Runs on 3 after standing

[adriangsmith]

Hi All,
Have an issue with my cb750.
If left for a week in the garage it will only run on 3 cylinders (number one not firing).
If ridden and then left to stand for 20 - 30 minutes it will start up and run on all 4 cylinders.
Pretty sure the carbs are good so maybe electrical based.
It has killed a spark plug on this cylinder already. totally dead, no spark at all!
Any ideas?

Cheers
Adrian

[Bryanj]

pilot jet or plug cap

[Mag1]

Also check the ht lead is tight at the coil, pull off the spark plug cap, snip the ht lead back say 5mm and rescrew cap back on then recheck for spark. Also plug gap check at same time.

[MarkCR750]

Could also be a sticking float, give the float bowl a few sharp taps with the plastic handle of a screwdriver whilst it's running on 3 cylinders and see if it kicks in.

[mike the bike]

The obvious thing to do is to swap HT leads and swap plugs and see if the problem migrates to a different cylinder.  You'll then know if its HT related, a dodgy plug or carb related.

[Spitfire]

Mine used to do that, I have a standard header pipe layout with a DS silencer, once I drilled a few holes it pretty much went away.
Usually it was the same two cylinders on the right as you sit on the bike that were affected, sooted up plugs, I put it down to the silencer being too restrictive.

Cheers

Dennis

[adriangsmith]

Hi Spitfire,
I think you may have cracked it.
Mine has been running rich. Will keep an eye on the plug condition for the next few weeks to confirm it.
Cheers

Adrian