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Author Topic: XS 750 brass venturi-Carb gurus chime in please  (Read 441 times)

Offline TranceMachineVienna

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XS 750 brass venturi-Carb gurus chime in please
« on: Sep 03, 2018, 05:41:05 »
Hey guys!

So my new cafe racer project will be a special project with lots of brass parts.Since my family runs a small brass workshop in Austria I try to learn as much as possible about brass and metal fabrication.
The project is a stock XS750 motor with a few minor mods including Keihin CVK carbs (off of a Triumph Daytona 900).
These brass venturis are originally turned from a blank piece of brass and are used as christening cups  ;D

I need youre help guys:

1.)How would you guys mount the cups on the carbs?

2.)How long would you guys leave the carb venturis?How will the length influence performance and peak power?

Thanks in advance!

Offline datadavid

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Re: XS 750 brass venturi-Carb gurus chime in please
« Reply #1 on: Sep 03, 2018, 07:29:49 »
I would use a touch of permatex gray to avoid leaks and use set screws, done it before and held up great, 2" of straight pipe between venturi and carb mouth works great, no point fiddling around with different length intakes unless you are racing the thing imo. I would consider filtration on a street bike, sucks to change worn valves and guides every 5 years.

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Offline TranceMachineVienna

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Re: XS 750 brass venturi-Carb gurus chime in please
« Reply #2 on: Sep 03, 2018, 07:32:33 »
These are the carb venturis.

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Offline datadavid

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Re: XS 750 brass venturi-Carb gurus chime in please
« Reply #3 on: Sep 03, 2018, 07:40:36 »
Hehe, thats awesome! I would just cut off the ends and hammer out a flange to fit over the carb mouth.

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Offline TranceMachineVienna

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Re: XS 750 brass venturi-Carb gurus chime in please
« Reply #4 on: Sep 03, 2018, 09:22:33 »
It's going to sound mean I guess :)

I had a different idea,cut off the brass,turn two rings with threads and screw the venturis on.
What kind of filter material would you use?

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Offline datadavid

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Re: XS 750 brass venturi-Carb gurus chime in please
« Reply #5 on: Sep 03, 2018, 09:24:51 »
Using generic foam filter socks on a couple of velocity stacks right now. Turning 2" dia fine  threads sound like a lot of work, but go for it if you want!
It's going to sound mean I guess :)

I had a different idea,cut off the brass,turn two rings with threads and screw the venturis on.
What kind of filter material would you use?

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Offline der_nanno

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Re: XS 750 brass venturi-Carb gurus chime in please
« Reply #6 on: Sep 03, 2018, 13:58:11 »
Can't you just have them stretched at your metal-stretcher so they will fit over the carb mouths? And see, I told you about 55mm, which basically is 2" plus 10 percent.  ;)

And unlike David, yes they will make quite a dramatic difference, that is if the exhaust side can support it. When still working at the raceshop, we used pipercross-socks over the velocity stacks of race car engines. Out of all available, these seemed to have the least detrimental effect on throughput. That being said, you need at least 0.5" (10-15mm) between the end of the stack and the sock, otherwise it will notably affect the flow. (If you have more room, it won't be to your harm at all...)
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Offline datadavid

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Re: XS 750 brass venturi-Carb gurus chime in please
« Reply #7 on: Sep 03, 2018, 14:08:03 »
Ofc it can and will make a great difference, especially in tuned engines! I just dont think a few mm's in this case will matter much. Variable intake systems are cool but unviable in bikes, otherwise we would have seen them i guess.
I noticed a good midrange increase in my previous xs750 when lengthening the intakes with ordinary oval pods somewhere around 2 inches, those cv carbs need smooth air.
Can't you just have them stretched at your metal-stretcher so they will fit over the carb mouths? And see, I told you about 55mm, which basically is 2" plus 10 percent.  ;)

And unlike David, yes they will make quite a dramatic difference, that is if the exhaust side can support it. When still working at the raceshop, we used pipercross-socks over the velocity stacks of race car engines. Out of all available, these seemed to have the least detrimental effect on throughput. That being said, you need at least 0.5" (10-15mm) between the end of the stack and the sock, otherwise it will notably affect the flow. (If you have more room, it won't be to your harm at all...)

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Offline TranceMachineVienna

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Re: XS 750 brass venturi-Carb gurus chime in please
« Reply #8 on: Sep 03, 2018, 14:11:07 »
Can't you just have them stretched at your metal-stretcher so they will fit over the carb mouths? And see, I told you about 55mm, which basically is 2" plus 10 percent.  ;)

And unlike David, yes they will make quite a dramatic difference, that is if the exhaust side can support it. When still working at the raceshop, we used pipercross-socks over the velocity stacks of race car engines. Out of all available, these seemed to have the least detrimental effect on throughput. That being said, you need at least 0.5" (10-15mm) between the end of the stack and the sock, otherwise it will notably affect the flow. (If you have more room, it won't be to your harm at all...)
I will ask him if he can do that,good idea.
Just measured the venturis they measure 10cm so I guess I will cut off nearly half of them :)


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Offline teazer

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Re: XS 750 brass venturi-Carb gurus chime in please
« Reply #9 on: Sep 03, 2018, 14:43:12 »
Those will do nothing positive for airflow or power.  Stop and look at the air path and work out what shape you want/need and how to mount them. 

I have run UNI pods, big bellmouths and special short intake rings on a GT750 Suzuki with CV carbs and at the strip the difference is hard to measure.

If you want those to work well, I would suggest you start with a billet mounting ring with a hole centered on teh air path and additional holes to match the ports that are outside the venturi.  Then drill a hole in line with teh venturi and mount a suitable bellmouth (velocity Stack) inside that hole so that teh air through the carb is unhindered.

Something like this: http://www.mapcycle.com/categories/carburation-intake/velocity-stacks/2036-126-stack-velocity-tuned-mkii-smoothbore.html

or this: http://www.teamhansenhonda.com/parts/photo029.jpg 

or: https://shop.tonupstore.com/tuotekuvat/700x500/Velocity_Stack_900_1.jpg