ALBANY VINTAGE &
CLASSIC MOTORCYCLE CLUB
Tricks of the Trade: Balancing
Twin Slide Type Carbs
By Colin White
*
Balancing twin cable carbs has always
been a bit hit and miss and you are never quite sure that
they are exactly right. The modern twin normally has a
pair of CV type carbs on an accurate bar linkage. These
have provision for a vacuum gauge to make sure that the
balance is perfect. They also stay in tune for a very
long time and rarely need adjusting.
The older twins however are opened by a
cable which forks into two and normally doesn't have a
bung to allow the use of a vacuum balancing system. Many
mechanics have found varied ways of adjusting these units
and I will attempt to pass on what I think is a pretty
accurate and fast way getting them right.
STEP 1.
The
first thing to do is balance the carbs at idle. I usually
make sure both idle mixture screws are set at the same
number of turns for a smooth idle when hot. To make sure
the slides are balanced I usually hold a loose bundle of
rag over the end of each muffler alternately listening to
the strength and regularity of the engine beat of the
other. The idle speed stop screws can be carefully
adjusted until you are happy with an even twin idle. Do
this outside the workshop so you don't gas yourself. You
can now stop the engine and let it cool down.
STEP 2.
The smoothness of running will now depend on the two
slides lifting off idle at exactly the same time.
Before you start, make sure that there is free play at
the cable adjusters at the top of the carbs and remove
the air cleaner. You will now be able to look inside the
carby throats and see the slides.
Unfortunately
you can't look at both at once! If you could we wouldn't
have to find sneaky ways to do the job.
Place your index finger into one of the
carbs and lightly wedge it between the cutaway edge and
the throat of the carb. Whilst looking at the other slide
gently open the throttle. When the slides start to lift
you will see and feel them lift off together. Your brain
will pick up even the smallest of differences. It is
surprising how accurate this is, the human touch is so
sensitive that you can feel movement as little as one
thou. of an inch. I usually swap a few times to test the
accuracy.
Adjust the carb top cable adjusters to
make any corrections. Make sure you are left with at
least 1mm play at the cable when finished. It's a nice
feeling to find your carbs were miles out and you've made
a huge improvement. Good luck!
Bob Rees' Note: Colin White died suddenly on Sunday
March 24. He was a great bloke, a family man, and a
motorcyclist since his youth. He worked in the motorcycle
trade as well as enjoying bikes recreationally. Colin was
a keen club member and a mine of information when it came
to the workings of motorcyles. On behalf of the club we
have sent our condolences to his wife Joanne and
daughters.
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