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Yamaha Tech Originally the Yamaha XT600 Tech Forum, due to demand it now includes all Yamaha's technical / mechanical / repair / preparation questions.
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Old 1 Jan 2007
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Why does the XT have twin carbs ?

Just thinking why ???

Iv had loads of large singles and none of them have had twin carbs. I know they dont have many problems but surely 2 is less reliable than 1. Iv had excelent fuel economy on single carbs too.

So why cant the XT function on a single carb ?

Can anyone explain the whole twin carb set up to me ?

Thanks guys
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Old 2 Jan 2007
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Well, consider that the XT and the XR 600s are very similar bikes. I brought my XT into a shop with an XR enthusiast mechanic and he asked me what year XR it was. In any case, they were competing models and both ran dual carbs. The simple answer is "airflow," as for the real answer, I don't know.
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Old 2 Jan 2007
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I believe the answer is something like this .....

CV Carbs are more efficient but suffer some problems on big singles.
1) They do not perform well at low revs when the venturi vacum is inconsistant and "pulseing" with each stroke
2) Again, at low revs, they do not respond to rapid opening of the throttle, because they rely on the venturi vacum.

So, Yamaha fitted a slide carb to deal with small throttle openings, and a CV to provide more efficient carburation when the throttle is wider open.
.... an attempt to have the best of both worlds.

Fow my part, I would rather just have a single big slide carb, my Suzi SP400 never seems to have a problem. But then, I would rather the XT had a nice heavy flywheel on the crank rather than that nasty chain driven counter weight that gets all "confused" at low revs
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Old 4 Jan 2007
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the XT500 had one big carb.

mine worked well for 20+ years, but I heard they had lots of low speed stall issues, opening the big carb too fast lost flow and dropped the engine.

anyway

on the XT550, they went to a big carb with a smaller carb for low speed ... like a pilot valve next to a flow valve. the small carb would handle idle to 25% the bigger carb started opening around 20% and went on to 100%.

they also went to the 4 valve head for better flow.

with the monoshock, they also had to maintain space, going from a single carb to a 2-to-1 intake to feed the two valve head requires more room.
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