Overheating cylinder

Is this the old style horizontally opposed air cooled twin with a carb' for each pot?

If so, check the left plug on the left cylinder...is it running weak? If so adjust the mixture for that pot to run it richer.

Appolgies if this is toooooo way out of date advice wise

Del

Yes it is old style boxer, will follow your advise and report the results soon.

Thanks for the memory..."Getting the carbs well-synched is a little tricky..." is an understatement. I've never owned a BMW, but synchronizing any engine with multiple carburetors requires lots of patience. It makes one a believer in single carburetors and manifolds.

Also, retarded ignition is a classic cause of engine overheating.

....and over advanced!!! Usually over advanced is the biggest over heater!!!

Ohmigod!- this is i of those things can be many answers to- mech cond, settings, tuning,means of measurement, location of measurement, reliability of measuring equipment- I owned & worked on over 60 m/c/s- in no case did i worry about or even attempt cyl temp measurement @ idling 1 min- what is needed is real world tuning- spark plug reading after full throttle acceleration- light brown- to set /balance carbs I used mercury columns(home made)- the advice Ken gives is valid, as one would expect.

I had the same on an old R65 ex Police bike. It took a while but I eventually found that it was a minute air leak at the carb/head gasket.

The thing that foxed me was that it would start and idle fine, and trickle along on the road. Any sort of acceleration would seize the piston in the bore. Extra heat load!!

Cooling off would allow it to, again, start, idle and run on light throttle.

Hope this helps.

Let's know.

Cheers,

Stu

I agree any of the posted items can cause hot running. Ken mentioned timing. I would like to add a timing issue (ignition or valve) on one of the cylinders can definitely cause overheating. Are you confident the non-stock electronic ignition is set up right? Was it done with BMW components or aftermarket? Check the timing at various RPMs to see the timing advance curve is what it should be.

If the engine was not exhibiting this problem before the last round of work (did you just rebuild the carburetors?) I would re-check that work. A lean fuel ratio can cause elevated running temperatures and this bike obviously has separate carbs. If this bike has CV carbs like the R75/5 I used to ride they can be quite tricky.

A lean fuel ratio can cause elevated running temperatures and this bike obviously has separate carbs.

As Dave said a lean fuel mixture could be the problem. Check for air leaks on the carb first. Good luck.

Previous owner had it stored in a shed for a year or more. So I do not know how it ran. Other than electronic ignition very thing is stock.Ignition system was checked by an auto technician, he says it is working fine but is reluctant to touch the bike.

All the checks that are suggested may find the problem. Obviously some of these checks require a good bit of understanding of IC engines. You might start with the simplest ones and go till you find the problem or exhaust your background and access to specialty tools.

As suggested I would first check for vacuum leaks. A "back yard" way to do this is: Get a 2-3 ft. long piece of hose about 3/4 in. diameter, like garden hose. Put one end to your ear and move the other end all over the carbs/intake/head with the engine running. You would be surprised how small of a vacuum leak you can find this way. Be careful where you put the hose, a large leak could hurt your ear. There are obviously other perils so be careful.

Next do all the simple checks you know how to do. With all simple inspections/checks complete, I would check the ignition timing. Did the auto tech just check for spark or did he use something like a timing light? If the overheating takes place at idle, I would set the idle speed per the book and check the ignition timing at that engine speed to see if it matches the manufacturer's specs. Check both cylinders separately.

BTW, if you have not already done so, be sure to clean the tank and lines and replace the fuel with fresh fuel. (The old fuel, unless it is really smells odd compared to fresh gas or has a lot of water or crud in it, can be poured into your car, provided you have enough fuel in the tank to dilute the old stuff: for example adding 2 gallons of old gas to half a tank of fresh gas won't hurt your car.)

Another BTW: just before setting the valve clearances, you should retorque the nuts on the studs that retain the rocker shafts. (You back off on the nut about a 1/2 turn, and then retighten to the factory setting.) You'll want to get a manual so you have the various bolt torques, clearances, etc.

Gas tank was cleaned twice, new lines and filters were installed. Fresh fuel is passing at about 26 oz per minute per side. Auto technician checked spark but does not know how to check timing of this bike and I am worse than him. I have Haynes BMW Twins and Clymers BMW manuals both of these are not much help, in trouble shooting.But I am still at it. I am sure with your help I will able to solve this problem.

Find out if the bike has on the flywheel or in the front alternator area a top dead center mark. I think I remember this engine correctly as having a "dry" flywheel on the output side and the alternator out front. If there is no TDC mark find TDC with a dial indicator thru the spark plug hole. If you don't have a dial indicator you can get close by sticking something thru the spark plug hole to "feel" the piston and very carefully turning the motor back and forth by hand. Now make your TDC mark on the flywheel and use a protractor to establish other points like -5 deg, -10 deg etc. A degree wheel on the front where the alternator is may be easier. Do this for both cylinders.

Now hook up a timing light and check timing at various RPMs on both cylinders. The two points you are most concerned with is at idle and at full advance (at several thousand RPM). You need the timing specs for this engine for at least these two points.

This method is somewhat "from scratch". If you had a BMW service manual it would be more specific although it may be a little iffy with the non stock ignition. If my crude outline doesn't make sense read up on how to use a timing light or better yet put a timing light on a car with the help of a mechanic friend so you fully understand what is being accomplished by putting it on a vehicle that is set up to be timed with a timing light. Old cars with points (and maybe some newer stuff) have their front harmonic balancer marked for TDC and other BTC angles along with a stationary reference tab. After you time a car set up for it, the process will make a lot of sense.

By the way, in the 1970's, it was a rare BMW that came from the factory set to run right. Different temps and sounds from side to side are par for the course.

This site has some good info. Worth noting is the fact that the /2 carbs had air screws for idle mixture control, while /5 and later had fuel screws. So on your's, you turn the screw out to richen the mixture.

I note that the writer in the linked site said not to remove spark plug leads on /5 and later. I went to the distributor's BMW school for /6 and we pulled plug wires, so I think it is probably ok -- although you may want to ask around. An issue is that when you pull a plug wire the voltage can get very high, and some people worry about arc damage to insulation, etc. If you have the literature (or can find it) for the ignition system they might have something to say. Seems I remember some people having spark plug terminal extenders, (used only during tune ups) which provide an uninsulated area that can be shorted out to ground with a plastic handled screw driver. Wear gloves -- the spark won't kill you but it can certainly make you jump.

In any case don't run it on one cylinder for too long, because the cylinder can eventually flood, and you can also accumulate a lot of vapor in the mufflers, which can go off with a bang when you reconnect the plug.

You can also, as the linked site suggests, do the synch just by listening in stereo. Also, feeling the exhaust pulses with you hand about 6" back from the outlets helps.

Carb synch gets less important the further from idle you are: .020" difference in slide height at idle is a big deal, but at high rpm and high load, it gets washed out by all the other things that are different from cylinder-to-cylinder.

The most common cause of your difference in temperature is lack of synch, and at least 50% of brand new BMWs would have noticeable differences from cylinder to cylinder, because when the carbs are out of synch, one cylinder is more-or-less "along for the ride".

If this drives you crazy, you're not alone. I think somewhere in the linked article it talks about new BMW mechanics going through a period when they pull their hair out -- but eventually they say "close enough is close enough".

Ken,

he has transistor ignition, if it is as good as the ones I used to build 35 years ago for friends, he could get electrocuted!!!!!

I put a switch on the units to return to normal ignition for setting up and to keep the mechanic alive!!!!!!

I think the mechanic has good sense, asking him to tune a horizontally opposed twin is right outside of his area of competence and he knows it!

To the original poster:-

By the way, if you have a gas cylinder, reducer and a length of runner or plastic pipe, let some gas out at low pressure (outside) with the engine running and placing the jet of gas where possible air leaks are present, will show up as the engine picking up speed when the gas gets sucked in. The "Tappet Brothers" on US radio do this on a regular basis....

To check for vacuum leaks I have heard of using water which could slow the engine speed down if the water was sucked into the cylinder fast enough. And some use a hand pump oil can with petro. Just make sure you do that outside on a cool engine and have the proper fire extinguisher very close. Your choice of clothing maybe of a benifit also.