bmw r65 1979, carb help

[manxr65]

hi there im new to forum and to bmw's so please forgive me for not knowing
i need some help with the carbs , i have made a cafe racer from a r65 and have put a pair of universal cone filters on and open exhaust , i cant seem to get bike to tick over , my air mix screw is almost out , would air filter and exhaust make that much difference , cables are fine and carb is clean , any advice would be helpful
thanks

[Bob_Roller]

I would say that the modifications you made, are most likely a
likely suspect in the engine not performing well .

About the only thing you can try, it's it's trial and error, is to try larger idle jets, and see what effect it has .

[Barry]

While the mods would certainly affect the overall carburation I'd be very surprised if they would prevent the engine from ticking over altogether without something else being wrong like a blocked idle jet or incorrect settings.

Having made sure the jets were clean I would start with stock carb settings.  One turn in from closed on the throttle stop screws and 3/4 of a turn out from fully closed on the idle mixture screw.  All else being OK like timing and valve clearances those settings should give you a fast enough idle speed when fully warm.

[darrylri]

my air mix screw is almost out

I believe that you have made your idle leaner, not richer.  The air screw lets more air in the further out it is.  

[GrimReaper]

my air mix screw is almost out

I believe that you have made your idle leaner, not richer.  The air screw lets more air in the further out it is.  

And a universal air filters and open exhaust make it even leaner

[Barry]

I believe that you have made your idle leaner, not richer.The air screw lets more air in the further out it is.  

This would be true if in fact it was an air screw. I have to admit I just assumed that we are talking about a stock Bing CV carb fitted to all R65's. That would make it a mixture screw not an air screw so out is rich and in is lean.

If an older slide type Bing has been substituted then it really is an air screw and works the other way around.

[montmil]

We've danced through the idle mixture screw issue on the CV Bings before. As for this particular post, Darryl is correct. He likely recalls the last discussion, too...

I checked the Bing Agency's Tuning Manual and, although you failed to mention the model year R65 you're cafe'ing, the base setting for the idle air mixture screw for the 32mm VC carb is one-half turn out from a gently closed position.

Closing down (tightening) the screw reduces the volume of air and richens the mixture. Your screw position that far out would be seriously over lean. Remember, this fuel circuit handles only idle speed, nothing more. The throttle stop screws should be just off the carb throttle lever at idle.

The Bing manual, at $11.00 bucks or so, is a valuable tuning reference for all model year BMW, Bing carbs, or otherwise.

The aftermarket air filters are allowing the engine to draw more air than the OEM setup thereby leaning the mixture. Open exhaust also effect the carb tune. Jetting will be a challenge, ie: seek and find. Avoid seek & destroy.

The idle circuit settings should be fairly easy to resolve.

Monte

[Barry]

Monte

3/4 of a turn out for the mixture screw was an educated guess on my part as my R45 is 1/2 turn out and I thought the larger engine would need a little more especially with less back pressure from the open exhaust although I doubt it has a very big effect at idle.

I'm going to stick to my guns and make a potential pratt of myself here:-

If the Bing manual says screw in the mixture screw to richen and out to weaken then it's wrong or Bing CV carbs don't work the way I thought they did. The old slide carbs have an air screw and CV carbs have a mixture screw.  They work in opposite directions more air weakens and more mixture richens and just to be absolutely precise in the case of a Bing CV "mixture" means exactly that. It's not fuel that the mixture screw meters but a pre-mix of air and fuel.  

Do I get a free copy of the manual from Bing if I'm right ?  

[montmil]

Barry, permit me to call your attention to page 2 of the Bing manual. The designated initial tune position for the Idle Speed Mixture screw on the 32mm is 0.5 (one-half) turn out from a closed position. The largest setting listed is only 1.0 turn out for the R90/6 running 32mm carbs.

Page 4 of the Bing manual has a lengthy paragraph explaining the idle speed mixture and how it functions in an idling CV carb with insufficient air flow through the venturi. Also included is a very nice cutaway rendering showing the Air Screw function in relation to the Idle Jet. As the Idle Jet is a fixed size -adjustable only by changing jets- reducing the air volume allows a proportionately higher volume of fuel to mix with the lower volume of air... doing so richens the idle mixture.

On occasion, tuning these neat Bing CV carbs can be counterintuitive. Often frustrating but seldom fatal. The Bing manual is definitely worth the eleven bucks you'll be sending to Kansas, Toto. [smiley=beer.gif]

Monte

[Barry]

Na Monte, the Bing manual is not worth the $11 if it's so misleading.  

The Bing CV carb mixture screws turn inwards for leaner. It just can't be any other way when you look at how they function.

Here's how I'm saying it works:-

Fuel flows through the pilot jet and is pre-mixed with a little air from a cross drilling at the air inlet.  This pre-mixture enters the venturi through a small port controlled by the idle mixture screw and mixes with a little more more air passing under the butterfly.

So turning the mixture screw clockwise inwards reduces the volume of pre-mixture entering the venturi and weakens the resulting idle mixture that enters the cylinder.

There is also another transfer port not controlled by the mixture screw but it doesn't  alter the way the mixture screw works.

If the Bing manual talks about air screws then I can only think it's referring to the slide type carb which does have an air screw and turning an air screw in richens the mixture.

Someone else chime in here maybe after consulting the Guru's.

[Dizerens5]

Can't this be settled just by trial and error? That is, try a quarter-turn, one way or the other, on the air/mixture screws and see what happens: rpm up or down? If up, then you have made the mixture weaker. If down, it's the other way. Or am I wrong there?

[Barry]

The problem is that both a too lean mixture and a too rich mixture can slow the RPM down so the test would not be definitive.

I think this is the reason why the mixture screw function can be misunderstood. We are all just aiming for the fastest idle so it doesn't matter which way is rich and which way is lean... but IN is lean  

...and here are some quote's from a couple of guy's who know better than any of us.

Snowbum
“ The Bing CV carbs mixture screws turn INwards for leaner, and the slide carburetors turn OUTwards for leaner.”


Duane Asherman
“Set it for maximum rpm and then go in (richer on the /2) about another 1/8th to 1/4 turn.  That last 1/4 turn makes it slightly richer, but any rpm decrease should be nonexistent or small.  On the /5 and later, out is richer.”

Duane is talking about /2 carbs which are slide type and /5 or later which are CV carbs.

[Dizerens5]

This is getting more and more confusing. Duane is certainly wrong about the carbs as my old /5 which was an R60/5 had slide Bings from new when I bought it.

[Dizerens5]

PS I have just looked at the cutaway drawing in the Haynes manual. It does indeed show that the "mixture regulating screw" controls the arrival in the inlet tract of whatever comes out of the pilot jet, not of air alone - but there is also an additional inlet from the venturi side of the butterfly which in idle position may or may not provide air only, I don't know - it would depend on whether any fuel enters the venturi with the throttle in closed position (does anyone know?). Anyway it does seem clear that the mixture screw works as Barry explains. But probably others on the site are better at reading engineering drawings than I am!

[Barry]

OK Sorry for the confusion. I think Duane was thinking of the larger engined /5  models which had CVs while  the R60 retained slide type carbs.  Your Haynes will confirm this. He was still making the distinction between the way slide type carbs having an air screw and CV carbs having a mixture screw worked.

but there is also an additional inlet from the venturi side of the butterfly which in idle position may or may not provide air only,

Yes that drawing is excellent and was one of the things I looked at to confirm my thoughts. The pilot air passage which provides the air for the pre-mixture is shown as a dotted line.
By "there is also an additional inlet from the venturi side of the butterfly which in idle position may or may not provide air only" do you mean the area labelled 5 ?
This is the other transfer port I mentioned earlier and I think would also provide a pre-mixture of fuel and air although its function during transition from idle may be more complex. I expect the Bing Manual might clarify this !!