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georgesgiralt
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Question about parts in carbs and their effects...
07/14/18 at 05:04:01
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Hello !
Prior to her total face lift, my '82 R65 was sucking more than 7.7 l/100 km. Awful. It was the fastest machine from a petrol station to the next one.  Angry
After cleaning and changing the brass inside the carbs, the rubber and a good check up, the figure get to 6.8 l/100 Still awful.
Sad
So I decided to move to the last BMW settings, with the asymmetrical top air box, hard springs above the pistons in the carbs and the corresponding jetting. I'm now at 6.1 ~ 6.3 l/100 km which is still bad.
I've been told to toss the springs and make bigger holes at the base of the pistons  Cry

So, BMW provides two different springs, a supple one up to 1983 and a harder one up to the end of production.

What exactly is this spring supposed to do and what kind of effect it's power has on the carb? What change will removing them make ?
I've excluded making bigger holes in the piston because the OEM part is quite expensive.... But will it change something and in what way ?

For reference, the carbs are the 308/307 and have the needle at notch 3, 138 main jet and 2.64 needle jet, with "hard" springs and asymmetrical top air box. The membranes are OEM and quite new (less than two years).
Supple Spring : 13 11 1 335 324. 33 turns
Hard spring : 13 11 1 338 134. 22 turns

I will appreciate a lecture on these matters and all the help i can get from you !
Many thanks in advance and have a good ride !
  
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Barry
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Re: Question about parts in carbs and their effects...
Reply #1 - 07/14/18 at 16:13:32
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georgesgiralt wrote on 07/14/18 at 05:04:01:
I've been told to toss the springs and make bigger holes at the base of the pistons 

What exactly is this spring supposed to do and what kind of effect it's power has on the carb? What change will removing them make ?


The springs slow the rise of the pistons and cause a temporary  rise in air velocity in the venturi which in turn results in a temporary rich condition or accelerator pump affect.  In the mid 80's the springs where stiffened to allow leaner mid-range settings without causing flat spots.  Logically if you are not running lean then stiffer springs shouldn't be need and might make fuel consumption worse. Conversely removing them might cause a lack of throttle response unless you are already running rich.


georgesgiralt wrote on 07/14/18 at 05:04:01:
I've excluded making bigger holes in the piston because the OEM part is quite expensive.... But will it change something and in what way ?


A larger hole reduces the damping of the piston and people usually do it in the belief it will sharpen throttle response.  Given you are trying to home in on optimum settings I don't think having something non standard would be helpful.


For what it's worth the order of priority I would address things when looking to improve fuel consumption wold be:

float levels
idle mixture (really this is the no1 area for improvement but you can't set it without correct float levels)
needle jet and needle settings
Last and very definitely least the main jet
 
  

Barry Cheshire, England 79 R45
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georgesgiralt
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Re: Question about parts in carbs and their effects...
Reply #2 - 07/15/18 at 11:37:34
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Thanks a lot Barry ! I've learned something valuable here.
I'll start fiddling !
Have a nice day.
  
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georgesgiralt
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Re: Question about parts in carbs and their effects...
Reply #3 - 07/18/18 at 01:59:10
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Hello Barry,
While we are at it, is there a tuition, a book, a course discussing these subject ?
Something I can read and annotate when I do changes.
All the information I've got came from many many places in many forms, the most important one being "I've been told" ....
Thanks
  
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Barry
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Re: Question about parts in carbs and their effects...
Reply #4 - 07/18/18 at 03:27:58
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Hello Georges,

The only book I have is the Haynes carb manual. I haven't read it for some time so i couldn't say if it covers the two aspects of CV carb operation you were asking about. The effect of the damper hole in the slide is well known enough as the Harley crowd often drill it out. The efect of the slide spring perhaps is more elusive and it took me some time to sift through varaious accounts of how it worked. In the end the reasoning behind a stiffer spring to counter overall weaker carb settings clarified and confirmed it's effect for me.

 
http://www.lunadesign.org/images/gixxer/carbs/Haynes%20Carburator%20Manual.pdf
  

Barry Cheshire, England 79 R45
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georgesgiralt
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Re: Question about parts in carbs and their effects...
Reply #5 - 07/18/18 at 04:47:07
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Thank you Barry, something to read when sleep would not come Wink
Have a nice day
  
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georgesgiralt
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Re: Question about parts in carbs and their effects...
Reply #6 - 08/11/18 at 00:29:38
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Hello Guys,
So I read the book. On paper because it appears I had it stashed with a lot of books I bought second hand at a car boot sale.
I still have a couple of questions, though.
I was amazed to discover that it seems a needle notch change has a large change on the whole range of operation, and that a needle jet change only plays on up to 50 % throttle and a less important influence. Seems to be a little different of the collective wisdom.
And then came the CD carburetor. Mr Shoemark tells us that on the low throttle openings, the idle jet play its part, that at full throttle the main jet plays alone, but he says that in between, one can play on the needle and needle jet and on the apparent weight of the piston (actual weight and strength of the spring above it) but he does not say in what way and what to expect...
So if one of you knows, I would be delighted to learn about it !
Have a nice day !
  
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Barry
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Re: Question about parts in carbs and their effects...
Reply #7 - 08/11/18 at 13:57:45
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I went to the trouble of measuring the needle taper at 1 mm increments and calculating the change in jet area produced by one needle position. It is a big change but not linear. Lifting the needle from P2 to P3 results in a jet flow area increase of 44% at initial lift reducing to 5% increase at maximum lift.

Needle jet changes are much smaller. Increasing the needle jet from 2.66 to 2.68 results in a jet flow area increase of 10% at initial lift reducing to 2% at maximum lift. The 10% change drops very quickly so that by only 4 mm of piston lift the increase has already dropped to 5 %.  Similar % changes apply for 2.64 to 2.66
You will often hear it said that one needle position change is equivalent to 3 needle jet size changes.  While the measurements I took show that it's not that simple, on average  the figure of 3 is not that far off.







  

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Calculations_001.jpg

Barry Cheshire, England 79 R45
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