Topic: Can someone tell me how these carbs work?

[azuredesign]

This was used in the 4 valve single cylinder Yamaha srx250, XT250, TT250, and I think xt350.
It a weird set up with 2 intake manifolds which appear to feed each intake valve. One carb has a mechanical slide, choke circuit, slow speed, needle and main jet, the other which feeds off of the float bowl of the first carb, has a vacuum actuated needle jet and that's about it. Why would Yamaha do this? It would be cool if the vacuum side acted as a secondary stage, like in a two barrel carb, but both carbs open simultaneously I think. FWIW, there are two exhaust ports which I guess are fed from each exhaust valve. Thanks for any help, the digital shop manual I received with the bike is not helpful.

[vanillagorilla]

I had an 82 xt550 and the carb setup was similar. The right carb seems to be a secondary intake that opens 1/2 to wot. I am not entirely sure, but thats what it seemed like. And you could feel when that one opened up, nice boost in power. Man I miss that bike... Fitted fenders from a 2002 cr250, she was a good lookin old thumper!

Nevermind the goofy expression... Guess thats just how I roll...

[Industrial Cafe]

goofy expression nothin, check out the devil eyes!!!

[azuredesign]

The right carb seems to be a secondary intake that opens 1/2 to wot.

Thanks for writing, I'm not exactly sure what you mean, but maybe it is functioning like the second stage in a 2 barrel carb.

[Industrial Cafe]

it is functioning like the second stage in a 2 barrel carb.

that's it.

[TomC]

Hi azuredesign
     As someone who has put a lot of miles on an SRX250. They work very well.
          TomC in Ohio

[Rampage1967]

I never rode one or worked on one, but, I do remember reading the factory literature on those carbs put out by Yamaha back in the late 80's. My memory is that they work just like a vacuum secondary four barrel on a car. The slide side opens up when you twist the grip and the CV side opens up as the engine demands more air flow.

It seems like a really neat, simple way to get a variable intake without any extra controls on a single cylinder engine.

[azuredesign]

Thanks guys, I'm looking forward to feeling how it works myself. Have a bit more work to do before then, but should have it out by December if the ground is not covered in the white stuff.