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  #1  
Old 30 Oct 2009
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Hi
The main difference is that I changed from a fron balnce pipe set up to no balance pipe into a y-piece. I will balance the carbs tomorrow and re-check the valve clearences and see if it makes a difference
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Old 31 Oct 2009
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Quote:
Originally Posted by simplemind View Post
Hi
The main difference is that I changed from a fron balnce pipe set up to no balance pipe into a y-piece.
That's not good...
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Old 31 Oct 2009
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Alibaba what would you suggest as a rough basic idea of rejetting to when doing as Simplemind have done by chaning the back-pressure so radically as he have done?

Whould a lift of the needle (one or two notches) and a change to next larger main-jet be fairly ok guess work, or would you recommend as I would => workshop with proper tools to adjust the mixture correctly?

I've tested s/s "megaphone" (made in Australia sold by www.midland.se in Sweden) once on my R80. Loved the sound! But... lost all low rev torque => opted to stick with "stock" mufflers/silencers instead (have regretted that choice ever since -ROWL). My reason was that I was very concerned about what the effect would have on the exhaust fume temps, and rejettings were beyond my scope of knowledge back then.
Sounds very much like what Simplemind have done with his beemer , i.e. altering back-pressure way too much without rejetting the carbs accordingly...

Most interested to hear about your thoughts in/on the jetting theories/practicalities -
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Old 31 Oct 2009
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Albert thanks for the advice. The valve clearences are set OK and the bike actually idles pretty smoothly only gets a bit poppy higher up the revs. Whilst balnancing the carbs with a vacuum gauge is within my capability - adjustment of mixture is something i would try but needle adjustment is beyond my ability without extensive reading up first (dont want to make matters worse). I take it from all this is that the mixture is too weak thereby causing overheating in the header pipe?
I would be happy to get the bike on the road without causing any permanent engine damage and then take it to a bike shop for them to sort out.
Mark
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Old 31 Oct 2009
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Begin by checking the Bing site:
BING POWER SYSTEMS GmbH
BING POWER SYSTEMS GmbH - Einstellblätter

On the side of the carbs there is a number 32/64/xxx or 40/94/yyy - type that into the box at Bing site => then you get the original Bing jettings for your carbs (each carb have its specific jettings - BMW may have altered the jettings to suit different markets!).

My guess is that you have 145 main jets if in europe and 135 if in the US.
As an acute remedy you could get main 150-160 or 145-155 jets to test with - what you risk is too rich mixture => just cause a soothing which is far less damaging compared to overheating. (I've tested both duirng the 1990:ties...).

Best is however to have a workshop fix the situation for you - and you should look at a racing shop then... something like MOTOSPEED.se - Your Performance Partner or MODELBOUWWINKEL & WEBSHOP - LIMBURG - MBS-RACING guys that know how to jett and tune bikes - beemer shops very rarely do...


When you changed form dual balans pipes to a collecting Y you trade off low rev torque for high rev performance - you were aware of this, and that was your intension ?


Looking forward to hear how it goes -
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Old 2 Sep 2010
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How did it go?
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  #7  
Old 5 Sep 2010
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How it went
Albert,
I took the bike to a BMW mechanic who stripped the carbs and found a blockage in one of the jets. The bike starts and idles smoothly but after 2000rpm the exhaust note is loud and raucous (banging noticably). I realise that the exhaust configuration will alter the exhaust note but the mechanic thinks that the baffles in the silencer (the 11yr old original) sound shot. I can pick up one for around £300 but want to make sure that it is the baffles. What is the life span of a silencer? I would imagine that if the mods to the exhaust were unsuitable I wouldnt have a smooth running accross the rev range particularlyat idle/low revs, any thoughts?
Mark
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