Topic: DOHC CB750 Carb question

[apehanger 550]

  I have a DOHC 750 that is very difficult to start when it sets for a time. If It sets with the fuel petcock left on,it is not an issue. It is not a vacuum petcock with a prime,just on,res,off. It will try to fire with full choke,but it takes many attempts before it catches and then a few minutes to warm up
  Other than that,it runs very well. It also starts easily if run every day or two.
  Your thoughts appreciated.

[trueblue]

I find with my 650 if it sits for more than 3 days I have to turn on the fuel tap, wait 30 seconds or so then try to start it and it starts pretty easy.  When cold, I pull on the choke give it 4 full twists on the throttle then hit the starter and it comes to life very quickly, leave it run on the fast idle until it starts to stammer a little from the choke, push in the choke and ride away.  After all this it usually has a few coughs and farts for the first kilometer or so then doesn't miss a beat.

[onepieceatatime]

Unless it has been removed, there is a vacuum valve between the petcock and carbs on the early model DOHC bikes. A lot of people remove them because of problems with fuel flow (usually due to bad internals). If removed, you should ensure you turn the petcock to the off position when not in use, just like the older bikes that did not have this added vacuum shutoff.

[City Boy]

Hi.I am not sure what you mean by sitting for a while but I find with my big brother 1100F it does not want to start after a few weeks if I don't drain and refill the float bowls first.I am astounded at the King's ransom we pay for the rubbish fuel we are offered these days ! My vacuum fuel valve has always functioned as designed on my unit.      Rock On