Cold start

[rusty nuts]

My 1975 cb 750 when starting from cold only runs on three cylinders then after about a minute runs on four and is fine any ideas anybody.

[Jore]

Is it always the same cylinder?

[rusty nuts]

Yes number three .

[Jore]

It might be a sticking float valve that frees itself with the vibration of the other cylinders running.

[Nurse Julie]

Another Newbie from Lincolnshire, welcome to the forum.

[rusty nuts]

Yes a newbie from the posh part of lincolnshire ?

[Johnwebley]

Yes a newbie from the posh part of lincolnshire ?

   that must be with the hills and valleys !!

 welcome to sohc fun palace !!

[Nurse Julie]

Yes a newbie from the posh part of lincolnshire ?

Is there a posh part? Woodhall Spa would be the 'poshest' I suppose. Ok if your thing is chasing  white balls around a field  Or is it Stamford?

[rusty nuts]

Yes a newbie from the posh part of lincolnshire ?

Is there a posh part? Woodhall Spa would be the 'poshest' I suppose. Ok if your thing is chasing  white balls around a field  Or is it Stamford?

'
?

[Nurse Julie]

Yes a newbie from the posh part of lincolnshire ?

Is there a posh part? Woodhall Spa would be the 'poshest' I suppose. Ok if your thing is chasing  white balls around a field  Or is it Stamford?

'
?

[Nurse Julie]

Yes a newbie from the posh part of lincolnshire ?

   that must be with the hills and valleys !!

Yes John
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[K2-K6]

Welcome to the forum.

Don't known what you've checked so far so some things to verify I'd do; swap plug leads 2 with 3 if they'll reach to reasonably eliminate the HT section of number three.  Start it in the dark and look to check for HT leads tracking to earth when you first start it.

Check to see the choke flap for no 3 is intact and going to position correctly.

Suspect the idle circuit on no3 if you're not certain it's clean and operating correctly. Try backing idle screw out on no 3 while it's running to see if it responds.

Swap the plugs on 3 and 4 to see if problem transfers.

Are the compressions equal for all four cylinders.

[rusty nuts]

Thanks will try that.

[shifter]

Yes Rusty ,Now that is the nice part of Lincs ,   

[Greebo]

Thanks will try that.

1st thing I would try is another spark plug in No 3 either swap one from another cylinder or a brand new one