LOUD backfire!!! Hawk 250

hnu13 · 01-17-2017, 08:23 PM

bike has a very loud backfire on deceleration..

- Mods -
stock pz30 carb
0.5mm overstock pilot jet
115 main jet
needle raised +2
mixture screw about 1 1/4 out
POD filter
spark arrestor out
baffle drilled out (about 8 holes)
cat removed

any ideas????

Sport Rider · 01-17-2017, 08:57 PM

did you check valve clearances?

Azhule · 01-17-2017, 09:03 PM

Usually "Popping/Backfire on deceleration" means : Pilot jet too small or Mixture Screw is off (usually a bit lean)... might need to check valve clearances too

Also... according to "Mikuni"... "some heavily modified bikes (free flowing exhaust) will always have some kind of backfire/popping on deceleration"...

You might try 1 1/2 to 1 3/4 turns or more on the mixture screw... make sure you go no more than 3 turns out... go to a bigger pilot jet if you go 3+ turns out

hnu13 · 01-17-2017, 09:22 PM

Quote:

Originally Posted by Sport Rider

did you check valve clearances?

yes, just did them last week

pete · 01-17-2017, 11:56 PM

air leak in exhaust most lightly..

air is getting in and igniting the unburn fuel
that gets in on a closed throttle....

Fuel / Heat / Air = BANG

It may have always done it... now yer gutted the muffler
& removed the CAT you can now hear it...

My XT660 pops/bangs like a bastard on a trailing throttle.
But it has a AIS (Air Injecton System) to let air in to burn the unburnt fuel....
Clean air bullshit and all that..
never head it till I put the Metal Mule pipe on...

....

**JerryHawk250** · 01-18-2017, 07:30 AM

I agree with Pete. Sounds like an exhaust leak.

'16 TT250 · 01-18-2017, 10:57 AM

If the bike has PAIR (exhaust air injection) blocking it off may cure it. Assuming it's tuned right and there are no defects this will likely stop the pop. If it doesn't have PAIR then you might have an exhaust leak or it might just be going lean on decel, not uncommon.

Weldangrind · 01-18-2017, 11:02 AM

Quote:

Originally Posted by hnu13

0.5mm overstock pilot jet...

Please explain what you mean.

hnu13 · 01-18-2017, 09:01 PM

Quote:

Originally Posted by Weldangrind

Please explain what you mean.

i drilled out the pilot jet to 0.5mm

Weldangrind · 01-19-2017, 11:14 AM

Quote:

Originally Posted by hnu13

i drilled out the pilot jet to 0.5mm

Thanks for the clarification. If I'm correct in my understanding, that equates to a #50 slow jet. Since the Keihin style uses much larger slow jets than Mikuni, I presume that size to be in the ballpark.

Do you have an air injection fitting or has it been blocked off?

hnu13 · 01-19-2017, 07:42 PM

Quote:

Originally Posted by Weldangrind

Thanks for the clarification. If I'm correct in my understanding, that equates to a #50 slow jet. Since the Keihin style uses much larger slow jets than Mikuni, I presume that size to be in the ballpark.

Do you have an air injection fitting or has it been blocked off?

whats that? lol
but seriously, I'm not sure what that is.

hnu13 · 01-19-2017, 07:49 PM

Quote:

Originally Posted by pete

air leak in exhaust most lightly..

air is getting in and igniting the unburn fuel
that gets in on a closed throttle....

Fuel / Heat / Air = BANG

It may have always done it... now yer gutted the muffler
& removed the CAT you can now hear it...

My XT660 pops/bangs like a bastard on a trailing throttle.
But it has a AIS (Air Injecton System) to let air in to burn the unburnt fuel....
Clean air bullshit and all that..
never head it till I put the Metal Mule pipe on...

....

i not sure where it would be leaking? i removed the header pipe last night and re installed it to make sure everything was proper, and it looked fine..

**JerryHawk250** · 01-19-2017, 08:29 PM

Quote:

Originally Posted by hnu13

i not sure where it would be leaking? i removed the header pipe last night and re installed it to make sure everything was proper, and it looked fine..

Did you replace the gasket?

hnu13 · 01-19-2017, 09:08 PM

Quote:

Originally Posted by JerryHawk250

Did you replace the gasket?

no sir.
to the head?

**JerryHawk250** · 01-19-2017, 09:18 PM

Quote:

Originally Posted by hnu13

no sir.
to the head?

Yes the one at the head

01-17-2017, 08:23 PM	#1
hnu13 Join Date: Oct 2015 Posts: 66	LOUD backfire!!! Hawk 250 bike has a very loud backfire on deceleration.. - Mods - stock pz30 carb 0.5mm overstock pilot jet 115 main jet needle raised +2 mixture screw about 1 1/4 out POD filter spark arrestor out baffle drilled out (about 8 holes) cat removed any ideas????
	0

01-17-2017, 09:03 PM	#3
Azhule Join Date: Dec 2015 Location: CO Posts: 1,525	Usually "Popping/Backfire on deceleration" means : Pilot jet too small or Mixture Screw is off (usually a bit lean)... might need to check valve clearances too Also... according to "Mikuni"... "some heavily modified bikes (free flowing exhaust) will always have some kind of backfire/popping on deceleration"... You might try 1 1/2 to 1 3/4 turns or more on the mixture screw... make sure you go no more than 3 turns out... go to a bigger pilot jet if you go 3+ turns out __________________ "Think as you like... but this self proclaimed Professor is always right" - Buckshot "You never know what someone is hiding beneath their smile..." - NinjaTom - R.I.P.
	1

01-18-2017, 07:30 AM	#6
JerryHawk250 Moderator Join Date: Oct 2016 Location: Houma, La. Posts: 11,846	I agree with Pete. Sounds like an exhaust leak. __________________ 2025 Lifan KPT 400 2024 Linhai 300 ATV 4x4 2023 Lifan Lycan 250 Chopper 2023 Venom Evader 2022 Lifan KPX250 2020 Kawasaki Vulcan S (Sold) 2004 Honda ST 1300 (Sold) 2016 Black Hawk 250 (sold) 2016 Cazador180 XL 2014 Coolster150 JerryHawk250.com My YouTube Channel
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01-17-2017, 08:57 PM	#2
Sport Rider Join Date: Jan 2016 Location: Mooresville NC Posts: 2,148	did you check valve clearances?
	1

01-17-2017, 11:56 PM	#5
pete Join Date: Jan 2012 Location: ChCh , NZ Posts: 2,266	air leak in exhaust most lightly.. air is getting in and igniting the unburn fuel that gets in on a closed throttle.... Fuel / Heat / Air = BANG It may have always done it... now yer gutted the muffler & removed the CAT you can now hear it... My XT660 pops/bangs like a bastard on a trailing throttle. But it has a AIS (Air Injecton System) to let air in to burn the unburnt fuel.... Clean air bullshit and all that.. never head it till I put the Metal Mule pipe on... .... __________________ 09 XT660R ... 06 TTR250 ... 80 Montesa H6 125 Enduro... 77 Montesa Cota 348 MRR "Malcom Rathnell Replica"... Current resto projects.. 81 Honda CT110... 80 Kawasaki KL250A1... 11 Husaburg TE125 enduro... "sold" along with another 31... Lifan 125 Pitbike.. "stolen" ... KIWI BIKER FORUM...... http://www.kiwibiker.co.nz/forums/content.php All the best offroad rides in NZ... http://www.remotemoto.com/ E-mail... xtpete1@gmail.com Last edited by pete; 01-18-2017 at 04:45 AM.
	4

01-18-2017, 10:57 AM	#7
'16 TT250 Join Date: May 2016 Location: Hacienda Heights, Ca Posts: 281	If the bike has PAIR (exhaust air injection) blocking it off may cure it. Assuming it's tuned right and there are no defects this will likely stop the pop. If it doesn't have PAIR then you might have an exhaust leak or it might just be going lean on decel, not uncommon.
	0