Idle screw

[Dizerens5]

Being new to R65 I'm not sure if this is a common problem. When I got my 79 model last year I set up the idle as carefully as I could and in fact it idled and started well after some trial and error. Recently starting got more and more difficult with engine firing up on one cylnder for a while. I knew the idle settings were ok so got ready to take the carbs apart but "just to be safe" I rechecked my settings and found that on the left carb the idle mixture screw had unscrewed itself over about 1,500 miles and was nearly 2 turns out. I returned it to about 3/4 turn out and all is fine again. There seems to be an o-ring there so I'll try replacing that, or maybe some non-setting thread lock would be better? I'm just glad the thing didn't fall out on the road!

[Bob_Roller]

I believe that there should be a spring on the idle mixture screw .

http://www.realoem.com/bmw/showparts.do?model=0364&mospid=47851&btnr=13_0464&hg=13&fg=05

Item number 4 .

It may still be stuck inside the hole where the adjuster threads into .

[Barry]

Definitely should have a spring in fact it would be harder to tune really precisely without one as the small amount of play in the threads would cause a hysteresis effect.

[Semper Gumby]

Definitely should have a spring in fact it would be harder to tune really precisely without one as the small amount of play in the threads would cause a hysteresis effect.

I'm going to have to look that one up...

Ah HA!

http://en.wikipedia.org/wiki/Hysteresis_effect

[Barry]

Wikipedia's explanation of hysteresis is awful complicated.

A simple definition would be "A phenomenon where a physical quantity  takes on a different value for an increasing input compared to a decreasing input."

What I meant was that any screw thread has a bit of clearance. this means that for say  1/2 turn out you can push or pull the screw and move it vertically by some very small amount as the upper or lower surface of the thread on the screw takes the load.  With a spring in place it's always the lower surface that will take the load so that any hysteresis effect is removed making the 1/2 turn a repeatable setting.

This is not bull. With the mixture screw fully in but no spring fitted I actually marked my carbs with a fine line of red paint so that I could accurately set 1/2 turn out.  With the spring fitted the screw slot came to a slightly different position relative to the mark.  Hysteresis occurs in all sorts of mechanical and electrical systems.

[Lucky_Lou]

Definitely should have a spring in fact it would be harder to tune really precisely without one as the small amount of play in the threads would cause a hysteresis effect.

I'm going to have to look that one up...

Ah HA!

http://en.wikipedia.org/wiki/Hysteresis_effect

AHH the joy of education i knew i came here for a reason [smiley=1drink.gif]
Lou

[Semper Gumby]

Barry -- I like your explanation better and now I understand the problem Bing fixed by putting the spring in.

(thanks)