R69S carburetor leak and heating on one side

I'm wondering if I can get some advise on a recently purchased a R69S. It appears that my carburetor float needles are not sealing properly as fuel leaks from the idling mixture adjustment screws after the engine is shut off and for a while after I shut the fuel petcock. Is that a correct diagnosis?, and if so would that necessitate a carburetor rebuild? Would this be relatively easily done by a fairly handy person?

An additional problem I've noticed is that while idling the right cylinder head and exhaust pipe gets much hotter than the left side. I've checked the valve adjustments and will check the timing. Does any one have other suggestions (fuel mixture?)?

Thanks for any advise provided.

One thing you should begin to make a habit of is turn off the petcock about a block or two from where you will shut off the engine. This helps the fuel from being too full and expanding due to the heat.

Check and be sure you're floats are not cracked and taking on water? Are they still brass? You might want to check with Vech at Benchmark Works to get his redesigned plastic floats.

Be sure the float can move freely. The rod going through the float rides in a hole in the bottom of the well. If there's too much corrosion, the float can't come back up. People have take something toothpaste and used it to smooth the needle to ensure a good seal.

What year model R69S? The later lever tops (fuel goes in at the edge) were supposed to be better about sealing.

You should probably do a carb rebuild followed by a carb synch. They're pretty simple...just be careful with the brass jets screwed into the carb pot metal. Confirm the jet sizes and confirm that the slide needle is in the proper position.

Thanks Kurt!

The bike is a 1966 with the fuel line in the center.

Have you had a chance to check when your bike was built? If you go the area about in the Vintage forum, you can look through the lists that I've compiled when I could find the info. You can also email BMW archives with the VIN and other information you have and they can check their records for more details if they exist.

Check this post out...an email to Fred Jacobs will certainly find the right person.

reachfar said:

I'm wondering if I can get some advise on a recently purchased a R69S. It appears that my carburetor float needles are not sealing properly as fuel leaks from the idling mixture adjustment screws after the engine is shut off and for a while after I shut the fuel petcock.

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Normal. The heat from the cylinder causes the fuel in the float bowl to expand where it dribbles int the carb and out the idle air mixture screw. It will be worse on warm days. As Kurt mentioned, turn off the petcock a ways before you kill the engine to minimize the problem.

reachfar said:

An additional problem I've noticed is that while idling the right cylinder head and exhaust pipe gets much hotter than the left side. I've checked the valve adjustments and will check the timing. Does any one have other suggestions (fuel mixture?)?

Click to expand...

You've hit the big ones: Valves, fuel mixture, carb balance, and timing. When checking timing I like to use my multi-meter in beeper mode and check BOTH lobes of the advance. I expect the beeping to stop just as the S mark passes the window. Rotate the engine one more turn and verify the beeping stops at about the same place. Some offset is not uncommon. A lot of offset can lead to problems, especially if the bike was timed to the less advanced mark.

Make sure your points and condenser are good then

Check the static timing first, Vech at Bench Mark Works is a great source of info on setting the static timing and the Clymer book that I have is pretty good to.

Check the valve clearance also but I don't think that would cause just one side to overheat.

A very important step to take before you go crazy with worst case scenarios is make darn sure your throttle cables are properly routed and in sinc with each other. If they are a little off or one hangs up more than the other during use that can cause overheating on one cylinder due to an unbalanced condition. You also have to make sure they are the proper length for the type of handlebars, to long and they hang up and get out of sinc, you will know it if they are to short.

Once you are sure your cables are routed correctly you can get down to the business of tuning the carbs which would include adjusting the throttle cables, idle mixture and idle speed. Remember that the /2 is an antique and I don't think they had Twin Max tuners back then, it was done by killing one cylinder and setting the carb on the other and then switching sides until you got it running good. Be very careful and make sure you have a fan blowing cooling air over the cylinders as you do your adjustments to prevent overheating and more serious problems.

Once again Clymers has pretty good information on how to do it.

After you have all that stuff figured out and set up correctly if the bike still heats up on one side start looking a little deeper.


And a +2 on the shutting the gas off about a block (1,500 feet) from you destination to drain the carbs down a little.

Probable causes of your problems R69S

Running hot on one side, and the symptom of one header pipe being a lot bluer, and for a longer distance, is differential timing. In other words, one cyl is running ahead of, or behind in timing.


For starters, read this:


The magneto basics
The initial set up of the magneto is important if it is to produce a strong spark. It is important, if a strong spark is to be produced, that the rotating magnet is timed properly to the body of the magneto. Check to see that the scribed line on the rotating magnet lines up EXACTLY with the bottom of the "V" notch on the brass plate on the front of the mag body, when the "S" mark on the flywheel lines up with the stationary mark in the window on the crankcase. If it is off slightly, loosen the 6 mm nuts on both sides of the mag body and rotate the body left or right until perfect alignment is obtained. Then NEVER MOVE THE BODY EVER AGAIN! To change the timing, move the breaker plate the points are mounted on.

items and procedures to remember. First of all, disconnect the plug wire from the coil, and using an ohm meter, check the resistance of the wire and the plug cap. It should be ZERO resistance!
Don't run resistor plug caps.


Remember, the faster a magneto spins, the hotter the spark. At the speed you can kick the engine over, the spark is relatively weak, and to introduce resistance with resistor plug caps and resistor plugs is to invite starting problems.

The condenser:
Another problem could be the condenser. ALWAYS carry a spare with you, for if the condenser fails, the mag will not produce a spark. The condenser doesn't have to be a genuine BMW part. The condenser used on a V-8 Ford, (such as a 19'73 351ci) comes with it's own clamp, and has a lead with a spade terminal already on it. The Ford condenser will produce a strong blue spark, and last many years. And, another problem can be with a poor ground of the body of the magneto. A poor ground will cause the magneto to kill condensers on a regular basis. Remember probably no one ever took the body of the magneto apart, and CLEANED where all the various parts touch each other (like the back of the brass face plate, the 4 beveled washers directly behind, the two stacks of steel plates and the aluminum body) after 50 years, dissimilar metals can and do corrode, producing a high resistance ground. When you put it back together after a cleaning, use a hand impact driver to get the 4 screws that hold it all together ting.


Timing
I have seen folks use a timing light to check the timing while the engine is running, and obtain the correct timing by moving the magneto body as previously described. This is NOT the way to set the timing. If you remove the centrifugal advance, you will notice that the points are mounted on a separate breaker plate, which is held down by two screws. That is the plate which needs to be moved when adjusting the timing. Rotating the plate clockwise will advance the timing while counter clockwise retards the timing.

Keep in mind that the timing of an air cooled engine is critical to the operating temperature of that engine. One can easily time a pre-1970 BMW engine statically. By aligning the "S" mark on the fly wheel to the static mark in the window of the crankcase, and using a test light or an ohm meter on the points (with the condenser and wire from the magneto coil disconnected) to set the breaker plate in the proper position, after the point gap has been set.


Which brings us to another problem that is easily overlooked or not understood.
Differential timing.
To test for it, pull out the spark plugs, disconnect the wire from the coil to the points. Attach one lead of the ohm meter to the nut and bolt on the points, the other to ground. put the ohm meter next to the observation hole in the case, so you can watch the flywheel and watch the ohm meter at the same time. Get a regular old lead pencil. Now, turn the engine s-l-o-w-ly in the direction it runs. The mag fires when the points crack open. The ohm meter will tell you exactly when this occurs. It should occur at the line above the letter "S". But for this test, where ever the meter reacts at the line or somewhere else, put a pencil line on the flywheel at that spot. Then turn the engine one complete revolution, and see if it does it again at exactly where you put that pencil mark. If it does not, put a second pencil line. Now the difference between the first pencil mark and the second is how much differential timing the engine has. Service bulletins of the era allowed 3 degrees. Linearly, on the flywheel that is approximately 3/16 of an inch. So if you see MORE than that, there is your cause of one cylinder running hot. It could have one cylinder running 8 or 10 degrees or more ahead, or behind the other cylinder. The engine will run, but not correctly. It will not be smooth. Setting the idle air mix and syncing the carbs is impossible under this condition. What causes the condition? 2 possibilities.
1. the shaft the advance is mounted on is wobbling, moving the advance closer and then further away from the points, making it fire early, then late, then early. Remember, the BMW uses the wasted spark method. Only one cylinder is on the compression stroke and fires even though both plugs fire simultaneously. The next revolution of the crank, the other lobe of the advance causes the magneto to fire again, when the other cylinder is on the compression stroke. So, pull the advance, rig up a dial indicator and check to see if the shaft is turning true.
2. the advance is worn unevenly.

Geez, I'm typing a book! Feel free to call me 662 465 6444 and I'll explain it all including the carb problems if you like....




Craig Vechorik
"Bench Mark Works"
662 465 6444
vech@benchmarkworks.com
BMW MOA Ambassador # 9462

Thanks Craig! You're a wealth of information! I'll put my timing light away and get out my ohm meter.