Engine trouble Help! Video of engine. Thanks!

[surfish95747]

Okay guys. I have a 1980 cb750k. I have rebuilt the carbs, with new air cutoff valves, accelerator pump etc, rebuilt the top end with new exhaust valves, new piston rings, valve clearances set to .005" each, and new gaskets etc. It has been running rich and the spark plugs were black as night when I pulled them out today. It was running totally fine until August 4th, last Thursday. Since then it will work okay when I start it up, but after warming it up, really just riding it to the gas station ( a mile away) it will start to run really rough. Almost seems like it is not getting any gas or running out of gas. However, carbs are getting plenty of gas. I know this because each float bowl will drain gas when I use the drain screw. Today I reset the air/fuel mixture screws to 1 and 1/2 turns (per FSM), they were at 3 turns. Oops. I also put in brand new NGK D8EA spark plugs. I also put a new o ring inside the petcock lock nut to stop it from leaking gas. So, take a listen to the video and see what you can hear. Real trouble starts around 1:40 and even for at the 2:00 minute mark. Thanks everybody!

[surfish95747]

Oh I forgot, All stock jets, with a MAC 4-2-1 exhaust and Cone Engineering cone muffler. Let me know if any other information is needed. Thanks!

[TOOLS1]

Carb sync?
TOOLS

[surfish95747]

Huh. Interesting thought. I had synced the carbs with the pilot screws turned out 3 turns. Do you think readjusting the pilot screws would throw off the previous sync requiring a re sync?

On Aug 8, 2016 9:07 PM, "TOOLS1 [via Honda CB750'S]" <[hidden email]> wrote:

Carb sync?
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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[TOOLS1]

Shouldn't, but a vacuum leak could cause this. You can hear the cam chain in the video, that is why I thought carb sync.
TOOLS

[surfish95747]

Okay. I'll look for vacuum leaks. Maybe in the air box boots? Yeah I can't seem to get that cam chain noise to go away. Maybe I should have put a new tensioner in there when I had it apart. Gah.

On Aug 8, 2016 9:37 PM, "TOOLS1 [via Honda CB750'S]" <[hidden email]> wrote:

Shouldn't, but a vacuum leak could cause this. You can hear the cam chain in the video, that is why I thought carb sync.
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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[TOOLS1]

That is how an engine sounds when the carbs are out of sync.
TOOLS

[Maz]

It sounds, to me, like it's dropping 1 or 2 cylinders. I don't know where you have your tach/hour meter connected (but I'd like to know 'cos I would like to do the same for setting up idle mixtures) but it drops to "0" which suggests it's not getting a pulse signal. If that is the case, I guess 1 or 2 plugs stop sparking. Just my thoughts.
Maz

[Re-run]

Have you tried to spit test to see if any pipes are running cooler than others? Good dab of spit on a finger and very quickly touch the header. Don't touch hard as you'll push the spit away and it can't protect the finger. If it sizzles, the pipe is hot. If it does not, then it is cold. You could do a laser temp gun, but I am a cheap ass.
Also, I would pull the plugs out as soon as the issue starts. Doing it this soon might reveal which cylinders are not playing nice.

What air filter are you running?

Have you verified your flaps in the carbs are opening and closing properly? The issue seems to start occurring as the bike warms up. Maybe there is some choking happening and it is causing the plugs to foul.