Topic: 1979 Honda CB650 Engine cuts off during acceleration

[Rhino08]

Hello I’m new to this forum, just joined and happy to be here.
I need some help with my recently acquired 1979 Honda CB650.
I rebuilt the carb and it fired up nice, warmed up the bike and tried taking it for a test ride, put the bike in first gear and as I open the throttle the engine cuts off.  Tried several times and same results.
I tried again opening up the throttle more and it does move about 10 feet and before I can change to higher gear the engine cuts off again.  Happens each time I restart and try to move the bike in first gear.  Any help would be much appreciated.


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[minimo]

Did everything go back in the same way upon rebuilding the carbs? Are you running everything stock? Air pods? Aftermarket exhaust?

This chart may help


Have you checked the condition of your spark plugs? That may be telling of whether you’re running a rich or lean mix.


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[Rhino08]

Everything is stock on the bike and carb, I tried test riding with the stock air box and clean filter as well.  The exhaust is also stock dual pipes from Honda.
I wanted to get the bike to factory specs before any upgrades with aftermarket parts.
Thanks for replying by the way!!
The spark plug does show a bit Rich in the mixture.
New plugs also.


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[minimo]

Did you replace any of the carb parts like Jets and needles etc in the carbs? We’re all the tiny hole passages cleaned with thin musical wire or the like? How about the o-rings? We’re those replaced? Did you check your float heights?

The way that your bike just shuts down sounds like it could be a super rich condition at about 1/4-3/4 throttle.


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[minimo]

Pull out a spark plug and see what it looks like.
Photos help.

Sooty deposits indicate a rich condition. Ashy grey/white May indicate a lean condition. Light tan will be about where you want to be

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[minimo]

Generally, try adjusting one thing at a time, but while you’re in there, verify how many turns out from full seating of the air/fuel mix screws you have across your carbs. Compare that against what it should be stock and make sure that they’re matched across all carbs.


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[minimo]

Also, to be sure, you’re not running with Choke on, correct?


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[Rhino08]

Thanks for the input.
I’ve replaced everything on the carb, main jet to 90 which is to specs. 
Pilot jet is also clean and all accelerator pump works as well.  After the engine has warmed up, choke is fully open.  And yes spark plug does show it’s a bit rich.   Running a little rich shouldn’t cause the engine to shut down like that or does it??  I’ve ran bikes rich before, and didn’t have results like this.  This is a problem I’ve not seen before.


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[jaytee-nz]

When you say "the engine cuts off", do you mean it stops immediately just like when you turn the key off or does it splutter / run rough and then die ?

[Rhino08]

It dies right away with normal acceleration, but if I rev high i can get the bike to move a few feet before it dies.



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[1976ssport]

Sounds like fuel starvation.  Could be petcock, floats too low, fuel line blocked.  Just throwing some things out there.

[BenelliSEI]

Gas cap vent plugged? Try setting off with the cap loose........

[Rhino08]

Those are some good ideas, and I’ll try them.
I actually just checked the float level, and it’s specs say 12.5mm and I didn’t check this before.  Mine is currently at 14.5mm on 2 of the 4 bowls.  I’ll make the change and try this and also try opening the gas cap as well.


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