Best mixture setting way off from spec

[Luca]

So I have to admit I haven't checked that the jets are stock sized, but this is a NE bike so I don't think it's seen much elevation.  I recently gave my carbs a quick cleaning so I thought I'd reset them and tune em up.  I decided to check the factory manual for base carb settings and found some discrepancies.

My 82 LS idles best with the mixture screws 1-1/4 turns out, stumbling at 1-1/2 and 1 turn out.  That's way off from the factory's 1/2 turn out base setting, and if I do set it to 1/2 a turn and crank the idle screws up it will hold idle speed, but sounds and feels terrible.

Anybody else find they're setting their mixture screws significantly fatter than factory spec?

[Red_Hen]

Luca - I think you've got it right on the mixture screws - check this out:

http://www.bmwr65.org/htdocs/yabbfiles/Attachments/Tuning_40mm_BING_CV_carburetors_for_BMW_motorcycles_by_ear.pdf

[Barry]

Anybody else find they're setting their mixture screws significantly fatter than factory spec?  

Mine are bang on 1/2 turn out or perhaps a touch less. That's with 45 idle jets.  Some US bikes have a 40 idle jets which would need more than 1/2 turn out. Perhaps you have 40's

[Luca]

Thanks for the info.  I think I'll have a look inside the carbs when it's convenient.  The owner's manual indicates a 40 jet as you suspect.  The factory manual has some variations for a number of things and a change in 1983 including the modified intake system...

...which will bring me to another thread about chasing a flat spot at around 4.5-5K RPM and the post 10/82 airbox cover with different snorkels.

[Justin B.]

The base setting "suggestion" in the book is assuming a carb has been totally torn down/reassembled, and the initial setting it just to get the engine cranked up.  As long as it stumbles each side of the "sweet spot" you should be fine.

[flybot]

Have you checked your valve adjustment? They affect the carbs.

[Luca]

The base setting "suggestion" in the book is assuming a carb has been totally torn down/reassembled

As Barry suggested, the settings I got from the manual are for 45 idle jets.  I haven't confirmed, but I think I have 40's.  Even at 1 turn out the bike doesn't idle nicely and the exhaust pops excessively when dieseling.  I fully rebuilt the carbs about 2 years ago.  Recently I just cleaned them out.  There wasn't much in there except for some black goo packed around the main jet, and cleaning had no noticeable impact on performance

Have you checked your valve adjustment?

Yep, valves were set before doing carb adjustments.  I also verified clean ignition electrical contacts, put fresh thermal compound on the ICU, and had a look at the plugs.

The bike has always had a flat spot at around 60mph in top gear.  This spring it's seemed a bit more pronounced, but that's completely subjective.  I've always had Snowbum's intake tuning article in the back of my mind, and I've noticed that BMW changed the R65 airbox snorkels in 10/82 and later used that design for other bikes.  The information isn't very clear, though...

My suspicion is that I've got a problem with the intake tuning.  Bike runs great below 4500 and above 5000.  It's just at 60mph in top gear that it briefly becomes a dog.

[Barry]

.. suspicion is that I've got a problem with the intake tuning.Bike runs great below 4500 and above 5000.It's just at 60mph in top gear that it briefly becomes a dog.

Snowbum may have covered this; there was a service bulletin issued about mods to the air intake. Bear in mind this is a European service bulletin.  There may also have been one for the US although I don't have it. I do have a US bulletin for the R80 which quotes the same part number for the replacement intake snorkel so I guess the mod is also applicable to a US R65..

[AlfromNH]

The Bing manual states a starting point of 1/2 turn out, but my '79 Riders manual says 1 turn out.

I'm still struggling with a flat spot at 1/4 - 1/3 throttle. I am in the process of cleaning/inspecting the idle ports for the 4th time.  

[Barry]

....but my '79 Riders manual says 1 turn out.  

So it does; I'd never noticed that before. I think the Bing manual is correct but again it will depend on idle jet size. I've being using info from this table which lists turns out for all of the Bing model numbers. Notice only 40mm carbs are 1 1/4 turns out and most every thing else is less than 1 turn.  

As well as idle jet size there are other variables that will affect mixture screw settings. Valve clearance have been mentioned; tighter clearances will need slightly more turns out and wider clearances slightly less. Idle mixture is quite sensitive to float height with lower fuel level in the bowls dropping idle speed particularly if the mixture is correctly set i.e. not too rich.  

Up to a point this can be used as a test of the idle mixture. Turn of the fuel tap a mile from home or whatever distance allows you to just about get home with the engine feeling reasonably OK and without stumbling. When you come to stop the idle speed should be lower than normal because the fuel level has dropped in the bowls. This is an indication that idle is sensitive to fuel level.  If the idle speed doesn't drop then the mixture was probably set too rich.  


1/2 turn
64/28/201, 64/28/202, 64/28/301, 64/28/302, 64/28/303, 64/28/304, 64/32/203, 64/32/204, 64/32/307, 64/32/308, 64/32/347, 64/32/348, 64/32/201 (US), 64/32/202 (US), 64/32/223, 64/32/224, 94/40/107, 94/40/108

3/4 turn
64/32/305 (UK), 64/32/306 (UK), 64/32/321, 64/32/322, 64/32/325, 64/32/326, 64/32/335, 64/32/336, 64/32/349, 64/32/350, 64/32/351, 64/32/352, 64/32/353, 64/32/354, 64/32/357, 64/32/358

1/2 - 1 turn
64/32/9, 64/32/10, 64/32/11, 64/32/12, 64/32/13, 64/32/14, 64/32/201 (UK), 64/32/202 (UK)

1 turn
64/32/19, 64/32/20, 94/40/109, 94/40/110, 94/40/113, 94/40/114

1 - 1 1/4 turn
94/40/103, 94/40/104, 94/40/105, 94/40/106, 94/40/111, 94/40/112

[montmil]

... I'm still struggling with a flat spot at 1/4 - 1/3 throttle. I am in the process of cleaning/inspecting the idle ports for the 4th time.

With a throttle opened one-fourth to one-third of full open, the needle, needle jet and slide cutaway are controlling fueling. The idle jet circuit has already completed its job and has passed it off upstream.

Perhaps adjust your search into the mid-throttle circuitry. It's not unusual for a bit of clogging to occur around the needle jet and its holder. Confirm both needles are extended the same distance beyond the slide.

I cured a similar flat spot by changing needle positions; first as an experiment, then as a final position.

You may find this 1978 Butler & Smith tech article to be of help:

http://www.bmwr65.org/htdocs/yabbfiles/Attachments/Understanding_theBMW_Bing_CV.pdf

[AlfromNH]

With a throttle opened one-fourth to one-third of full open, the needle, needle jet and slide cutaway are controlling fueling.

Hmmm... right you are. Nothing like barking up the wrong tree.

The needles and jets are new, but I'll confirm that they're extended beyond the slide the same amount.