Topic: CB650 Bleeding Carbs

[Wayfarer]

I disassembled, cleaned and reinstalled (w/o the airbox, for testing) the carbs of a 1981 CB650C.   My aux fuel tank is a funnel suspended about 18” above the motor.  When I added fuel, it began flowing out the rear of all four carb bodies.  When started, the engine raced at first, then settled down to normal.  I added a little more fuel to the running engine and it began racing again. 

When I cleaned the carbs, the floats and needle valve looked good, and I find it hard to believe that all four failed simultaneously.  Could it be that the pressure of the fuel coming from so high up was able to force it past the closed needle valve?

Also, this bike has a very odd petcock.  The petcock itself is the normal older on/off/res type, but the fuel then flows to the vacuum shutoff valve which is mounted on top of carb #3, after which it then flows to the carbs.   I left this off intentionally but am now wondering if the diversion and subsequent U-turn also serves to regulate the fuel pressure.  Comments?

TIA,
Tim
 

[Bob Wessner]

You're pouring raw fuel into and open funnel, 18" above a running motorcycle engine? 

The height shouldn't matter. I've run an aux. tank 18" or more inches higher than the carbs and no similar problem.

[eldar]

It is hard to say why the engine would race but how large in diameter is the line you used from the funnel? My thought is that if you used a larger line, gravity would pull more fuel down into the carbs but I dont see how that would make them race. try a different aux tank. try having someone hold you fuel tank a few inches higher and see what happens.

[Wayfarer]

You're pouring raw fuel into and open funnel, 18" above a running motorcycle engine? 

The height shouldn't matter. I've run an aux. tank 18" or more inches higher than the carbs and no similar problem.

Well, it is attached by a fuel hose to the carb fuel intake.  I was careful to pour very carefully, just a few ounces.  Is that different from adding fuel to a normal aux tank in the same position?  (not being sarcastic;  my question is literal).

Thanks,
Tim

[Bob Wessner]

From my perspective, yes, any tank, aux. or otherwise, contains the fumes/vapors with a cap or lid. You would be surprised how quickly and large an invisible cloud of fuel vapor can form from an open container, particularly when it's warm.

[Wayfarer]

From my perspective, yes, any tank, aux. or otherwise, contains the fumes/vapors with a cap or lid. You would be surprised how quickly and large an invisible cloud of fuel vapor can form from an open container, particularly when it's warm.

Good point.  Thanks.  I'll lose the funnel.