I was originally taught to tune the bike by setting the
tappets exactly and then balancing the carburettors by listening to the
tappets: the lazy cylinder is noisier. I was never able to get the hang of
this, but did find it a good indication of the state of tune of the bike (if
I'm riding the bike and one cylinder is noisier I know that it's time to do
some work). Back when fuel was leaded I used to aim for a slightly coppery
colour at the back of the mufflers, but this was due to the lead burning out of
the fuel, and doesn't happen in these days with unleaded.
I use shorting adaptors to balance the carbs. For years I
used car-type spark plug leads: they have exposed metal around the top of the
spark plug that allowed them to be shorted to the cooling fins of the cylinder
head. However swapping them in each time I needed to balance the carbs was
tedious and there is some discussion that the silicone suppressed cable will
damage the electronic ignition of the bike (better to stick to the standard
5kohm caps). I now use adaptors that I built myself (see separate blog page).
I'm told that older bikes (R75/5 vintage) could be tuned
using this method by removing the spark plug cap rather than shorting the
(attached) leads. However, this can damage the ignition on the more modern
bikes, so leads should remain attached to the spark plugs throughout.
I've tried to use vacuum gauges, but didn't get great
results. It seems that the choice between the vacuum gauge method and shorting
method is a religious difference between different riders: they use one method
or another and are convinced of its superiority with religious fervour.
I currently use the following procedure to tune the
bike. Snowbum’s page, http://bmwmotorcycletech.info/synchcarbs.htm,
is much more informative and authoritative.
- Set the tappets. Usually I'm lazy and don't re-tension the heads, so there's only one or two tappets to alter.
- Check the timing using a strobe light. I don’t often need to alter the Hall effects generator on the camshaft.
- Warm the bike up. I find that anything less than 3km and whatever tuning I set changes when the bike is properly hot. Other people recommend 10km.
- Install the shorting adaptors. These need to be tightened with pliers so they don’t get loose during the process.
- To do the earthing I use a big screwdriver with insulating tape wrapped around the shaft and rest it on the exposed metal at the top of the spark plug with the blade wedged in the fins. Because of the electronic ignition there is very high voltage, and if I'm not careful I get the tingling feeling of it earthing through my hand.
- Balance the cylinders at idle by shorting first one cylinder and then the other. I’m looking for about 1000rpm with both cylinders working, and about 700rpm each. I adjust the throttle stops so that both cylinders are even and together they’re 1,000rpm. I’m relying primarily on the tacho for this.
- I adjust the idle mixture by screwing the mixture screw in to the point where the engine drops revs or falters, and then back it off 1/8 turn. This is with both cylinders running, nothing shorted. Repeat for the other cylinder.
- Go back to the previous step and confirm 1,000 together, same revs with one cylinder shorted. If any adjustment is necessary re-check the mixture, then recheck the balance and so on.
- Once I’m happy that both cylinders have their idle set correctly and are balanced I adjust the throttle cables for off-idle operation. I recheck the slack (both cables should have some). Then I hold the bike just above idle and short out the cylinders alternately: they should each drop by the same amount. I adjust the cable housings to get them even.
- I check this last step by picking the revs up a bit, somewhere in the range 2,000–4,000 rpm, and ensuring that the cylinders are still balanced.
- Throughout the process I occasionally blip the throttle a few times to ensure that the cables are seated in the sheathes.
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