Vacuum pipes on CB750FO

[Dave B]

I have got to the stage of trying to start my bike which came as a box of bits. IT won't start.
Carb jets were all very blocked.Slides seized.
I have cleaned the carbs and all jets are clear and slides and needles running free.
All four float bowls are filling with fuel but fuel does not appear to be passing through jets around the needles.
I have a good spark and good compression.
I have attached a picture of two tubes that i believe just vent atmosphere.
However when i apply compressed air to open tube end it will not pass through to inside of carb.
The tubes are new.
I have twice stripped the carbs and checked jets and floats.
Any ideas greatly appreciated,
Dave.

[McCabe-Thiele (Ted)]

I am relatively new to this forum but my thoughts are - When you blow down the main jet emulsion tube or up from the carburetta base is it a free flow of air ? I would use some rubber or plastic tubing to ensure a flow. If that is good then I would be looking at things like is there a decent level of suction on the intake side. Have you got the choke on plus air filter etc.

[K2-K6]

To give a separation of cause,  lift the throttles and spray something like carb cleaner,  e-z-start into the throat, then try to start it.

If it goes then you know you're just looking at fuel rather than another problem mixed into the non-start.

The tubes are usually to vent the float bowls and stop them air locking into vacuum which would make the jetting variable in use. They should have clear passage into the carbs to work effectively. 

For any carb,  if the float bowls aren't ventilated properly, then the vacuum in the venturi can't overcome that created in the float bowl, so no flow will occur.

[Dave B]

Yes main jet tubes are clear Air Filter and box in place Choke on

[Dave B]

I will get some easy start ordered
Thank you

[Bryanj]

Plastic pipe def not a good idea, are the slides closing all the properly and are the slides in the correct carbs, if mixed up you can fit the with cutout on wrong side

[K2-K6]

I will get some easy start ordered
Thank you

If you've got all the airbox assembled,  then you can just remove the filter holder to spray in there and not remove the upper assembly.

It's really just to see if it'll fire with an independent supply of flammable vapour,  then allowing you to concentrate on fuel (if it runs) rather than having to consider the ignition. 

Hopefully you'll getting an answer to allow further diagnosis. 

[Dave B]

Thank you i will let you know

[Nurse Julie]

There looks to be a very lage gap under the carb slides that I can see in your picture and they don't look of an even lift. Have you bench synced them yet?

[K2-K6]

There looks to be a very lage gap under the carb slides that I can see in your picture and they don't look of an even lift. Have you bench synced them yet?

They look to be unmounted from the "bar" and won't have any return spring pushing the slides down so all in different places as far as I can view.

[Dave B]

Thats true,
They are sinked when connected

[Spitfire]

Did you check the filters in the float bowl inlets, mine were full of rust.



Cheers

Dennis

[Dave B]

Yes they were solid as were the main jets and idea jets.
Just a thought, is there any jet holes in the brass fitting above the main jet that houses the needle
Dave

[philward]

Do you mean the emulsifier tube (needle moves into this and main jet screws into) - here's an example
https://www.ebay.co.uk/itm/Reproduction-Needle-Jet-Holder-Set-of-4-Honda-CB350F-CB400F-Super-Sport-/172644532354
If so - yes they have holes in

[Bryanj]

There are holes but not "jrts"