Weber downdraft tuning

gdub0717

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I know there are plenty of threads on this subject. Can someone send me.some link scar runs realtively well but would like to gain some additional technical knowledge on these carbs and check/tune them.
I have one of these syncrometers that I used on myside drafts.
Looking to use and adapt.for use on the down drafts.
As usual thanks for the input and advice!
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Ohmess

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For sync, start with a check of your linkage. Disconnect it at the top of the rod from the bellcrank and check everything from the top of that rod back to the accelerator pedal. Make sure it all moves smoothly, with no binding and that the pedal stop is in the right place. Then, with this still disconnected, check the linkage between the carbs, again making sure it moves smoothly and also checking that both throttle plates move together in the same way when you move the linkage.

In order to use that synchrometer, you need an adapter like the first one pictured here:


For technical knowledge, start with the Pierce Manifolds tuning guide:


Their videos are good too:



Then, look through Stevehose's stuff posted here.
 

Stevehose

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For downdrafts (downdraughts?) you'll need this with your synchronmeter:


Haynes aslo makes a good book on the downdrafts:

 

Honolulu

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Using the Search function, you can see that home-made adaptors have been made by some of us. Some of those are surprisingly low-tech.

(Some) BMW R-bike guys use a Harmonizer, essentially an electronic differential manometer. I've used a (borrowed) Harmonizer; used the factory synchronizer, a Uni-Syn, some aquarium tubing filled with ATF. The Harmonizer was easiest to use, had excellent sensitivity but can be damped easily, and overall impressed me the most. They may be out of production but turn up in the usual places, or get together with a local R-bike group, someone is likely to have one and you may be able to use it.

NOTE the Harmonizer and some of the devices above measure vacuum at the base of the carb rather than the actual airflow through the carb. This seems to be the more common method of synch-ing carbs, although it's a secondary (though probably sufficient) measurement strategy. After all, it's the relative depression in the venturi that draws the fuel, not total airflow.
 
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