Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: kthree on October 12, 2019, 11:19:43 AM
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Hi all. I have a rebuilt Honda K7 motor currently fitted with a set of Kehins with cheap POD air filters.
The original K7 card set up was PD42 pumper style carbs. I have a set of them and an airbox to suit.
I'm told the full airbox is the way to go - but which carbs will give the best performance.
Any points of view on which will be the better carbs to run with?
Cheers and thanks.
G
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PDs are notoriously difficult to get properly clean and need to be on a bike that is used regularly, minimum weekly and not just for a quick ride
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Accepting the above as consideration, I'd favour the PD carbs with standard airbox.
They just run a far more optimum air fuel ratio at close to ideal for light load, then supplemented by the pump when needed. As opposed to generally a bit too rich all the way through on conventional non pumped carbs.
Flow enough for highest factory rpm on SOHC for the F2, benefit by not pushing so much unburnt fuel into engine oil, and give good snap acceleration whichever rpm you wind it on from.
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Thanks.
I've heard the standard airbox is definitely the way to go so I have found one in good conditions. Carbs have been disassembled, soda blasted, all parts ultra-sonic cleaned and reassembled with new gaskets and seals throughout. So with standard airbox and filter, an 836 kit in the K7 motor and a reasonably open 4 into 1 exhaust system -does anyone have any clues on the optimum jetting setup?
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Not a good idea to soda blast PD's. They have a breather on the side of the carbs that everyone misses when cleaning them. They will get clogged with blast media then there is no way of getting the carbs to run correctly. Will see is I can find a diagram to show the breather. Can't stress enough that it's vital they are cleaned completely.
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Regarding jetting, I feel it's often missed that a big bore kit will just suck more air through the carb, accompanied by more fuel.
Initially it's partially self correcting if using the same intake system so not such a dramatic change of jet sizes as often projected.
Suggest try with std, plus raise needle by one notch (fourth groove down from top of needle) shift idle adjust screw 1/8 to 1/4 turn more fuel.
Also run with either NGK 8 plugs or even 9 grade initially. This will mitigate against getting the plugs too hot if it runs lean, to help in avoiding pre-ignition risk.
Try it from there, have a feel of behavior and good look at the plug's colour to see where you are.