Topic: CB750k 78 misfiring on hard acceleration

[jeffthompson55]

So about a week ago, I replaced the spark plugs, cleaned the carbs, and replaced the seals and gaskets because I had a small leak from the float bowl on one of the carbs. After I finished, I did an audible tune (no vacuum lines, just tried to dial it in by ear temporarily) and it ran great. Come a week later, when I hit highway speeds or pull the throttle more than 1/2-3/4 it feels like one of the cylinders is misfiring. It will still run and pull, but at about 75% ish of the normal power which really affects how fast I accelerate. All of my plugs are gapped to about .26mm, and it ran fine before this, just a bit rich. Now I'm not sure where to go from here. Should i pull the carbs out again and replace all the jets as well? Help me I just want to ride

[Paintedseat]

Did you sync

[calj737]

It sounds to me as though your fuel level is a bit low in the bowl, and thus as you crank up the motor under sustained load, you're starving it for gas. Establish the actual fuel height using the Clear Tube method to verify it, do not rely on setting the floats by measurement.

[Yamahawk]

A fluid level is an old way of checking not only fuel levels, but a level of two objects that are separated by some distance, too. I have used clear hose filled with water to level large piping for steam lines in a boiler house, (32" hot reheat) to get the piping at the same height. It works amazingly accurately. Same principle as the float level check Cal is showing you here. This was before laser levels were widely available...
Charlie

[calj737]

A fluid level is an old way of checking not only fuel levels, but a level of two objects that are separated by some distance, too. I have used clear hose filled with water to level large piping for steam lines in a boiler house, (32" hot reheat) to get the piping at the same height. It works amazingly accurately. Same principle as the float level check Cal is showing you here. This was before laser levels were widely available...
Charlie

First used by the Egyptians. Long before Sochiro ever became a scratch in the crotch.

[Yamahawk]

A fluid level is an old way of checking not only fuel levels, but a level of two objects that are separated by some distance, too. I have used clear hose filled with water to level large piping for steam lines in a boiler house, (32" hot reheat) to get the piping at the same height. It works amazingly accurately. Same principle as the float level check Cal is showing you here. This was before laser levels were widely available...
Charlie

First used by the Egyptians. Long before Sochiro ever became a scratch in the crotch.

Yep
Charlie

[PeWe]

Very sudden and abrupt misfires? Can be ignition like a loose/bad ignition wire, bad points.

[Paintedseat]

I would sync first

[calj737]

I would sync first

What’s the point of synchronizing your carbs if the fuel level isn’t verified? No external adjustments will overcome improper fuel availability.

[Paintedseat]

Because I don't think float level is the issue. He did an "audible" tune after having the carbs apart. Main clue is that he is twisting the throttle 1/2 to 3/4 it starts missing. I doubt that is enough to empty the bowls. Sounds like a sync issue to me.

[Paintedseat]

Calj737 how do you like that M unit?

[2wheels]

That happens when the points gap needs adjustment.

[calj737]

Calj737 how do you like that M unit?

I like them very much.

As to the carb issues, moving the throttle up the needle taper creates additional need for fuel as the vacuum increases, not something normally attributed to a synch. Who knows, until he verifies his fuel level...