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Colin WhiteTricks 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.
Twin carbsThe 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.
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.
Slide cutawayUnfortunately 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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