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May 12, 2015 11:04:01 GMT
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Sorry for the late input but I just found this thread. Regarding choke size on carbs, I believe the purpose is to create sufficient flow velocity to create the venturi effect on fuelling. As far as I know, it has nothing to do with suction. The venturi effect has nothing to do with creating a low pressure area in a conventional sense.
The potential issue (that I am experiencing) is that throttle bodies that are too large do not create sufficient velocity to mix fuel with air (on a fuel injection sysytem) when trying to cold start. I just spent a miserable winter trying to tune cold start (i.e. get it to start) on my Lotus 912 engine and I think a contributory factor is that the TBs are 50mm rather than more like 40mm, so when funds allow I may go for new TBs.
ATB Richard
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May 12, 2015 20:50:04 GMT
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A 58mm itb will have about 52mm usefullness (for lack of a better word) because the butterfly restricts it. Hence why roller bodies are good.
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May 13, 2015 15:10:47 GMT
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A 58mm itb will have about 52mm usefullness (for lack of a better word) because the butterfly restricts it. Hence why roller bodies are good. Thanks. The Dellortos were nominally 45mm (DHLA45) but were fitted with a choke size of 37mm. On a carb does the butterfly fit inside the choke further restricting it?
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May 13, 2015 19:48:33 GMT
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Yes, anything in the path of the air restricts it.
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slater
Club Retro Rides Member
Posts: 6,390
Club RR Member Number: 78
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Are your injectors in the throttle bodies? Most engines would have the injectors either in the head or manifold. Basically as close to the inlet valve as poss. Maybe you could consider a move? If your injectors are not in the TB the TB size is of little importance (within reason) It's the velocity once the fuel enters that's important.
Also the carb uses a venturi (or choke) to create a pressure drop across the float chamber so that fuel is sucked out. That is the Venturi Effect. Air speeds up as it goes through the restriction and drops the pressure just like the wing of a plane That's how a carb works in essence. The throttle is after the Venturi otherwise the manifold vacuum would just suck fuel out when the throttle was shut.
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sparkyt
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Posts: 1,767
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May 14, 2015 11:10:31 GMT
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Gas speed is the key . . If you can keep the size of the pipe the same throughout the inlet manifold system it will help keep the fuel in suspension as a vapor (not normally a problem with injection cars as the injectors are normally close the the inlet valve ) chokes in carb are used to speed up the gas flow over the fuel delivery pipe to aid the fuel vaporization.
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May 14, 2015 20:33:45 GMT
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The injectors are in the TB (Jenvey) which is why the size may be important for consistent atomisation with higher velocity.
Thanks for comments, Richard
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sparkyt
Posted a lot
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Posts: 1,767
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May 14, 2015 23:20:16 GMT
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Maybe ... .
The fuel it ripped of the surface of the fuel delivery pipe in a carb with nothing more than the speed of the air . .
Something you don't have a problem with
I can see your thinking . That the size of the port in the itb may be to big for the engine
All you can do mate is give it a try and rr the results .
Good luck . .
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