in and try that out


Had the same problems, William's info helped a lot. Big thanks to you bro .

Hey William,

I've put my 45 pilot jet back in and it's blowing flames again.

I also had the fuel screw all the way in.

The bike also ran without the choke on and its idle sounded normal

Twisting it out 2 turns made NO difference,

If i fully closed it i only JUST heard a slight difference in idle - bearly.though

Any ideas? :thumbsup:

Thank you

*** Also forgot to mention earlier im using the red needle to suit my altitude on the 4th clip down as per JD instructions

Edited by xsmilinx


I've just realised.

When I removed the stock fuel screw only the spring and washer came out - no o-ring.

When I installed the zipty fuel screw I put the o-ring on not knowing one was already in there.. I guess I've doubled up now.

Unless there is NO o-ring to begin with?

Anyway i got a mirror and got the o-ring out - but i cant see a 2nd one in there.

Would that cause the problem?

*sigh* :thumbsup:

Update: so I can pretty much confirm there is only 1 o-ring. Reinstalled the fuel screw but no change at all. :blink:

I'm going to buy a different fuel screw to see if that helps.

Edited by xsmilinx

Buy a brass or stainless steel screw.

Read the instructions I posted a few more times plus my previous posts in this thread. To recap, the pilot jet and fuel screw are for idle ONLY. They are not for adding fuel on decel. If the bike idles with the fuel screw all the way in on a 45 pilot, that means the pilot is too large and you need a 42. It also makes me suspect the ACV is hanging up.

Did you ensure the slow air jet passage was clear? Float height is right?

Thank you William.

I'll get a 42 pilot at the same time as the screw.

I have not checked the float height as of yet - I will though. Bikes almost brand new so never messed with the float height on it.

Appreciate your patients - first time i've ever played with a carby :rolleyes:

This is one area id like to do myself rather than take it to the bike shop :)

42 Pilot jet is installed along with new fuel screw - It now has an effect.

1 turn out is where it stalls.

Will do the rest over the weekend and let you know how it goes..


Appreciate the help William - I think it's time I took it to get properly tuned.

42 pilot and the fuel screw cuts off about 1mm from being closed.

I'm not confident enough to pull the carb out myself to check the float height.

I did check the ACV - the thing with 2 screws and the spring inside - there was a little air passage in that and confirmed it was clear. There was also a circular rubber plug inside which was also clean. As far as testing it I'm not too sure how.

Spent the last 3 nights trying to do this - the bike runs fine with all of these settings but it's still blowing flames (only when the header glows red though)

Cheers all!

Again - appreciate your time in helping me try and fix this.

Edited by xsmilinx

Hey Guys

Figure I'd update this post! and hopefully get a little more help. :rolleyes:

I took the bike out to a friends place who has a lot of property to test ride on.

Altitude is roughly 800 feet and temperature is about 12 - 20c (or 53 - 68f)

168 main jet

42 pilot

Red Needle 4th clip down

Fuel screw stalls the bike about 1/8th of a turn out so I have it set to 1/4 turn out. (Thinking I need to drop in the Pilot again?)

FMF Powercore 4 Slip on

Grey Wire Mod

Bike runs great and has good power all the way to WOT.

Decel popping is very minimal now (still get a few quiet ones when slowing from high revs. Not sure if flames are gone - bit hard to check when riding - I know this only happened when I let it get too hot)

I changed my main to a 165 and took it for a ride and noticed it was slightly missing at WOT so I put my 168 back in which improved it a lot. So I'm assuming my main is jetting correct.

I moved my needle position up 1 notch (into #3) and took it for a ride, had more backfiring, moved it up again (#2) and it was worse, I then put it into #5. Out of all of those #4 felt the best.

I'm happy with it's overall performance but I'm getting a light dry black soot around the end of the pipe and on the spark plug after no more than a 500 meter test ride. This happened on ALL my jet Changes I listed above.

I figure it was a bit Rich that's why I went down on the main and needle for testing. I still got the soot when using the lower jets/clip.

I then took it for a ride and went no more than 1/4 throttle and in 1st/2nd and 3rd gear - no soot.

Seems I only get it when I open it right up.

I don't know if this is a proper form of testing but figure I'd put it up here anyway.

I can confirm there's no blockage in my AVC and the diaphram is in good condition.

My Air filter is clean

I have properly sealed my Muffer to my header with High Temp Silicone as recommended by FMF.

I have not checked my float level though - if this is a must then I'll take it to a mechanic. (Not confident in removing the carby)

Do you think the soot is something to worry about?

Thanks again for your help :thumbsup:

No, all sounds good. The soot is probably from the AP. The AP makes things very rich each time you crack the throttle. The soot would only go away if you went on an extended, steady throttle 'speed run'.

Great - Thanks William.

Im taking it out this Saturday to see what mileage I get.

Appreciate the help once again TT

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