Vintage BMW Motorcycle Owners

Hey Gang,
I bought a non running conversion that has the R75/5 motor.
After doing the rebuild on the carbs, when I barely give it throttle, the left carb maxes out asap.
Both carbs have even idle..
Mix screw on left is 1.25 out, right is 1.5
Both cables have 1/4" slack
As mentioned, when I just start to barely give it throttle, the left carb maxes out either carb sync tool.
I'm talking going from 1000 rpm to 1100 rpm.

Any ideas why this would be happening?
Everything else on the bike has been adjusted right ( valves, timing etc etc.. )
Carbs are spotless with new insides..

Thanks for ideas..

Oh yeah...
When I say max out, what I mean is that the flow dials on the carb sync too max out and the carb seems to bog, not the actual throttle response...

Regular CV carbs? What else did you do to the carbs during the rebuild? The cable slack of 1/4" is way too much...should only be around a couple of mm. With that much slack, you will have to turn the throttle that much more to take up the slack before the carb responds.

Kurt is right, too much slack, you only need enough to assure the carbs go to idle and stay there when the handle bars are turned and such. One or two mm.

Re check your work particularly in the malfunctioning carb. what is different between it and the other side?

Did you install new diaphragms? If not, check for a hole or tear. Is the top of the carb properly sealed, check for a vacuum leak. If you did install a new diaphragm, check to be sure it is in the correct position. They are marked and have a notch system to line them up in the carb.

Not for nothing, is your gauge system working properly? a leak in a hose or bad fit on the vacuum port will cause problems. Test your gauges by reversing them or on something else.

Good luck. St.

Thanks guys...
The carbs are the 64/32/09 and 64/32/10
On rebuild new diaphragms, o rings, jets, floats, needles, etc etc.
I've been through the carbs twice now double checking stuff.. floats are both at perfect level.

I've also messed with cable slack from the 1-4mm
Switched sync tools back and forth..
Installed the Bing manifolds by the butterfly valves.
Sprayed carb cleaner around both valves, no leaks

When using the coned shaped sync tool, the left carb reads like it has maxed out vacuum with slightly touching throttle.
Using the Carbtune Pro, rather than rising the indicator, it dips and stays.

I'll go through everything again this weekend.. thanks again for suggestions!

Sometimes the sync tools cause more problems than they are worth, LOL, My old BMW mechanic/dealer scoffed at them and did everything by ear and feel. Over the years I have tried different vacuum sync things and to be honest, found myself chasing after gold at the end of the rainbow. Gauges would read weird things yet, the bike would run great?

So, if the bike appears and feels like it is running good, the carbs appear to pull off at the same time, the engine doesn't stumble, ditch the sync tool and ride it.

One of the nice things about your bike is unless you have an after market ignition system using a electronic box, you can pull the spark plug off of one side and run the bike, working on getting the smoothest operation possible. This shouldn't be done if you have electronic ignition instead of points.

I can't think of anything you have missed, you have valves, points, timing, and last carbs all accounted for. Ten to one it will be something very simple solves the problem. Cheers, St.

Thanks again guys...
Took the carbs off once again and did another deep clean/check.
All looks great.
Fired it up again and same issue.. left carb just seems like it draws way too much air compared to the right. That's according to both of my sync tools.
It's like this even after a long warm up.

Not sure if this video will post or not...

https://youtu.be/PXXNbTVm4e8

Second video..
https://youtu.be/v5lOhfWbSLI

Note:
Idle is a bit low.. but that's just temporary for this.. I have it set for 950-1000rpm normally.
Basically you can see how the right carb responds great with the throttle gently opening up.
The left carb just peaks no matter how soft I try to open throttle.
Both cables have same amount of slack.
Now with that..
The bike feels like it runs good at half throttle and higher while riding down road.. but the sync doesn't feel together.
It's only after the idle does it start to pop and seems to be running rich.
I've tried adjusting the air/fuel screw to the recommended 3/4 out on both sides, but they both seems to be wanting to be at 1.25-1.50 out.. seems like that's rich.

I do see where the Bing manual suggests 3/4 turn for these type of carbs. But in the end, the mixture needs to be what the carb wants. Are the jets, etc., all stock inside the carbs?

As a starting point, how many turns of the idle speed screw does each carb have? This screw lifts the slide up from the bottom so that at least a little air is going under the slide.

Have you had a chance to observe the movement of the slides when the vacuum gages are removed? It might be interesting to see if the left slide does something different just as you pull on the throttle. To better see how they move, you could take a some chopsticks and gently slide them so they are held in place by the slide. Then when you turn the throttle you can see if the chopsticks are moving in synch with each other and by how much. Maybe this visual would show some unusual movements of the slides.

Well, I am running out of ideas as to a cause and solution.

One thing I had happen to me a few years back was the top of my carb started leaking, This prevented the proper vacuum. The leak was around the insert in the top of the carb. A rare place for a leak. I am surprised if yours is leaking it doesn't show up in a leak test?

I believe you are on the right track looking at the carb issue, but, I will throw in another thought.

What condition is the engine itself in? Do you have good compression, and good valves? I don't want to insult you as the bike looks pretty darn nice, and you know what you are doing but, as I said, I am running out of ideas.

Again, I hate to say it but is the ignition system 100%? Are you sure, the engine is getting proper spark when you rev it up?

You have the ability with a point system to run one side at a time by just pulling the spark plug on one side. My old mechanic used to tune all the point equipped bikes that way. Get the bike running well on one cylinder then adjust or sync pull off with both running so the carbs worked together.

Sorry I can't help more, piddly stuff like this drive us crazy sometimes especially when we are so close to getting a bike to the point of riding it. Keep the faith, a solution will come. Good luck. St.