1979 Honda CB 750, help a new guy out?

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1979 Honda CB 750, help a new guy out?

weaseltree19
Hey there folks,

Before I start I'm fairly (totally and completely) new to the game of motorcycle maintenance. I have no hands on experience whatsoever. I read through The Essential Guide to Motorcycle Maintenance over the summer, but this bike is my first hands on experience. I would like to learn how to do most of this stuff myself and ideally avoid a shop unless I have to.

Ok heres what I got. 1979 Honda CB 750. Not running. Charge in the battery, I'm assuming fuel in the carbs because the carb overflow is flowing with gas when the petcock is open. Cant get the bike to turn over, something catches and it sounds like something is spinning in the gearbox but no combustion. Spark plugs are new, not sure if they're getting electricity though, checked for spark and didn't see one, but not sure if I did it right.

Im absolutely sure that Im doing a number of things wrong, so go easy.

Thanks mates
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Re: 1979 Honda CB 750, help a new guy out?

weaseltree19
Just tried to start it, had a huge backfire so we have a spark I guess.
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Re: 1979 Honda CB 750, help a new guy out?

TOOLS1
Administrator
Sorry were were not here waiting for you to post. We do check in ever few days or so.
First you need to do something about the gas coming from the overflow tubes. With backfiring, you can catch the bike, and other things on fire.
Here is a link to a thread on rebuilding and adjusting the CV carbs your bike has (good luck finding this on the twins forum) http://honda-cb750-s.456789.n3.nabble.com/Cleaning-and-adjusting-a-carberaters-best-videos-for-dummies-This-ought-to-be-pinned-at-the-top-td3905817.html
Also most of the time when one of these bikes backfire like that, it is because someone has switched the plug wires, or coil wires (find this on the twins 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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Re: 1979 Honda CB 750, help a new guy out?

weaseltree19
Alright, carbs were clean before I opened them up to check.  I'll move onto the correct layout of the plug wires.
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Re: 1979 Honda CB 750, help a new guy out?

TOOLS1
Administrator
If gas is running out of them, they are either dirty, the float needles are dried out, or the floats have dried out, and are no longer boyent. On the plug wires, the two wires on the left coil go to cylinders No#1,4 and the two wires on the right coil go to cylinders No#2,3. They can be crossed between those pairs since both of those cylinders in the pairs fire at the same time. One cylinder is on the exhaust stroke when this happens, and is called a wasted spark. The other thing to check is the (I think they are blue, and yellow) wires going to the coils. If they are crossed, it will make the ignition 180* out of time, and this will make for some exciting backfiring.
Also you can download the Honda Factory Service Manual from the manuals thread at the top of the home page here.
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: 1979 Honda CB 750, help a new guy out?

weaseltree19
Alright, I'll check the carbs again tomorrow.  The plug wires were configured correctly, I'll check those blue and yellow wires connected to the coils tomorrow as well.  Thanks for all the help mate, really appreciated.
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Re: 1979 Honda CB 750, help a new guy out?

weaseltree19
In reply to this post by TOOLS1
Alright, so I'm back at it.  The yellow/blue wires first.  I cant seem to find any layout or diagram on the internet indicating which one goes to which.  Any suggestions?  I remember that my floats were a little dried up when I pulled the carb out yesterday, any tell-tale signs of them loosing their buoyancy?  If so, where do you suggest purchasing new ones?  The spark wires are all attached correctly as well.
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Re: 1979 Honda CB 750, help a new guy out?

shinyribs
Administrator
Just moving this from the Write-ups area to the general discussion area.
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Re: 1979 Honda CB 750, help a new guy out?

TOOLS1
Administrator
In reply to this post by weaseltree19
If you look at this thread, http://honda-cb750-s.456789.n3.nabble.com/NEW-Repair-manuals-Other-write-ups-too-Updated-04-26-2014-td685918.html you will find a wiring diagram for your bike, and a free download of the Honda Factory Service Manual, which will also have a wiring diagram for your bike. You can kind of check your floats by putting them in some gas, and comparing how well they float with each other. If one or some do not float as high as the others, you know you have some bad ones. Honda I think still has these floats, and I am sure vintagecb750.com has them also. One thing you might want to try is to clean the float needle valve seat with a Q-tip and chrome polish. Some times this will cure an overflow problem.
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: 1979 Honda CB 750, help a new guy out?

icerigger
In reply to this post by TOOLS1
You did check the drain screws, right?
Livin' my life like a song.

1985 Honda Rebel 250 - "Birdie"
1979 CB750K - "Behemoth"
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Re: 1979 Honda CB 750, help a new guy out?

weaseltree19
In reply to this post by shinyribs
Hey tools, its been a few days but I've got the bike running.  You were right, it was the wiring to the ignition coils.  Ive taken the bike on a few spins around the block so far but nothing big.  

Still with the fuel overflow though.  Im not sure whats going on.  Im almost certain that the floats are buoyant and the carb is clean.  I was thinking, could it be something with the fuel lines?  I made them myself and they're a bit messy but they work.  What is the standard diameter for the lines?  I cant run the bike with the petcock fully open or the bike will flood and die, so I've just been running with it half open.  Any suggestions?  
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Re: 1979 Honda CB 750, help a new guy out?

TOOLS1
Administrator
Is the gas coming from the fuel line connections? Or is it coming out the overflow tubes on the bottom of the carbs? The fuel line/petcock have nothing to do with metering fuel. That is the job of the float/needle valve, and seat.
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: 1979 Honda CB 750, help a new guy out?

weaseltree19
In reply to this post by icerigger
Ok explain to me what that means
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Re: 1979 Honda CB 750, help a new guy out?

TOOLS1
Administrator
Fuel line connections - where the ends of the fuel line connect to the nipples on the petcock, and on the carbs.
Overflow tubes - there are little nipples on the bottom (float bowel) of the carbs. There should be small diameter tubes coming off these nipples running down the back of the engine by the swing arm. If gas is coming out of these, you have a problem with the float, the float needle valve and seat, or the little brass tubes in the float bowels have cracks.
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: 1979 Honda CB 750, help a new guy out?

icerigger
In reply to this post by weaseltree19
Okay, if the drain screws are in good condition and properly screwed in, no cracks in the overflow tubes, listen to Tools. He is wise in this.

True overflow can only happen if the needle valve doesn't stop the flow of fuel correctly. Not many things can go wrong with this. Dirty valve seat, degraded or damaged needle face, needle sticking in the tube, float not floating, or float improperly adjusted. All of this stuff is easy to check and covered in  the FSM or Seans book linked to early in this thread.
Livin' my life like a song.

1985 Honda Rebel 250 - "Birdie"
1979 CB750K - "Behemoth"