Vintage BMW Motorcycle Owners

I'm very adept at tuning carbs on R100 series but i'm really struggling for some reason on tuning my R50/2.
It'll tune and run, but i've never been happy/satisfied with it for the last 5k miles since the build.
I'm scratching my head and wondering if i'm applying late 80's tuning expectations to mid-60's designs here and need to adjust my expectations.

Baseline:
re-torqued heads
valve lash set
points are good - harms plate
timing differential resolved (love tap!) and verified with dynamic timing gun
no vacuum leaks
proper carbs full dip & soak rebuild with all new jets, needles, etc., meticulously ensured that jets, mains, needles, etc., are correct for the bike.

Went for an 18 mile ride and then put it on the center stand while running.
When i go after the idle screw and turn it inward to where the engine stumbles, i then turn it outward and both sides take 9.5-10 complete turns to where idle drops down again.
QUESTION 1 - Is this normal? Seems excessive.

I set it to halfway and tightened the lock nut and then tackled the idle setting screw. I like around 1000 rpm or when the gen light just turns off.
My carbs have the brass ports in the flange against the heads for hooking up a manometer. I have a digital one that is very nice.
However, I cannot - CANNOT - get the suction balanced between the 2 sides by adjusting the idle RPM screw (the big round one). Even if i turn the left cylinder way up and the right way down, it still shows the right pulling harder.
QUESTION 2 - is this normal and should i give up on a manometer and just do the shorting method?

So i went with the classic shorting method to move along and got things balanced at idle and then moved on to the throttle cables which is easy but without being able to empirically verify with the manometer, i'm really doing it by feel and smooth turn off idle.
It's subjective and I'm not satisfied with it.
Advice or thoughts welcome.

That many turns outward is not correct. Either you have a vacuum leak (non-flat carb mounting surfaces) or you have not turned the opposite carb idle screw out enough to take the other side out of the equation.

See the procedure here: https://vintagebmw.org/forum/viewtopic ... 448#p77448

sprayed all around the flange-head interface with carb cleaner. can't find any leaks. Guess i'll check again.

Duane Ausherman has a page on adjust /2 carbs. Look at the first picture...be sure that the center section is now working its way up and out.

https://w6rec.com/adjusting-the-2-bing-carbs/

Set your air screws 1 3/4 turns out to start. Buy some empi gauges. Easy peasy.

If you’re turning the air screws 9-10 rotations, something is not adding up.

Datchew wrote: ↑

Fri Jul 23, 2021 11:33 am

sprayed all around the flange-head interface with carb cleaner. can't find any leaks. Guess i'll check again.

I am not a fan of this method either. I take them off and with 600 grit wet or dry on a flat plate sand them a bit and see if they are flat. I will hazard a guess they are not unless you just did them. Over tightening those nuts bends the ears dramatically.

Checked flange flatness at last build 5k miles ago and sanded for visual verification. Good to go (then.)

I'm a fan of loctite 567pst for fuel resistant fastener torque retention vs overdoing it with the Kung Fu grip so I'm skeptical that they'd warp at all, much less warp so quickly, but I'll check them if it comes to it. Thx for the reminder. I overlooked that.

Made another run after setting idle mix at 1.5 turns per side. This time I pulled a plug wire and tried to adjust mixture via idle speed by ear on each isolated side. Didn't do this before as I was using a manometer. Perhaps the later CV carbs are sensitive enough to allow mixture tuning via idle drop w both sides running (and they are) and type 120's not so much. Again... managing expectations.

Anyway, I attempted to tune each side mixture to best rpms and then drop the idle adjust a little lower each time and iteratively repeat until no more progress was apparent, I.e, isolate the idle mix jet from the main.

When I realized I was picking the flyshit out of the pepper I tied it off and went after the throttle cable balancing. Ended up around 2 turns out each side.

Will ride again today from cold start and report back.

My guess is that your air screws are not actually seating fully when you turn them in until they 'stop'. Did you chase the threads in the carb body? Often there is build-up that prevents the screws from fully seating, hence the false 'turns out' prediction.

The base number of turns out for your model is pretty close if all's right. The engine should stumble with just a partial turn of the screw either in or out. I didn't think there were 9 or 10 turns even available without the screw falling out.

Dave

@Daves79x:
I scratched my head on this and finally had to perform the laying on of hands to satisfy my curiosity.

It turns out (see what i did there?) that the lock nut was "hung up" on the last outer threads of the air mix screw and when i was turning it in, it was staying in position and masking things with a false "bottoming out."
I held the nut in place with a 9mm wrench and with judicious screwdriver usage, found the screw had more to go.

Voila. I found the sweet spot where it chokes down the idle going in (closed), the high rpm spot and the rpm drop as you unscrew it.

Fixed! thanks much.
M

GREAT TO HEAR YOU FIXED IT!!!

It is typical that those last threads are messed up because of the screwdriver slipping or people making things too tight.

Good to know and good for others information on the forum.