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SOHC.co.uk Forums => CB750 => Topic started by: neilg on November 03, 2018, 05:31:15 PM

Title: One for the Engine Tunners to ponder
Post by: neilg on November 03, 2018, 05:31:15 PM

In a moment of madness I have created a monster using an F2 motor with a K7 exhaust. So I ask myself, where should I start with the carbs. Should I start with F2 jetting due to the combustion chamber design. Or K7 due to the 4 into 4 exhaust design. I have to use PD carbs to keep in with the theme of the build so swapping for earlier types is not an option.
I need to rest now cos my headaches are starting again 🤔

Neilg

Title: Re: One for the Engine Tunners to ponder
Post by: hairygit on November 03, 2018, 10:32:08 PM

Go with whatever the K7 uses as stock.

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Title: Re: One for the Engine Tunners to ponder
Post by: K2-K6 on November 04, 2018, 09:18:35 AM

I'd go the opposite as it's the F2 cam, valves,  combustion chamber shape,  compression and rpm ceiling that hold much more relevance to carb setup.

In addition,  the F2 carb's are PD42A with the K7 using PD42B spec which looks like a different throttle slide geometry to match the engine spec and lower rpm expectations of the K.

Title: Re: One for the Engine Tunners to ponder
Post by: neilg on November 04, 2018, 04:35:35 PM

Mmmmm now you see why my head hurts 😢

Title: Re: One for the Engine Tunners to ponder
Post by: SumpMagnet on November 04, 2018, 05:42:05 PM

I'd go with the F2 setup as a start point.

My thinking would be the F2 would need bigger jets and draw more fuel. If you start at the richer settings, it's safer and you can work down to optimise running. The opposite could be running lean which is far less desirable.

Title: Re: One for the Engine Tunners to ponder
Post by: hairygit on November 04, 2018, 05:46:04 PM

The F2 has smaller mains than the K7 though, due to the 4 into 1 exhaust, same with the F1, smaller jets than the four pipe K6.

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Title: Re: One for the Engine Tunners to ponder
Post by: SumpMagnet on November 04, 2018, 07:44:11 PM

Didn't know that :)

Well....my logic is still the same. Go with the larger jets and work down to whatever you need.