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can someone give me a idea of what might be going on with my bike i have a 1981 cb 750 custom that i got last fall , i did some work on the bike ( fork oil , carb cleaning ,new tires ) the bike has 18000 miles on it , it was running fine up til last week then it started back firing out of the 3&4 cylinder mufflers , i check the spark it's fine in both cylinder and also took the carbs off again and there fine , i put about 50 miles on the bike when it started acting up on me ,
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Administrator
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Backfiring is usually caused by timing. Have, you checked that? Also check your cam chains.
Welcome to the forum.
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)
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timing is bang on and i did adjust the cam chain, but the bike did this to me once before but went away as quit as it came this time its here to stay. when the bike is running you can hear the carb pistons pumping then the rpm's go up then it drops back down and starts to back fire when it 's running about 2000 rpms??
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and the compression is good all 4 cylinders are in the upper 140 psi
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Administrator
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Next I would start looking, for 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)
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My 81K was doing that and it just ended up being the idle wasn't adjusted properly, which was weird because it acted just like a vacuum leak like tools said and I messed with the timing for a week. My dad came over with his timing gun to check it, said it was dead-nuts on, then checked the idle screw and kept messing with it while it warmed up, then it was totally fine. Not sure if that is the case with you but something else you could check.
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Why would the idle screw fix it?
I'm kind of having a similar issue. My bike is backfiring around 5-7K RPM's and on deceleration until it warms up for about 10 minutes, then it gets much better.
It still pops on deceleration, but nowhere near as bad.
I did recently adjust my idle down to within factory spec (1100), but now I'm curious to see if I put it back up to 1500-1600 if it'll fix my backfiring issue.
1981 CB750K with 900 cams
90K KM's, rebuilt head, rebuilt carbs, upgraded valve stem seals
My wife's recipe website that I'm trying to help promote: Strawberries for supper. Yes, I am a lucky man.
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even my exhaust pipe dont get hot like 1&2 cylinders , it's like its not getting anough fuel but the carbs have been off the bike 3 times and all the jets are clear and the floats are moving well , you can hear the piston in the carb pumping , you can also put your hand on the carbs and feel the pumps working before backfiring
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Administrator
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Just remembered your carbs have n "air cut valve". When these go bad it causes the engine to back fire. There is one on each carb.
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)
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Those are the ones that can be blocked, right? Sent from my BlackBerry device on the Rogers Wireless Network
Date: Tue, 1 May 2012 20:14:09 -0700 (PDT) Subject: Re: back firing out of #3&4 muffler
Just remembered your carbs have n "air cut valve". When these go bad it causes the engine to back fire. There is one on each carb.
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)
1981 CB750K with 900 cams
90K KM's, rebuilt head, rebuilt carbs, upgraded valve stem seals
My wife's recipe website that I'm trying to help promote: Strawberries for supper. Yes, I am a lucky man.
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Popping on deceleration= mixture rich. Adjust mixture screw.
Popping when trying to give it throttle= lean condition check the idle jets.
You have to take them out and look through them.
As others have mentioned look at the rubber boots that hold the carbs on and make sure they have not slipped off.
Vacuum leaks cause leaness+popping. -LUCKY
On a Roadstar Adventure.
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Administrator
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One more thing to check is the exhaust pipe connection. There should be some copper or aluminum gasket rings between the pipe and the head. Also make sure the mounting bolts are tight.
That being said, I'd still look at an intake leak or mixture adjustment. Pull the carbs and double check the insides for cleanliness. Set the mixture screws according to the manual, and adjust from there. Might be a carb sync issue, so I'd look into that towards the end of your diagnosis.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
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Administrator
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One thing that sometimes gets overlooked: the screws that cap the vacuum ports used to sync the carbs. Snug, but not overtightened.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
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I REPLACED THE NGK PLUG CAPS WITH RACING WIRES AND NEW PLUGS AND THE BIKE IS NOW RUNNING BEAUTY'S , ONE GUY TOLD ME THAT THE RESESTOR IN THE CAPS MIGHT BE 2 MUCH FOR THE COILS TO HANDLE????? BUT THE BIKE IS NOW WORKING WITH THE RED RACE WIRES
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