R75/6 Part 6. BMW, Air Head Boxer, R Series service. Tune and Balance the Carburettors. Road Test.
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- Опубліковано 28 вер 2024
- BMW, Air Head Boxer, R Series service. 1975 R75/6.
This bike has been off the road for over a year.
THIS IS PART 6 of a series of six videos, they document the service work required to get the bike back on the road.
In this video, Part 6, I will show you how to set the Bing carburettors to their basic settings, then tune and balance both carburettors as the engine is running. Last of all it’s a road test.
The other videos in this series will show in detail the following tasks:
IN PART 1: • R75/6 Part 1. BMW, Air...
This video show clips of what’s covered in the other five videos, we will then prepare the bike for the service: Remove the engine crash bar and leg shields. I’ll show you the best way I have found to remove petrol tank. Then we will remove both carburettors.
IN PART 2: • R75/6 Part 2. BMW. Air...
Drain and replace the oil from the engine, gearbox, shaft drive & final drive housing. How to remove and refit the engine oil filter. How to remove the Air filter and petrol pipe assembly.
IN PART 3: • R75 6 Part 3. BMW Air ...
How to completely strip, clean, and assemble the Bing carburettors, I will show you every air and petrol way within the carb and how to make sure they are clear. I won’t be using an ultra-sonic cleaner in case you haven’t got one, they are not an essential tool, but they are rather good at cleaning carburettors!
IN PART 4: • R75/6 Part 4. BMW Air ...
How to change the points, set the points gap, then adjust and set the ignition timing.
IN PART 5: • R75/6 Part 5. BMW Buil...
How to cut and build a new petrol pipe assembly. How to fit a new air filter. How to assemble the carburettors to the engine, connect their cables, and adjust float height in the float chamber.
IN PART 6: See above.
The most patiently explained procedures, thanks Jim.
Hello Jamshed. I’m glad you found my video useful. Thank you for your comment. Take Care Jim
I used to remove ht while engine running. Now I realise the ht electricity must be shorting SOMEWHERE. Potential damage to leads or coil. Or even “carbon tracking” . Suggest fitting a metal rod from each cap to its plug and shorting it to engine earth with a screw driver. Lets ht go somewhere and stops plug firing, which is what you want.(and you don’t risk a shock too) I have this model Beemer. Like your knowledge and enthusiasm 👍
Hello Nigel. Thank you for your comment. You are absolutely correct, the power from the coil must go somewhere. The ignition coil is capable of producing many tens of thousands of volts, its only the spark plug gap and the conditions within the cylinder that keeps it down to the 10,000 - 20,000 volts you see as the engine is running. Pull off the suppressor cap and the voltage from the coil will increase until it can find another path to the bikes ground. On modern bikes if you pull off a suppressor cap from a spark plug as the engine is running, the large rise in ignition voltage can lead to the electronic ignition system being damaged. Your idea to short out the HT pulse at the spark plug is a good one, you know exactly where the pulse is going, and as you say the HT leads and the coil will not get stressed due to HT voltage spikes, and you remove the risk of causing carbon tracking where the high voltage breakdown occurred. But, and there is always a but, I bought my bike in 1974 that's 50 years ago now. The manual told me to pull the HT lead off the spark plug when you are setting up the carburettors, and that is what I have been doing for all of those 50 years. I have never had a problem with an ignition coils, but every ten years or so I change the HT wires and the suppressor caps. Yes shorting out the spark plugs has got to be the safest way to go, but just pulling off the suppressor cap on the 1974 models of 'R' series BMW boxers does not seem to cause a problem.
I think it's best to earth the removedplug cap, so no damage is done
Hello Nick. I think you're right. By just pulling off the spark plug cap as I do, the insulation of the HT lead, and the coil are under the stress of thousands of volts. By shorting the plug out, that HT voltage can't build up, so cant damage anything. The current produced by the ignition coil is very small, so I don't think that would damage the coil windings if the coil is shorted out. Thank you for your comment. Take Care Jim
Re needle position, how do you adjust the needle so the bike runs weaker at the minute my bike (R75/7) runs too rich, have adjusted both air mix screws from 1/8 - 1/2 turn out each side, bike will start without choke been in cold weather or been stood for several weeks, any suggestions please, Thanks Paul
Hello Paul
Thank you for your comment.
My R75/6 needs the choke to start almost all the time.
If the engine started without the choke being engaged, I would know there was something wrong.
It sounds like the carburettors on your bike are allowing too much fuel into the engine.
If your bike was working OK and now this problem has just arrived, then I would suspect the following things:
Float height in the carburettor has become too high. I show how to check and set this in Part 5 of my BMW service series. See: ua-cam.com/video/umaPjTaSNGU/v-deo.html .
This may be due to ware or bending of the tab on the float that operated the float needle valve.
I did hear a story of a float losing its buoyancy, that made the engine run rich.
Now, in the forty-nine years I have owned my R75/6 I have never had to adjust the carburettor needle height. Because of this I’m not 100% sure how to do it. I have just called a friend, it seems the needle can be in one of three, or four positions.
My bike has the Bing Carburettors you saw in the video. What I’m about to say is correct (so I have been told) for that type of Carb, but I’m not sure if your carb is exactly the same as mine.
The Carburettor needle is held in the slide of the carburettor. To get to the slide you need to remove the top of the carb and pull out the diaphragm and slide. You would have seen this in Part 3 of my BMW service series, URL: ua-cam.com/video/WhnsFUzRLJQ/v-deo.html
Now to adjust the height of the needle hold the needle firmly between your fingers and twist the slide. The needle should click, and its position will move. I’m sorry to say I don’t know which way to turn it. After a search on UA-cam, I did find the following UA-cam video:
ua-cam.com/video/vFnTVxm0FVw/v-deo.html
Its not very clear but should I hope give you a clue on how to adjust the needle height.
Please let me know how you get on. Take Care Jim
Great video but i have one question. Can you just remove the spark plug connector without damaging the ignition? Because in my BMW manual it is recommended to use a „Beru EP1“ to prevent damage.
I hope you can help me
Greetings from germany
Hello Maxiking3487. Thank you for your comment. The answer to your question is a little involved. I have to go out now so I will answer it tomorrow. Sorry for the delay in my reply. Take Care Jim.
Hello Maxiking3487. Thank you for your comment and question. Does pulling the sparkplug cap off the spark plug as the engine is running damage the ignition? The answer to this question is, "It depends". My bike was built in 1974 it still has the original standard 12volt coils and mechanical ignition points. My Haynes workshop manual tells me to pull the sparkplug caps off to check the tick over. I have been doing this for 40 years or so and never had a problem with the ignition. I have friends with similar age BMWs, and they pull their plug caps off with no bad effects. I’m quite happy to carry on doing this on my bike.
Now let’s get technical for a moment and look at how the ignition works. This is a simplified explanation. The points in your ignition close, current starts to flow into the primary winding of the ignition coil, that current produces a magnetic field which builds up through and around the secondary windings of the ignition coil. The points are then opened by the cam, at that point no more current can flow into the primary coil so the magnet field it had produced starts to collapse. As that collapse happens because of the way the coil works a large voltage is induced into the secondary windings of the coil. The high tension (HT) from the coil is wired to the top of the spark plug. The voltage quickly builds until it reaches the breakdown voltage of the sparkplug air gap, let’s say 10,000 volts. At that point a spark will be produced in the spark gap of the plug, current will flow through the spark into the ground of the engine and back to the case of the ignition coil. If you make the spark plug gap wider the voltage from the coil will carry on increasing as the magnetic field collapses, it will eventually reach a high enough voltage to jump the larger gap. The ignition coil is capable of producing voltage spikes well over the 10,000 volts required for the engine to run. Now when you pull the HT lead off the top of the spark plug with the engine running, very high voltages are produced by the coil. These voltages are way in excess of the normal 10,000 volts. They are so high in fact that they can break down the insulation on the wires within the ignition coil causing permanent damage. I don’t know why but the early BMW coils do not seem effected by these high voltage spikes. As I said before I have been using this procedure for 40 years with no problems.
Now, with the later coils and electronic triggering systems you really don’t want to run the engine with the HT cap off the spark plug, the high voltage spikes that can be produced will happily burn up the inside of the coils or fry the triggering system.
I hope this explanation helps. Take Care Jim
Hello Jim, wow thank you for your fast and very detailed explanation. I have a R75/5 from 1972 so i think i will try pulling the spark plugs off as the engine is running. Hopefully nothing is damaged.
Thank you so much you really helped me
Greetings Maximilian
Will this work on a 1982 R100?