Question about BMW Motorcycles
A well tuned carburettor would usually aim to use the middle notch of the needle with the other notches reserved for "local conditions" such as operating at altitude and general wear of the carburettor and engine.
The trouble you are facing is fitting open exhausts that throws "the book" out of the window as without the back pressure from the exhaust that is designed into and is greatly affected by the valve timing, the air consumption of the engine can be radically changed.
It is only at or near full throttle that the main jet is in complete control and between a quarter and three-quarter throttle the slide cut-away has a major influence and the idle jet and air passages can influence the mixture up to a quarter throttle. Successfully tuning a carb after a modification is almost a "black art". The needle and needle jet and needle position are "influencers" of the overall tune and should not be given too much importance and the needle characteristic tends to be more important than the position. One type of variable venturi carb has around 2000 different needles to choose from and another had five different slides to choose from - each with a different size of cut-away.
A vehicle manufacturer invests huge resources into ensuring the best compromises are made for carb tuning to provide smooth running, reasonable power and economy over a wide range of engine speeds, loads and throttle openings.
A home tuned carb is rarely "right" and at best provides a completely different set of compromises, usually achieving only marginal power improvements over a limited speed range while increasing the fuel consumption - much of it being expelled only partially burned through the exhaust while increasing the rate of engine wear.
I suggest you research the subject before you go further...
Posted on Oct 14, 2018
Buy a clymermanual
Posted on Oct 14, 2018
this should get you close, will still have to fine tune due to exacting conditions.....humidity, temperature, pipe, silencer and reeds....however try this:
Needle Top Clip
Air Screw 2 Out
Posted on Oct 08, 2009
Stock size for the slow jet is 35. Some guys put in 38's to make them less prone to getting plugged. I would say that you still have gunk in the slow jets. Also turn the pilot screws out 2 1/4turns. If the exhaust pops and you have no exhaust or vacuum leaks turn the pilot screws in 1/4 turn at a time till it goes away.
Did you change the plugs and the air filter? How about checking and or replacing the vacuum lines?
You might try a full bottle of Techron fuel system cleaner in a tank of gas. It may clear the problem up.
I had to clean the jets on my 1999 I/S twice and it's not a fun job.
Hope this helps.
Posted on Oct 14, 2009
There is only a manual for an XR250 but it may have the information you need. Go to http://www.carlsalter.com/motorcycle-manuals.asp , and download a (free) manual.
Posted on Jan 01, 2010
Sounds like you PILOT jets are gummed up. try to run it with the choke on and see if it runs better. what happens is, when the pilot jets are clogged they dont let enough fuel through to allow it to idle. also, when you roll the throttle on then the venturi lets more air through and the jets aren't allowing enough fuel to accomodate the increase in air flow causing the bog down.
Posted on Apr 30, 2010
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