throttle bodies

J

jan nelder

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Feb 18, 2018
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anybody know what the internal diameter of the throttle bodies is? And if a carb swap was done, what size carbs?
 
TonyA said:
They are listed as Jenvey 38MM in the Norton spec sheet.
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throttle bodies
 
City Garage said:
Where are the photos of bikes Ollie converted and what did he use for ignition?
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I’ve not seen any photos, best to ask Ollie.

I believe he uses a Power Commander, and that it takes a long time dial each bike in due to huge differences between the bikes, but again, best to contact him for details.
 
Fast Eddie said:
I’ve not seen any photos, best to ask Ollie.

I believe he uses a Power Commander, and that it takes a long time dial each bike in due to huge differences between the bikes, but again, best to contact him for details.
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That's my point...its a unicorn. No one has seen it.
 
TonyA said:
Hi Cliffa , I have measured them before and agree with your measurement . The 38mm comes from the point at the throttle valves.
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Hi Tony, actually from what I can see the butterflies are even smaller. Maybe 35mm, but it's difficult to measure them.
 
so if the head has 36mm inlet connection, and old Commandos work well with 30mm carbs, why not use 36/37mm carbs ( Keihin/ Mikuni are what I have been looking at)??
 
jan nelder said:
so if the head has 36mm inlet connection, and old Commandos work well with 30mm carbs, why not use 36/37mm carbs ( Keihin/ Mikuni are what I have been looking at)??
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For best performance the carb / throttle body should be bigger than the port that’s closer to the valve. The bigger carb will then ‘force‘ more charge through the smaller port, thus increasing charge velocity, thus increasing cylinder filling.

Of course, this is a gross simplification and there are many, many, variables in play.

If the ports are 36mm, I would personally be looking at 39mm carbs (well, that’s exactly wot I done on my Seeley) it’s only a 1.5mm difference in radius.

Whether or not the manifold connecting the two works best with a smooth taper, a sharp taper, or a step, can only really be ascertained via experimentation.

This is all on an ‘old’ Commando motor, but as examples of the unpredictable variables, and experimentation, I swapped out the genuine FCR velocity stacks for some stubs designed to take pod filters… shudda been a backward step, but I gained nearly 2bhp. Then I played with different ‘reverse cone’ ends on the 2:1 pipe to reduce noise, this shudda been a backward step too, but I gained another 5bhp. So experimenting with peripheries, even ones that ‘should’ have caused reductions, actually gained me 7bhp, and all in a couple of hours!

39mm FCRs on an ‘old’ Norton:

IMG_6210.jpegIMG_6212.jpeg
 
Last edited:
TonyA said:
Hi Voodooo , Yes , I would like to hear more about this .
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Theil sells a big bore kit. I know he does much more, but one thing that is clearly not his strong point is communication / promotion !

I was talking to him a long time ago about the 2:1 that BritTwit is patiently waiting for. He went through serval designs but basically could not get it to perform right, I’m guessing he may have canned the idea now, or at least put it to the bottom of the priority list. This shows the guy is serious in his work and has integrity, he could easily have pushed them out to the market irrespectively.
 

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