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Hi all, bit of help needed, after completely stripping the carbs down AGAIN, on my 1979 cb 750, she will not tick over, it revs up and down to about 3000rpm then drops back down and stops if I don't catch it on the throttle, I have fitted new all new seals, air cut off valves, float needles, and new intake rubbers. Now I am at a bit lost on why she wont run smoothly, I have checked the timing but only visually as I cant use the strobe light as it wont tick over. Any ideas please anybody, thanks.
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Administrator
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3 things can make an engine do that. First is the ignition advance, but since this started after the carbs were worked on I really doubt this is the problem. The other two things are the carbs being way, way, way out of sync, and a vacuum leak. I am betting on a vacuum leak myself. I would check all the rubber O-rings in the carbs to make sure that they are sealing, and especially check the tiny O-rings and washers on the fuel/pilot screws. I work on carbs all the time that have them missing, or improperly installed.
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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thanks for the reply Tools, I have had a look at the little o rings on the idle mixture screws, they are a sloppy fit over the end of the needle and they are not as thick as the ones that came off, do they have to be a snug fit over the screw tip, for example if you put them on and turned the screw over they would fall off, i also read somewhere that for all 750 Hondas that you turn the screws out a full 3 turns but in the book it says only 1, 1/2 turns. I am also pretty sure it is running on the rich side. any info greatly received. thanks.
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Administrator
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If that O-ring does not fit and seal the engine will be able to suck as much fuel through the passage as can get past the O-ring, and will also suck air past it. O-rings are there to make a seal for a purpose. I try to NEVER use the brass, and rubber parts from aftermarket kits. The fitment is very poor. I reuse all the original brass parts, and source the rubber parts from Honda.
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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Administrator
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IIRC, don't the dohc carbs also have washers against the o-rings for the idle mixture screw? If those are missing the o-rings get chewed up, or pushed around and deformed pretty easily.
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Administrator
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Yep.^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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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Hi thanks for replying. I have replaced the tiny o rings on the idle mixture screws and now it is only firing on number 1 and number 4 pots, there is no fuel getting through 2 and 3 carbs, could I have blocked something with the mixture screw as all four were firing before I changed the o rings, or is it as I suspect that the float needles are probably stuck. all 4 mixture screws have been wound back 1and a half turns. All info needed as I am running out of ideas, not to mention temper. thanks.
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Administrator
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I doubt the mixture screws would effect it to where it would not run on those cylinders. To check if the float/needles are working, just loosen the drain screws in the bottom of the float bowels to see if gas comes out. If it does then the needles/floats are OK. Since it is #2 I would check to see if the wires to the coil might have been pulled off while you were taking the carbs on/off.
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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Thanks for that Tools. I have a spark on all 4 cylinders, all 4 float bowls fill with petrol, It was running on all 4 pots before I removed the mixture screws, I removed and replaced these while the carbs were on the bike, a bit awkward to say the least, I removed the starter motor to give me a bit more room to work.I have not yet checked any other wiring yet. any other ideas please. Thanks.
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The wires from the pulse generator, which is inside the LH engine cover, connects at one of the red plug/connectors just below the starter solenoid. Check that, and the wiring that you can see, which would have probably been disturbed, if you were working on the starter motor.
1982 CB750F
1978 Triumph Tiger 750
197? Yamaha DT175
197? Hodaka ACE 100
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Administrator
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Will the two cylinders fire with a shot of starting fluid?
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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Hi thanks for the extra info and ideas, after shooting some easy start into the engine and it still would not fire on 2 and 3 cylinders, and after spending quite a few hours trying to trace an electrical fault, I found out it was the SPARKPLUGS at 2 and 3 cylinders, both plugs packed up at the same time, very strange. I just swapped plugs 1 and 4 around with 2 and 3 and 2 and 3 fired up while 1 and 4 didn't. so another set of new plugs to be bought tomorrow. hopefully this time when I fire her up she will run ok and tick over. thanks again.
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