Twin to single carb.

Message

Stratus311 · #1 Post by **Stratus311** » Thu Jun 15, 2006 2:26 am

So I've been thinking lately about going from 2 carbs to one. I know I'll have to fabricate a new manifold which I'm fine with but I'm curious to know what size carb to go with. It's a 1980 Yamaha XS 400 that currently has 2 Mikuni 34mm carbs. What single carb would be adequate? Thanks in advance.

-Holiday · #2 Post by **-Holiday** » Thu Jun 15, 2006 3:42 am

i was thinking about this the other day, and I dont really understand why you'd do this on a bike. What are the benefits of running one carb instead of two aside from maintentence?

Wouldnt the one carb have to have a CFM = to carb 1 + carb 2?

Ninja Geoff · #3 Post by **Ninja Geoff** » Thu Jun 15, 2006 5:08 am

Stratus311 · #4 Post by **Stratus311** » Thu Jun 15, 2006 5:57 am

Well, I've thought about it for simplicity. And remember it's just a thought. Just trying to get some ideas. By the way, Harley's have 1 carb.

And Geoff: While I like the idea of running a Holley, I just don't think it could meet my demands for fuel and raw horsepower from my XS 400.

paul246 · #5 Post by **paul246** » Thu Jun 15, 2006 7:17 am

I can't see the benefit of going to a single carb, either. Fabricating a good non-leaking manifold is too much trouble for a start. Carbs are easy to keep in sync, too. If you don't have a device for balancing the carbs you can always set the throttle plate gap in each carb to match each other using a small drill or wire as a "gauge" and go from there.

Remember that the main difference between the old BSA Thunderbolt and the Lightning (both 650cc machines) was the fact that the Lightning came with two carbs instead of one. The Lightning was the stronger of the pair and is still the more sought after of the two bikes.

Instead of spending money to achieve simplicity why not spend 89 bucks for a Twinmax (sp?) ?
http://www.adventuremotogear.com/twinma ... 5TC7G1GR78

Dichotomous · #6 Post by **Dichotomous** » Thu Jun 15, 2006 7:20 am

well just in terms of pure air capacity you are looking at 48mm. Thats squaring both current carbs and combining then taking the square of the result, pi is constant and can be eliminated.

#7 Post by **9000white** » Thu Jun 15, 2006 10:16 am

Dichotomous wrote:well just in terms of pure air capacity you are looking at 48mm. Thats squaring both current carbs and combining then taking the square of the result, pi is constant and can be eliminated.

somebody contact Starfleet Headquarters and tell them we have found Spock!!

Stratus311 · #8 Post by **Stratus311** » Fri Jun 16, 2006 12:32 am

Dichotomous: Thanks so much for the reply. That's basically what I was looking for.

Synching the carbs is not a problem at all. That's gravy. (By the way, that TwinMax is very cool). It's just about simplicity. As in, easy to diagnose any problems, 1 carb to tune, doing something different(which is why I'm building an XS400 in the first place), etc. Building a leak-free manifold is also easy. Good welding and pressure testing is key here. As far as power is concerned, I'm not building a drag bike, but something fun to ride.

Thanks for your opinions.

Dichotomous · #9 Post by **Dichotomous** » Fri Jun 16, 2006 6:34 am

well now that I think it over a little more, i dont know that you would need that large of a carb. Air is only going to each chamber one at a time, not really both. So the air demands are going to be different, dont know if you would need the full combined air capacity since they wont draw at the same time. Would splitting the difference help? I am just thinking that larger carbs would cost more and such. Though with that much airflow I would think building a drag bike could be in the posibility, as long as it can actually grab that much air, otherwise I wouldnt think it would run as well.

flynrider · #10 Post by **flynrider** » Fri Jun 16, 2006 6:43 am

Stratus311 wrote: It's just about simplicity. As in, easy to diagnose any problems, 1 carb to tune, doing something different(which is why I'm building an XS400 in the first place), etc.

I'm with you Stratus. Not a huge deal when I had two carbs, but now that I have to deal with a rack of four carbs, it can sometimes get to be a pain.

Carb rebuild? Prepare to do it 4 times, and get each carb set up exactly the same. A single carb would make maintenance and troubleshooting a piece of cake. I don't see any logical reason for having a separate carb for each cylinder. I'm sure there must be one, though. I would expect that 4 carbs are more expensive for the manufacturer than 1 big one.