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Backfire 71 CB

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KarlM

Backfire 71 CB

I have backfire "on the way down". 71 CB 750 K1
#4 cylinder backfires on deceleration. Runs perfect "up top"....
What causes backfire in one cylinder????
I can only think air/fuel mix or a different baffle in a new HM300 exhaust. No backfire in 1,2 or 3 cylinder so it' not likely to be electric.
My question: Is the backfire "leftover unburned fuel"? Funny how I just noticed this after I installed the new muffler!
Just for my own head....What causes backfire???
Thanks
Karl

rrgunslinger

Re: Backfire 71 CB

In my case it was caused by a lean condition. Are you saying you put a free flowing muffler on? If so you may need to raise your needles and adjust your air screws. Each bike is different. Took me awhile to get mine right. I have a 1973. What I did is under the project area.

American by birth. Cowboy by choice! Vero Beach, FL http://i373.photobucket.com/albums/oo172/grandpaslinger/HD%20Road%20Glide/IMAG0046.jpg

KarlM

Re: Backfire 71 CB

I guess. It's a HM 300 stamped on all the mfflers including the new one.
When I put the carbs back on I just "eyeballed" the adjustment on top of the carbs. I adjusted so a few threads were showing after the lock nut.
Karl

rrgunslinger

Re: Backfire 71 CB

Check how to bench sync carbs post.

Harvey

On Sep 11, 2012 12:11 PM, "KarlM [via Honda CB750'S]" <[hidden email]> wrote:

I guess. It's a HM 300 stamped on all the mfflers including the new one.
When I put the carbs back on I just "eyeballed" the adjustment on top of the carbs. I adjusted so a few threads were showing after the lock nut.
Karl

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American by birth. Cowboy by choice! Vero Beach, FL http://i373.photobucket.com/albums/oo172/grandpaslinger/HD%20Road%20Glide/IMAG0046.jpg

TOOLS1

Re: Backfire 71 CB

Administrator

In reply to this post by KarlM

It sounds like, you have a vacuum leak on that cylinder. Spray some carb cleaner around the rubber boot. If the RPMs increase, you have a vacuum leak.
TOOLS

Life is not about the number of breaths, you take, but the moments that take your breath away.
I don't have an anger problem. I have an idiot problem. Hank Hill
Never confuse education for intelligence.
Happiness is a belt fed weapon.
I just can't imagine what could go wrong.
No fire? No explosions? So whats the point of your story?
Those who are too smart to engage in politics are punished by being governed by those who are dumber. ~Plato
It couldn't be done, but the darn fool didn't know it, and did it anyway.
We all got problems. Ksharp
I like vintage bikes because they take me away from the clutter of technology that I work with everyday and back to a simpler time of mechanical elegance and simplicity.. "ninadm"
Darkwing Duck: The worst part of public transportation is the Public.
"That is awesome shit there" Re-Run
"Fear nothing, attack everything" Eric Berry
" Oh, you read that on the internet? Clearly it IS a massive problem. Of course it CAN’t be normal operation."

1976 CB 750-A X 2
1977 CB 750-A X 4
1977 CB 750-K
1976 CB 750 F
1981 CB 750
1966 Kawasaki SG 250
1981 KZ 750 LTD
1973 CB 350
1979 CM 185 Twinstar
1982 Honda XL 80
South of Eden (Kansas City MO)

KarlM

Re: Backfire 71 CB

Thanks Tools

I tightened up everything yesterday. I guess I'll spray it.

Maybe the boot has a crack??

Thanks

Karl

----- Original Message -----

From: [hidden email]

To: [hidden email]

Sent: Tuesday, September 11, 2012 1:14 PM

Subject: Re: Backfire 71 CB

It sounds like, you have a vacuum leak on that cylinder. Spray some carb cleaner around the rubber boot. If the RPMs increase, you have a vacuum leak.
TOOLS
Life is not about the number of breaths, you take, but the moments that take your breath away.
I don't have an anger problem. I have an idiot problem. Hank Hill
Never confuse education for intelligence.
Happiness is a belt fed weapon.
I just can't imagine what could go wrong.
No fire? No explosions? So whats the point of your story?
Those who are too smart to engage in politics are punished by being governed by those who are dumber. ~Plato
It couldn't be done, but the darn fool didn't know it, and did it anyway.
We all got problems. Ksharp
1976 CB 750-A X 2
1977 CB 750-A X 2
1981 CB 750
1995 KZ 1000 P-14
1981 KZ 750 LTD X 2
1973 CB 350
1979 CM 185 Twinstar
1987 Honda TLR 200
1982 Honda XL 80
2005 Yamaha PW 50
South of Edan (Kansas City MO)

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