78 CB750F won't start after carb clean and spark plug tune-up

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78 CB750F won't start after carb clean and spark plug tune-up

Rockwork
I just did a carb clean and serviced the spark plugs and now CB won't start. It started sputtering this week at above 2500 rpm, so I figured it was either a bad spark plug  connection or obstructed gas flow.

I checked the fuel tank cap, cleaned and re-gapped the spark plugs, clipped the coil wires and reset the connectors, and then pulled out the carburetor and cleaned all the jets and valves. There was a lot of sludge, and maybe some obstruction. but after cleaning, it seemed all the orifices were clear, I set the pilot screws back to where they had been (2 1/3 turns). There's fuel coming out of the petcock, but it doesn't seem any is getting into the engine.

When I try to start it, it just cranks, there is almost no fire -- every once in a while a sputter. I'm jumping it off of a car, so there is plenty of charge.

Thank you for any help diagnosing this problem.
"To strive, to seek, to find, and not to yield."
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Re: 78 CB750F won't start after carb clean and spark plug tune-up

TOOLS1
Administrator
Could you have installed the floats upside down?
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: 78 CB750F won't start after carb clean and spark plug tune-up

Rockwork
Good questioned, but I cleaned each bowls individually, so that I would put everything back the way I took them apart. (Floats flat side up, hull down into the bowl, right?)

I'm still not confident I'm getting a proper spark, either. I have NGK caps with the needle pretty recessed, so when I took one off last night I couldn't see a spark. Not sure if it's just not close enough to the plug.
"To strive, to seek, to find, and not to yield."
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Re: 78 CB750F won't start after carb clean and spark plug tune-up

motogrady
Administrator

take the battery out, put it on a charger for 3 hours, see what happens.

if it pulls amps when first charging, and then fall slowly, the battery is taking charge.
the slower, the better.

if the battery doesn't pull the amps when charging, especially at first, totally charged, or bad battery,
or bad/dirty electrical connections.
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Re: 78 CB750F won't start after carb clean and spark plug tune-up

TOOLS1
Administrator
In reply to this post by Rockwork
The flat part of the float goes towards the bowel, or down when the carbs are right side up.
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: 78 CB750F won't start after carb clean and spark plug tune-up

Rockwork
Correct. Sorry I mis-stated earlier. When reassembling and the carbs are upside down, the flat part is up - facing the bowl when it is put back on and then turned right side up, which is what I did. Any other reason it might not be getting gas?

I'm also going to pull a plug now and make sure there's a good spark.

Thanks.
"To strive, to seek, to find, and not to yield."
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Re: 78 CB750F won't start after carb clean and spark plug tune-up

TOOLS1
Administrator
Have you loosened the drain screws on the bottoms of the float bowels to see if gas comes out? Also when you cleaned the carbs, did you remove the pilot jets to clean them?
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: 78 CB750F won't start after carb clean and spark plug tune-up

Rockwork
Alright, after some serious jumping it started, blue out a bunch of smoke, and kept it running at a high RPM, then got the idle below 2K and runs smooth. I had cleaned out all the jets. Gunna take it out now to see if I corrected the sputtering issue I set out to fix. Then after recharging the battery will see how it idles at 900.

Thank you.
"To strive, to seek, to find, and not to yield."
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Re: 78 CB750F won't start after carb clean and spark plug tune-up

Rockwork
On the road it ran very smooth. I think the sputtering is gone, but it stalled at stop lights a couple times, I think because the idle fell below 2K and there's no charge in the battery to keep it running.
"To strive, to seek, to find, and not to yield."
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Re: 78 CB750F won't start after carb clean and spark plug tune-up

motogrady
Administrator


I'd say 2 grand is too high at idle, 1 grand is what I strive for.

Ride the heck out of it for a day or so, maybe then back the air screws (pilot screws, whatever
one calls them) out a bit to give it some more fuel at idle, then back the idle screw out just a bit.

Got to keep that battery juiced up, it gets weak, everything goes to hell.