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Old 06-01-2010, 04:50 PM   #31
squish
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Quote:
Originally Posted by outfit
The "new" spring is harder than the "old" one, so a probable explanation is that the new spring reduces the pressure fluctuations that occur at low rpms inside the carburetor. A possible reason for the better "response" at low throttle is that the harder spring yields a reduced efficient cross-section when you open the throttle. This yields a higher air intake speed, which in turn leads to better filling and thus more "punch".
To which Beemerboff wrote
Quote:
Originally Posted by Beemerboff
Amen

AH HA!. Now I understand where you are getting your logic.

You are confusing a more complete intake charge with a more rich intake charge.

What's going on with the smaller diameter venturi is a higher intake charge velocity, which in turn crams both more air and fuel into the combustion chamber.

I don't disagree a whit that changing the "weight" of a slide changes the effects and the response of a carb.

What I disagree with is that a heavier "weight" makes the charge more laden with fuel. There is no physical way it can. The needle at the bottom of the slide blocks more fuel from coming up the tube.

Short of putting a carb into a test fixture to prove this and posting the results I'm done with this discussion.

Thanks for your time and the explanation of your theory.




as for the assertion that the throttle doesn't matter. Well it does, since it's one half of the system that controls the fuel and air mixture in a cv carb.
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Old 06-02-2010, 05:32 AM   #32
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Amen , Amen
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Old 06-02-2010, 06:12 AM   #33
AntonLargiader
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So Beemerboff, does this mean that a carb with a torn diaphragm is running too rich rather than too lean, since the slide is not lifting? This should be easy enough to test.
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Old 06-03-2010, 08:13 AM   #34
Beemerboff
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Please , read my posts, I did say that when the piston is opening slower that the butterfly is opening the mixture is richer, but if the rubber is torn so much that the piston is not moving that statement clearly does not apply.

I have never had a torn diaphragm, SUs dont have them and I replace the Bing items as a service item - so I dont know just what happens, but I would imagine some fuel would be sucked in through the pin pricks, but there wouldnt be sufficient air flow to atomise it , so it wouldnt burn very well, if at all.

So you could have a decent bit of fuel performing as if it wasnt there.

Kinda like the brains of some of the posters on this forum.
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Old 06-03-2010, 08:28 AM   #35
lkchris
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The brains that count are those at BMW and Bing, and NOBODY on ANY forum can do it better.

Look up your OE specifications and stick with them.
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Old 06-03-2010, 08:45 AM   #36
AntonLargiader
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Quote:
Originally Posted by Beemerboff
...but if the rubber is torn so much that the piston is not moving that statement clearly does not apply.
I dunno about that; it might very well apply. I can totally see your overall point. If the effect of lifting the needle only increases the orifice by let's say 20%, and you're getting 100% more velocity by virtue of the slide being lower, sounds like a recipe for more fuel rather than less.

Doing the calcs wouldn't be that difficult but I think proving/disproving it experimentally would be easier. Thanks for the ideas.

Quote:
The brains that count are those at BMW and Bing, and NOBODY on ANY forum can do it better.
Because BMW has always gotten stuff like this exactly right.
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Old 06-03-2010, 12:32 PM   #37
Hawk Medicine
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Quote:
Originally Posted by bikecat
Came across this thread as I am starting a carb rebuild.

Any further reviews on the EUBMW rebuild kit? I realised that I bought them about a year back when the bike was still very much work in progress.

Will get the jets etc from Motobins, I reckon. Pound is low now and might as well take advantage of that.

Cheers
I installed EUBMW's carb rebuild kits in my R90s carbs and ran with them for several+ years and had no problems or complaints. I also installed a set in my newly rebuilt R100s carbs and they appear to be working well also.

All-in-all I find EUBMW to be a good bunch of folks to work with and for stuff like valves, springs and other hardware, you save a little $$$ and get the exact same parts as BMW provides.

LAstly... What other parts supplier will ship you your parts before they receive payment? Thats service!!!
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Old 06-03-2010, 05:20 PM   #38
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thanks for all the carb info, a true learning experience

I too have used EUBMW for lots of things and have had no problem
my last bike I used thier head gaskets, v.c gasket carb kits and pushrod seals . my current bike I have used thier carb kit on, no problems wiht any of it
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