79 cb750 dohc only running on cylinder 3&4

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79 cb750 dohc only running on cylinder 3&4

Parzival
Recently got a 79 cb750 dohc, haven't had a chance to really dig into it yet But here's what's going on. The previous owner didn't know how to rebuild the stock carbs so he slapped a set of kehin special cr's on it, outside of that everything seems stock and original. The issue I'm having is it's mainly running on cylinder 3&4, number 1 exhaust is warm, 2 is stone cold, and 3&4 sizzling hot after running for a minute or so. Any help is welcome, I'd also like some suggestions on how to jet these carbs for pods and a 4 into 1 exhaust. Pods came with the bike although the stock airbox is on it currently.
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Re: 79 cb750 dohc only running on cylinder 3&4

TOOLS1
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Even with the CR carbs, it will still run better with the stock airbox then with PODS. As for jetting them, you will need to do plug chops and figure it out. You could have another bike just like yours with the CR carbs that runs perfect, but yours might run like crap with the same jetting.
As for the cylinders that are not working. Since 1-4 and 2-3 are on the same coils, I think we can rule out the ignition. However, you should check for spark, and do this with the plugs that are in the cylinders right now. Do not use another plug because the ones in the bike could be bad. Other things to check are vacuum leaks, plugged jets, and compression.
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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Re: 79 cb750 dohc only running on cylinder 3&4

Parzival
Finally had a chance to check all my bases today. The plugs were terribly fouled/bairly could get a spark. So with new plugs I found that 1&4 had a pretty week spark compared to 3&4. Installed and cranked up running on two occasionally 3 cylinders. Pulled the plugs after about 2 minutes at idle for a look. 1 was lightly covered in black soot, 2&3 heavily covered and very black, number 4 clean as a whistle. After this I ran a compression test #1&2 140psi, #3 135psi, and #4 0psi.
I double checked that zero psi reading a few times to make sure it was correct (which it was) before dumping some oil in the cylinder which brought it up to 30psi. So that's where I'm at with it today. Again I appreciate the help
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Re: 79 cb750 dohc only running on cylinder 3&4

TOOLS1
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Sounds like a burnt or bent valve. Time to take it apart.
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)