T H E O L D G R E E N B E E M E R
RIDING A 1974 BMW R90/6 FLAT TWIN
Setting the carburettors is not a hard task, but first make absolutely sure the innards are
clean, all passes, all bores, all jets, best to let 'm have an ultrasonic bath. Especially
note the two tiny holes near the butterfly valve. Renew all seals and make sure
the floats aren't soaked. Check float level height, should be the same for
each carb ( Float closing fuel valve at the point of being in line with
carbs flange ) Dail in basic setting: Butterfly valve 0,5 mm open
( measure with a drill bit ) Mixture screw 1 and 1/4 revolutions
out ( counted from all the way in position ) start the engine.
This carburettor is modified to be adjusted by hand instead of by screwdriver.
Put on your Helmet and run the bike a few miles till it's warm. Adjusting the carburettors
while the engine is running choked is not possible so first ride it. Put it on the stand,
and use your ears. First make sure the accelerator cable adjusters are slack a
few mm. Then adjust the idle screws ( left arrow ) so the engine ticks over at 8
or 900 rpm. Next, turn the mixture screws ( right arrow ) to find the highest
tickover speed. Adjust the Idle screws to return the correct idle speed.
Repeat this process a few times until the engine runs smooth. I know this all seems
like hocus-pocus at first but eventually you will get the hang of it. As a final touch,
readjust the mixture screws as far as possible to Rich or Fat ( Out - Anti Clock
wise ) while maintaining a smooth idle. This to prevent the exhausts from ma
king banging sounds under decelerations, this points to a weak idle mixture.
Check your spark plugs after a ride, they schould be colored like coffee.
If darker or blackish mixture too rich, if whitish mixture too lean. The
last is bad as the engine than runs too hot. Correct immidiately.
Could also be: Ignition timing off or Spark Plug type too hot.
Continue to Listen up
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