Author Topic: Why this backfiring?  (Read 4979 times)

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Offline CB750wannabe

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Why this backfiring?
« on: May 20, 2012, 11:54:57 AM »
My 71 CB750K is backfiring on de-acceleration.  It sounds like its coming from the right side rear muffler.  I just had a 3000 mile tune-up.  It was doing the same thing before the tune-up on occasion during long mountain rides.  Any ideas?   
1971 CB750K Candy Garnet Brown
http://forums.sohc4.net/gallery/28485-110112203145.jpeg
1975 CB550 K1 Candy Jade Green

Offline killersoundz

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Re: Why this backfiring?
« Reply #1 on: May 20, 2012, 12:05:10 PM »
Not 100% sure but I don't think it's anything out of the norm, just a little unburnt fuel hitting the hot exhaust
My project thread:

http://forums.sohc4.net/index.php?topic=107447.0

My CB750K4 Starting up for the first time after a seized motor and rebuild!


Offline Duanob

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Re: Why this backfiring?
« Reply #2 on: May 20, 2012, 12:05:52 PM »
did you check your plugs ? My guess is a lean running cylinder. Probably a vacuum leak.
"Just because you flush a boatload of money down the toilet, doesn't make the toilet worth more",  My Stepfather the Unknown Poet

1974 CB360T
1976 CB550K2 Resurrected
1976 CB550F2 Barn Find
1979 CX500 VG "HONDA-GUZZI"
2007 Moto Guzzi Breva 750ie
2015 BMW F700GS
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Offline chewbacca5000

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Re: Why this backfiring?
« Reply #3 on: May 20, 2012, 12:09:32 PM »
did you check your plugs ? My guess is a lean running cylinder. Probably a vacuum leak.

+ 1 Short little pop pop pop on decel is typically a lean condition.

Offline killersoundz

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Re: Why this backfiring?
« Reply #4 on: May 20, 2012, 12:13:09 PM »
Can anyone explain why? I would think that would be a rich condition
My project thread:

http://forums.sohc4.net/index.php?topic=107447.0

My CB750K4 Starting up for the first time after a seized motor and rebuild!


Offline Stev-o

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Re: Why this backfiring?
« Reply #5 on: May 20, 2012, 12:26:57 PM »
What exhaust do you have? Baffle??
'74 "Big Bang" Honda 750K [836].....'76 Honda 550F.....K3 Park Racer!......and a Bomber!............plus plus plus.........

Offline lucky

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Re: Why this backfiring?
« Reply #6 on: May 20, 2012, 12:28:47 PM »
did you check your plugs ? My guess is a lean running cylinder. Probably a vacuum leak.

Backfiring on deceleration is too rich of a mixture.
Try turning the mixture screws 1/4 turn leaner. That should help.

Offline phil71

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Re: Why this backfiring?
« Reply #7 on: May 20, 2012, 12:43:45 PM »
got any holes in your exhaust? is your exhaust stock?

Offline CB750wannabe

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Re: Why this backfiring?
« Reply #8 on: May 20, 2012, 12:52:13 PM »
stock exhaust with baffles. no holes.  plugs are new and look good. 
1971 CB750K Candy Garnet Brown
http://forums.sohc4.net/gallery/28485-110112203145.jpeg
1975 CB550 K1 Candy Jade Green

Offline phil71

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Re: Why this backfiring?
« Reply #9 on: May 20, 2012, 01:11:09 PM »
If you have no vacuum leaks, and no exhaust leaks, you likely have a restricted idle passage, or jet. This is assuming you've dialed the air screws in best you can already.

Killer, the physics behind lean condition causing pops on decel are very complicated.

Google those words in various combinations , maybe look up scavenge too.

Offline Stev-o

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Re: Why this backfiring?
« Reply #10 on: May 20, 2012, 01:28:38 PM »
Are we talking pops on decel or an actual backfire?
'74 "Big Bang" Honda 750K [836].....'76 Honda 550F.....K3 Park Racer!......and a Bomber!............plus plus plus.........

Offline CB750wannabe

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Re: Why this backfiring?
« Reply #11 on: May 20, 2012, 01:31:04 PM »
pops on decel not actual backfire
1971 CB750K Candy Garnet Brown
http://forums.sohc4.net/gallery/28485-110112203145.jpeg
1975 CB550 K1 Candy Jade Green

Offline CycleRanger

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Re: Why this backfiring?
« Reply #12 on: May 20, 2012, 01:41:41 PM »
If you have no vacuum leaks, and no exhaust leaks, you likely have a restricted idle passage, or jet. This is assuming you've dialed the air screws in best you can already.

Killer, the physics behind lean condition causing pops on decel are very complicated.

Google those words in various combinations , maybe look up scavenge too.

What Phil says. Check the slow jets. You can do it without removing the carbs if you are careful. Small hands will help.
Do you have a copy of the Honda Shop Manual or Parts List for your bike? Get one here:
https://www.honda4fun.com/materiale/documentazione-tecnica
CB750K5        '79 XL250s     CL350K3
CB750K3        '76 XS650      '76 CJ360T

Offline TwoTired

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Re: Why this backfiring?
« Reply #13 on: May 20, 2012, 01:46:04 PM »
stock exhaust with baffles. no holes.  plugs are new and look good.

I'll need more than "look good".  Describe the deposit patterns.

Is the air filter stock or modified?

Backfire on decell is almost always too lean,
Here is why:
The engine RPM is driven by rear wheel on decel, while the throttle is closed and providing minimum fuel and air.  (Mechanical slide carbs.)
If the mixture becomes too lean, it will not fire at all on some power strokes and be pushed into the exhaust system unburned on the exhaust stroke.  It can do this for several cycles accumulating enough air AND fuel to support combustion, an ember then ignites it and you get pops on deceleration.

To cure this, the idle mixture is enriched, so even during decell, combustion take place on every combustion stroke.

Decel popping can also happen with exhaust system leaks and accumulated fuel in the exhaust.  Extra oxygen from the leaks mixes with unburnt fuel from the incomplete power cycles, again creating a combustible mixture.  Here comes that carbon ember again, and pop.

If you are satisfied that the exhaust header gaskets and system are air tight.  Check the Pilot mixture screw position compared to book value.

Today's modern shops are taught to adjust them to minimum positions, or as lean as possible, which is not correct for the SOHC4 carbs without accelerator pumps.

Cheers,
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

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bollingball

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Re: Why this backfiring?
« Reply #14 on: May 20, 2012, 02:43:48 PM »
did you check your plugs ? My guess is a lean running cylinder. Probably a vacuum leak.

Backfiring on deceleration is too rich of a mixture.
Try turning the mixture screws 1/4 turn leaner. That should help.

Lucky Can you explain how or why you believe this to be a rich condition

Ken

Offline phil71

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Re: Why this backfiring?
« Reply #15 on: May 20, 2012, 06:09:39 PM »
Lucky is entitled to his beliefs ;) This is america.

Offline lucky

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Re: Why this backfiring?
« Reply #16 on: May 20, 2012, 06:18:59 PM »
I will explain.

The difference is this:

If you give it throttle and you hear a popping sound WHILE trying to give it gas it is usually lean. Richen the mixture screw just a little and it will go away most times.

If you give it throttle and it revs clean and perfect then while the engine is spinning down you hear popping, OR you are driving the bike and you are slowing down to a stop and you hear popping sounds that can usually be mitigated by leaing the mixture screw just a little.

Quote: from TwoTired,
"Decel popping can also happen with exhaust system leaks and accumulated fuel in the exhaust.  Extra oxygen from the leaks mixes with unburnt fuel from the incomplete power cycles, again creating a combustible mixture.  Here comes that carbon ember again, and pop."
« Last Edit: May 20, 2012, 06:22:02 PM by lucky »

Offline phil71

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Re: Why this backfiring?
« Reply #17 on: May 20, 2012, 06:25:33 PM »
I just bought some popcorn.

Offline Stev-o

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Re: Why this backfiring?
« Reply #18 on: May 20, 2012, 06:27:14 PM »
I just bought some popcorn.

I just finished mine, guess I'll make some more!
'74 "Big Bang" Honda 750K [836].....'76 Honda 550F.....K3 Park Racer!......and a Bomber!............plus plus plus.........

Offline CB750wannabe

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Re: Why this backfiring?
« Reply #19 on: May 21, 2012, 11:05:34 AM »
Well, I'm sure sure what the references to "popcorn" mean but perhaps it's an inside joke?
I have some things to consider to solve the pops on decel issue (I think).  It seems the consensus is "too lean" but still not exactly sure how to resolve that but will "check the Pilot mixture screw position compared to book value".  Thanks Two Tired and to all for your responses.   
1971 CB750K Candy Garnet Brown
http://forums.sohc4.net/gallery/28485-110112203145.jpeg
1975 CB550 K1 Candy Jade Green

bollingball

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Re: Why this backfiring?
« Reply #20 on: May 21, 2012, 11:38:43 AM »
Lucky is entitled to his beliefs ;) This is america.
Phil You are absolutely correct. Do you have a problem wanting to know why right or wromg. Or do you just do what people say and not question why.
« Last Edit: May 21, 2012, 11:42:04 AM by bollingball »

Offline Duanob

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Re: Why this backfiring?
« Reply #21 on: May 21, 2012, 11:51:46 AM »
Lucky is entitled to his beliefs ;) This is america.
Phil You are absolutely correct. Do you have a problem wanting to know why right or wromg. Or do you just do what people say and not question why.

Unfortunately beliefs don't keep your bike running good. I'm with TT. In my experince my popping on decel was from a too lean condition. I was trying to cure a too rich condition but moved the needles too much. I had lowered my needles too low and my plugs were as white as brand new and the popping was horrible. Turned out to be crud on the needles holding them out of the emulsion tubes preventing them from seating. Once I cleaned them and put the needles back in their original clip position all was good.  It took a couple of tries on the needle position but while it was running too lean the popping was definitely noticable.
"Just because you flush a boatload of money down the toilet, doesn't make the toilet worth more",  My Stepfather the Unknown Poet

1974 CB360T
1976 CB550K2 Resurrected
1976 CB550F2 Barn Find
1979 CX500 VG "HONDA-GUZZI"
2007 Moto Guzzi Breva 750ie
2015 BMW F700GS
Another 1976 CB550K Cafe?

  __o
_- \_<,
(*) /' (*)

Offline OneWheelDrive

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Re: Why this backfiring?
« Reply #22 on: May 21, 2012, 11:55:30 AM »
Subcribed :-D
1975 CB550 cafe
1971 CB500 stocker
2008 Ducati Hypermotard 1100S *sold*
1973 CB350F *sold*
1975 CB550K Project "Keeper" *sold*
2010 Ducati Monster S4RS *sold*
1976 CB360T *sold*
1974 CB550K *sold*
1973 CB750K *sold*
1978 CB550K *sold*
2007 Vespa LX150 *sold*

Offline CycleRanger

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Re: Why this backfiring?
« Reply #23 on: May 21, 2012, 12:00:02 PM »
Well, I'm sure sure what the references to "popcorn" mean but perhaps it's an inside joke?
I have some things to consider to solve the pops on decel issue (I think).  It seems the consensus is "too lean" but still not exactly sure how to resolve that but will "check the Pilot mixture screw position compared to book value".  Thanks Two Tired and to all for your responses.

Getting popcorn is an internet colloquialism for preparing to sit down and watch the fun, i.e. debate.
In other words, everyone here chimes in that popping is caused by a lean condition and then Lucky jumps in to say otherwise this is almost guaranteed to start a perhaps lengthy back-n-forth between members.

Yes, check the pilot screws but also pull the slow jets and make sure they're clean too.
Do you have a copy of the Honda Shop Manual or Parts List for your bike? Get one here:
https://www.honda4fun.com/materiale/documentazione-tecnica
CB750K5        '79 XL250s     CL350K3
CB750K3        '76 XS650      '76 CJ360T

Offline CB750wannabe

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Re: Why this backfiring?
« Reply #24 on: May 21, 2012, 12:23:40 PM »
Oh!  I get it!!!  Thanks for the info!
Sometimes you get someone who likes to argue for the sake of arguing....
1971 CB750K Candy Garnet Brown
http://forums.sohc4.net/gallery/28485-110112203145.jpeg
1975 CB550 K1 Candy Jade Green