Head scratcher

[Dougie]

I'm having an issue with my 81 cb750 that I can't seem to figure out..
It was running smoothly till about a month ago till it started to biog out and die around 3000 rpm..
It idles nice but as soon as I crank it like I said she just dies out.
I've changed the stator and rotor
Changed the ignition coils
Changed the sparking units
Have rebuilt the carbs with the non adjustable floats
75/110 jets
Change the pilot screw o rings with 2.5 turns
New needles
The battery is new
New spark plugs

The only thing I haven't checked is the timing because one bolt is stripped and will take some work to remove.

Any ideas what the issue is? I'm supposed to head to this chop meet next week. Grrr

[TOOLS1]

Have you changed the air filter? Is it the stock airbox? How about fuel flow to the carbs?
TOOLS

[Dougie]

Yes I have changed the air filter, stock airbox and there is fuel dripping from all the carbs if I loosen the overflow screws.

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On Jul 4, 2015, at 12:30 PM, TOOLS1 [via Honda CB750'S] <[hidden email]> wrote:

Have you changed the air filter? Is it the stock airbox? How about fuel flow to the carbs?
TOOLS

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NAML

[TOOLS1]

What kind of air filter did you install? Also just because there is fuel coming from the carbs with the engine off and the bike not moving, does not mean it is getting enough fuel to run 3K RPM.
TOOLS

[Dougie]

I forget what kind but I ordered it straight from the vintage cb750 site which allowed me to select the specific kind for the era. I still have the original so I could switch back. 

The thing is I was running the stock floats the first time it started to bog out. I still have them so I could also try and go back to those.

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On Jul 4, 2015, at 12:41 PM, TOOLS1 [via Honda CB750'S] <[hidden email]> wrote:

What kind of air filter did you install? Also just because there is fuel coming from the carbs with the engine off and the bike not moving, does not mean it is getting enough fuel to run 3K RPM.
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NAML

[TOOLS1]

If the filter came from VintageCB it should be alright.
TOOLS

[TOOLS1]

It just dawned on me that you are working with CV carbs, and you could have switched the pilot and main jets. It is a common thing to do with those carbs.
TOOLS

[Dougie]

The primary main would be the 110?

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On Jul 4, 2015, at 3:26 PM, TOOLS1 [via Honda CB750'S] <[hidden email]> wrote:

It just dawned on me that you are working with CV carbs, and you could have switched the pilot and main jets. It is a common thing to do with those carbs.
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NAML

[TOOLS1]

Yes, the larger number is the main.
TOOLS

[Dougie]

Yeah like I said it was running fine before..

Update though... I switched back to the 68/102 combo... From the manual  turned the pilots 1.5 turns and I can get it to idle and when I choke it outs bit and release the choke I can get it up over 3000. It sounds nice! 

But it bogs a bit 1/4 throttle turn out.

I took it down the back alley and when I come to a stop it dies... Is this just a fine tuning process with the pilot screws? Is it still starving for fuel? 

I don't know what else to do hehe

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On Jul 4, 2015, at 5:21 PM, TOOLS1 [via Honda CB750'S] <[hidden email]> wrote:

Yes, the larger number is the main.
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NAML

[TOOLS1]

It really sounds like a lean condition. Have you checked for vacuum leaks?
TOOLS

[Dougie]

I replaced the boots... I haven't done a check at the airbox rubber mounts but what your saying makes sense.. 

I chopped the frame and lowered the seat exposing the battery and removing the battery side panels allowing more air to be pulled in.... 

I figured before with 75/110 combo it would compensate for the air increase... It seemed to work for awhile.

Maybe I'll have to go back to them.. Maybe I'll need to go bigger... 

Thanks for the input by the way.

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On Jul 4, 2015, at 6:11 PM, TOOLS1 [via Honda CB750'S] <[hidden email]> wrote:

It really sounds like a lean condition. Have you checked for vacuum leaks?
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NAML

[Dougie]

Could it be the accelerator pump?

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On Jul 4, 2015, at 6:11 PM, TOOLS1 [via Honda CB750'S] <[hidden email]> wrote:

It really sounds like a lean condition. Have you checked for vacuum leaks?
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NAML

[TOOLS1]

At that RPM no. The accelerator pump only works when you are twisting the throttle from the idle position. By 1/4 throttle the accelerator pump is no longer working. You have a lean condition, caused by a vacuum leak, plugged main jet, plugged fuel line, incorrect float height (not likely,) or you have the pilots and main jets switched.
TOOLS