Loud "Popping" (backfire?) on overrun?

I am getting bad & loud "popping" on decel or heavy overrun. Any ideas?

I recently modfied my '01 WR426's stock muffler, by opening it - drilled 16X 3/16th holes in a circle around the edge of the original 22mm core. All stock otherwise - (Aus., no airbox lid, WR timing)

After messing around a bit I wnet to the local dyno with the following results:

Final setup 42 PJ, OBELN#4, 165 main (otherwise stock) - air screw at 1.5.

He also got good results with stock needle - 42PJ, OBDRR#4, 165 main.

When he put in the OEBLN needle he got a slightly higher curve which starts a little earlier in the rev range. The curve is smoother than the OBDRR and higher in the mid range.

Increasing the main to 172 brought on the power slightly earlier agian, but the curve was a little lower, not as smooth, and finished a bit earlier.

When I tested my bike it seems pretty good overall though on the motocross track I had bad, loud "popping" on decel or heavy overrun. It is absent or far reduced on the more open stuff.

I didn't mess with anything as i need it running properly next weekend but the air screw is at 1.5 out (same for all the dyno tests) and .25 to .5 turns further out may clear up some of the popping.

There is another bloke who is running a similar mod to the stock pipe (replaced 22mm core with 40mm core). With wr timing he is running 38PJ , 65 PAJ, drm needle #4, 155 main jet, 200MAJ - without serious backfiring.

Any suggestions on how to reduce the "popping" would be appreciated.



Too lean on the pilot circuit, richen it up.

What happens just before you switch to reserve....it'll start popping & banging because its starved of gas. Same thing here.

thanks missile,

in theory I would agree with the to lean on pilot circuit - but i am running a 42PJ

go to 45PJ?

a lot of people seem to be running smaller PJ's - 38's?

Remember this is on heavy overrun that it pops.

I am still learning.

possibly the MJ being too large and unburnt gasses. if it was good for 165MJ on the dyno leave it there.

have you done your snap wheelies and adjusted your PS? go do it now!!!!!!

good to hear that the TT settings are now worldwide and on your mates bike.

why worry about it anyway? you've probably got a big arse or your hairstyle is out as well.

my bum looks big when i ride

Taffy :)

Taffy offered a table of ratios of PJ to PAJ, this may be where you are off. If you dont have a PAS (Pilot Air Screw - a screw doodad with infinite PAJ settings which replaces individual PAJ's).

Look for a thread called Jetting Q's and the table is somewhere in that. From early this year I think.

Good luck


Tested my bike post dyno on a fairly open motocross track - ran crisply but still popping a bit on decel. I was a bit concerned as I had done little high speed testing (limited by venue) but went with Dyno settings - they are the experts?.

Arrived for Hattah Desert Race (3 hour enduro), to find plug completely soiled from previous weekends test ride - new plug for prologue, ran ok over 3.85km's open(ish) whoopy stuff.

Rode down the road 1 km and got real bad coughing and spluttering before fouling plug and stopping - plug was excpetionally fouled - way to rich up top?

Dropped needle one click - clean plug and it started fine.

Started race and it was ok on the open stuff, doughy in the tight stuff and then coughing and spluttering when going back onto open bits.

It seemed to clear after revving until my fuel stop after three laps - it started first kick (to my surprise) but never revved out and stopped at the end of the pits.

Pushed back, changed plug (not 30 second job!) and got one more lap in. The plug was fouled to hell - enough fuel in there to run a Leyland P76! - rich on pilot?

I am pretty annoyed not to have it right yet but will be dropping the pilot jet size during the week and trying that.

I am pretty sure that the popping is not (at least on my bike) the traditional backfire due to lean mix, rather it is unburnt fuel ingniting within the pipe cos the bike is running to rich.

Is/was the dyno worth the $'s? I am fast learning more than what I want too about jetting fourstrokes (rather be riding) and think the dyno would be best suited for testing the bike once you have set it up. No substitute for testing by riding. The dyno gives good info eg - showing whether you have a smooth power curve, flatspot or what. I will be expecting more tests to get my monies worth as the setup post dyno definatley was not right.

Any more suggestions?

Yes - I have read and even printed jetting Q's, and done a fair bit of fiddling - just haven't got it right yet.


Jetting 95% fixed!

OK, heres the wrap up:

I was at 42PJ, 75PAJ, 165MJ, 200MAJ, OBELN #3 FS 1.5 out. It was still fowling plugs in the tight stuff and popping and farting a lot.

I went back to the dyno, and put in the 40PJ and it still gave a good smooth curve. I had definately learn't more about jetting a WR426 than the dyno bloke and he lost interest a bit when a big arsed computerised superbike turned up so I took it home to test. Still popping and burping like hell initially so I went in to 1 turn out on the Fuel Screw (FS).

Tested on Saturday morning and once I found a plug that wasn't fowled and gave it a bit of a run (some backfiring and farting as it cleared itself out), it went well, snappy off the thottle in all rev ranges, "snap wheelies" in most gears.

Still a slight bit of a burp when cracking the throttle open from idle but I haven't done to BK Mod to reduce the fuel squirt (no plans to at this stage).

Rode again on Sunday on a slightly faster track and found there was still a bit of popping and farting on heavy overrun but it dissapeared when I put the FS in to 3/4 of a turn out.

Thank god for a good ride (no fowled plug) with that extra bit of mid range snap the OBELN needle is supposed to give. Mininmal to no popping!

I may yet try the 38PJ with FS @ 1.5 which should give a similar result and may let me get the needle to clip #4.

Thanks for the suggestions.

You really should consider doing the bk mod.

I have almost exactly the same jetting as you and my 400 runs really clean. Before I did the bk mod she would load up with fuel in the tight stuff and the plug would come out black and sooty.(my Ap squirt lasted for about 5 seconds stock)

Try to imagine riding through a good tight section of switchbacks that has say 5 straights about 50 meteres long linked together with tight right angle turns between them. You're on the gas for the first straight for say 3 seconds.(AP is squirting for the whole straight)close the throttle, Brake , turn, on the gas for the second straight-say 4 seconds,(another 4 seconds of AP squirt)close throttle, brake turn, on the gas again for another 4 seconds( 4 more seconds of AP squirt) etc etc etc. Without doing the bk mod you're AP is squirting fuel in for the entire time you're on the gas in the tight going and after a while you start to get some blubbering and eventually the plug fouls. If you hold it wide open for a good long 20 second blast or ride at a constant throttle for a long time she will clear out again, but everytime you close the throttle the AP resets and is primed for another 5 second squirt.Yes this means every gear change!

If you look at how much fuel is being poured in by the AP compared to how much the motor can pull through the jets you will realise that its way more than whats required and confuses your jetting symptoms bigtime. Give it a try, you wont be dissapointed in the improvement and if you are, you can easily return it to stock setting with the turn of a phillips head screwdriver.

MOTMAN393's website has some good pictures and info that make it really easy to understand.

hope this helps you understand how it causes problems.

Thanks Michael,

when I say no plans to do BK mod at the moment I guess i am saying I am going to enjoy riding for a couple of weeks before I venture into the next mod.

It was good tonight to catch up on another couple of jobs in the shed instead of messing wiht my jetting (refitting bull bar on yoot).

I am sure the BK mod will be on the list one day.

Visited Cairns a few times over the last couple of years - always without bike on work trips, so I am a bit jealous of your neigbourhood.

What the hell is overrun?

What I am calling overrun is heavy engine braking when there is load on the motor from the momentum - ie as go from wide open throttle (WOT) and to closed throttle heading into a berm or corner.

I guess overrun means the engine is running "over" the pace at which it otherwise would at that throttle position.

make any sense?

Originally posted by Snapper:

make any sense?

Yes. Quite.

What size PAJ do you have?

Currently running 75 PAJ (stock) see signature.

If I go to the 38 PJ I may need to go down on PAJ (ideally) see Taffy's tables in jetting Q's.


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