It Runs!

The CB 750 sprang in to life this avo. Thanks to all who offered advice.

I took the carbs off and cleaned them again. I'm not sure this had much
effect.

The CB750 has a vacuum operated fuel valve in series with the manual
fuel tap. It only allows fuel through when the engine is running. If you
can't start the engine owing to a lack of fuel - you're stuffed.

OK, that wasn't the main problem but it sure didn't help. Hmm. Thinking
about it, the vacuum device stopped a whole tank of fuel from
evaporating out of the float chambers... So, perhaps a double edged sword.

Slight leak from the sump plug. Tightened it slightly - felt ominous.
I'll be thoroughly F'd O if it's stripped.

So I'll stick it all back together tomorrow night, get an MOT and it'll
be the eighties all over again.

Cheers,

Nick

 

That's what the Prime position is for, though.

 

Yeah. I've heard of bike with a "prime" position but mine has "res",
"off" and "on".

The vacuum valve is physically separate from the manual tap and it has
no controls of its own.

But there is clearly something I'm missing - unless cranking it is
supposed to create enough vacuum to fill the float chambers??

Cheers,

Nick

 

Then it's an ordinary gravity tap.

No, that's not how it works. The float chambers will only fill if fuel
from them is being sucked into the engine. It's like a lavatory
cistern: the float does exactly the same job as a ballcock.

 

Same setup as my 1981 CB900C. Not one of Mr. Honda's finest designs.

If the throttle plates are fully closed and your battery is strong, you
will *eventually* pull enough vacuum to open that valve, but it's quite
a marginal thing. Somewhat hard on the starter though, so if you have
any mechanical sympathy at all, pull off the vacuum line to the tap and
suck on it by mouth (least amount of faffing about), or use a Mityvac
pump. You could also pull the fuel lines off the input and output of
the vacuum valve and connect them together.

 

Ah, I misread that. So there's a manual tap connected through a vacuum
tap. How bizarre is that?

 

if it was mine I'd remove the vacuum one.

 

Very tempting. But I'm too lazy to turn the manual one off when I leave
the bike for ten years. :->

Cheers

Nick

 

On this bike a leaky float valve results in a pool of fuel on the floor
as it runs out of the overflow pipe. I guess if the overflow was blocked
it might find its way into the cylinder.

I think the vacuum valve saved my bacon while the bike was sitting. I
prevented the whole tank of fuel from evaporating through the carbs.

Cheers,

Nick