Tuning carb adjusting and timing r50
Posted: Sat 5. Jun 2004, 20:19
What is the order of adjustment? I have a rebuilt motor and it needs to be adjusted. What first, the valves? the idle? the timing?
OK, everyone is different but this is how I do things with rebuilt motorbikes, whether /2 BMWs or old British iron. This horrifies some of my purist friends who treat BMW instruction manuals like holy books but I ride hard, fast and for quite long distances and haven't blown a BMW motor up yet so I must be doing something right.
You should begin by checking the valve clearances as the motor should be cold for this. The book gives the clearances as 0,15mm for the inlet and 0,20mm for the exhaust with the engine at Top Dead Centre (TDC). However, I have better results on my R50 and R60/2 with 0,20mm for the inlet and 0,25mm for the exhaust.
Get TDC by removing the rubber plug from the aperture in the fly wheel housing near the oil filler hole and turning the motor over until the 'OT" mark lines up with the mark in the aperture. To turn the engine, you can engage 4th gear and bump it around with the rear wheel with the plugs out but it is better to remove the magneto/dynamo cover and turn the crank with a wrench.
To check the ignition timing: just before the OT mark on the flywheel, there is a line and the letter 'S'. Line up the 'S' line on the flywheel with the line in the aperture. The pistons are now at 9° Before Top Dead Centre (BTDC). The contact breaker points should just be opening. You can follow the instructions in the manual involving timing lights, voltmeters or stroboscopes if you like.
However, the R50 is not a sports or racing motor and the old-fashioned way, involving a cigarette paper between the points, also achieves a good result for far less effort. With the motor coming up to and passing the 'S' mark, the points should just release their grip on a strip of cigarette paper. Check this on both cylinders by turning the crank 360°. Sometimes one cylinder is more advanced or retarded then the other, which can be due to various causes, from wear to the points cam to the magneto rotor being unevenly seated on its taper on the camshaft.
But let's assume that your rebuilt motor is as it should be! Turning the engine back, you will see a second line with the letter 'F'. This indicates the position of the crank when the contact breaker points should just be opening with the ignition fully advanced.
As the manual says, it is best to check the timing at full advance with a stroboscope but if you do not have one, you can also check it statically using the timing light, voltmeter or humble cigarette paper. Place the paper between the points, then open the spring-loaded bobweights of the automatic timing device fully. The points should just release their grip on the paper with the crank set at 'F' and the bobweights snapped open by hand. Alternatively, jam the bobweights in the open position and bring the motor up to 'F'. As it passes 'F', the points should release the cigarette paper. Check this on the other cylinder as you with when checking the timing at retard.
It must be stressed that the fully advanced timing position is more important than the retarded timing position. If the timing is too advanced or too retarded with the crank at 'R' and the ATD bobweights fully open, this must be rectified.
It is also worth checking the timing on each cylinder because the cam that opens the points often wears unevenly and while one cylinder can be timed accurately, the other can differ by several degrees. Too much advance on one cylinder and you may end up with a holed piston and a 250cc single for the ride home.
If this is the case, there are various remedies. There are also various potential causes. But see how you get on with this advice. Do not worry if there is a difference of a few degrees between the cylinders. If, for example, the lefthand cylinder fires at 39° BTDC - points just opening with the crank at 'F' - and the righthand cylinder is 5° slower - with 'R' at or just past the bottom on the crankcase aperture - compromise by aiming for the mid-point. 37° BTDC won't harm your motor. In fact, it will probably run better on the poor quality gasoline sold in Europe these days. If it is more advanced, adjust it to the correct point.
Once you are satisfied with the valve clearances and the timing - not forgetting the contact breaker points gap, which you must check on both sides of the cam before checking the timing, and spark plug electrode gaps - you can start the motor, warm it up for a minute or so and adjust the idle. Set a fast idle of about 800 rpm while running in the motor so that the oil pump can push the lubricant around. Make sure the carb slides are lifting off equally. Don't worry too much about full throttle synchronisation as you won't be using full throttle for at least a couple of thousand careful kilometres! Just get the motor running smoothly.
Start by setting a fast idle then remove one plug cap. Make sure beforehand that the little pointed tabs about 1 cm below each plug lead terminal on the magneto coil are set EXACTLY to 11mm from the terminals. With the mixture screw about 2.5 turns out, bring down the throttle stop screw until the engine runs on one cylinder at about 3 beats per second. Adjust the mixture until you have a steady beat with a clean pick-up as you pull the cable to raise the slide. Repeat the exercise on the other cylinder. Replace both plug caps. The motor will idle too fast. Gently turn down the idle screws until you get a nice steady tickover around 800 rpm. You will be able to reduce this to 500 or 600 rpm - the 500cc motor likes to tickover faster than the 600 - when the motor is run in. Adjust the cables with about 1.5mm of free play and check that both slides lift off at the same time as you twist the throttle. Reconnect the air filter tubes and take her out on the road.
Good luck!
Prosper Keating
Paris