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OK so I have done plugs, shims, fuel tap works, no vacuunm lock in the tank, stripped and cleaned carbs, replaced float needles and seats, cleaned all passageways and ensured they cblow through, cleaned all jets - done everything.

Still wont start when hot unless you hold the throttle wide open.

The bike went for a run and got nice and warm, then I stripped back the tank to sync the carbs and after running without airbox I shut the engine down and turned off the ignition and heard some strange breathing sounds from the carbs - upon closer inspection there seems to be either pressure or vacuum in the bowls causing fuel to be pushe into the venturi from either the main jet or pilot circuit - there is no fuel pump working, no power to the bike, only the heat from the cooling engine.


https://youtu.be/4Pp6qX09B8w


Any ideas ?
 

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Carbs lean out when hot. EFI is the opposite. That's why you always used to see carbed big singles refusing to start after a hot stall. Something tells me if you pull the plugs after one of these no-start incidents, they will be pretty white.

Raw fuel can actually boil in the intakes when the engine's hot. Might be the noise you're hearing.

If it's anything like a ZZR 1200, and i suspect it is, fresh o-rings that seal the jet bodies to the float bowls are critical.
 

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Discussion Starter #4
Carbs lean out when hot. EFI is the opposite. That's why you always used to see carbed big singles refusing to start after a hot stall. Something tells me if you pull the plugs after one of these no-start incidents, they will be pretty white.

Raw fuel can actually boil in the intakes when the engine's hot. Might be the noise you're hearing.

If it's anything like a ZZR 1200, and i suspect it is, fresh o-rings that seal the jet bodies to the float bowls are critical.
Thanks for the reply and apologies for the second topic about the ame issue.

Well the float needles seats and seals were replaced as part of the carb rebuild I did a few months ago and it didnt cure the issue.
Aside from that the float valve seal as expected when tilting the carbs and blowing down a pipe connected to the fuel supply T on the bank of carbs.

Today I increased the float hight from 13 to 15mm and will fit them and see if it has any influence.

As for the colour of the plugs after a failure to start wen hot - sopping wet with fuel. Im now of the opinion that its not starting due to wet plugs which in turn is due to the fuel bubbling out of the carbs and into the inlet ports as well as some coming out of the rear vents on the back side of the venturi.

Really struggling to solve this one so any assistance is helpful.
 

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Sounds like maybe ur fuel tank might b pressurizing w the heat. Causing the carbs to overflow w fuel. Maybe get it hot n pop the cap open when u shut it off c if that helps any. Or petcock isnt shutting off fast/at all. Check ur oil too. Make sure its not saturated w gas. If it is ur washing the cylinders...
 
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