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Old 03-28-2011, 02:22 PM   #1
Smallwheelsgood OP
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Single Carb Airhead

Hello all, newbie to this site, I have lurked for a while and have found a wealth of information and experience, plus some cracking pictures.
I have 2 no. R100RS's, 1 x 1978 in bits, 1 x 1983 running reasonably well. I have long wondered about converting to a single carb setup, does anyone know of this ever having being done? If not, what would be the pros and cons of such a setup. I have heard that carb icing may occur with such long inlet tracts, other than that I envisage that it would offer less complication and maintenance, and perfect balance with only a small reduction in top end performance. Any help would be much appreciated.

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Old 03-28-2011, 02:32 PM   #2
Houseoffubar
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It will work, if you are OK with harder starting, and about 27HP, but it will "work" I would recommend avoiding such thoughts
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Old 03-28-2011, 04:37 PM   #3
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"small reduction in top end performance"

If you are talented enough to pull off this mod you have to be talented enough to balance your carbs every 5-10,000 miles.
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Old 03-28-2011, 05:16 PM   #4
Tosh Togo
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Originally Posted by Houseoffubar View Post
It will work, if you are OK with harder starting, and about 27HP, but it will "work" I would recommend avoiding such thoughts
Quote:
Originally Posted by kadesean View Post
"small reduction in top end performance"

If you are talented enough to pull off this mod you have to be talented enough to balance your carbs every 5-10,000 miles.
What they both said, and you WILL need carb & manifold heat, unless you want it to chug to a stop on rainy days. If you'd like to see how to do it, take a peek at any 1970 or older stock VW beetle for a simple carb-heat setup. Even enclosed under the rear lid, Bugs had to have heat unless it was a nice dry day. As an added bonus, the intake pulsations from that looooong path to each intake valve will make jetting a real treat, and throttle response will be soggy at best.


If you wanna go slow that bad, buy a Harley.
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Old 03-28-2011, 07:11 PM   #5
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Hmmm....

The last time I saw one of those conversions was back around 1990 at the MOA National Rally. A guy had converted an R65 using what looked like a Ford single pot sitting where the airbox used to be, with long intake tubes going to the heads. The install was very simple IIRC.

The bike ran quite well. I don't remember seeing any provisions for carb heating, so I can't comment on that aspect of things but I have held that conversion in the back of my mind, thinking that the info might be useful one day.

search around the net. Someone has to know something concrete because it has been done and more than once.
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Old 03-28-2011, 07:33 PM   #6
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First, I love the creative out of the box thinking. I'll never scoff at an idea that is different if only that it helps us learn and play.

Buy, please explain this to my like I'm the village idiot (I know, I know...), why would we want to do this? Is it simplicity of operations? Available materials? Efficiencies?

Thanks.
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Old 03-28-2011, 09:33 PM   #7
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BTW I agree with the "out of the box" thinking. This is a bad idea, but only because the design of the engine does not lend well to such a modification, however, I'm planning to use a single carb on a XS650 twin, and hardly ever call myself an idiot for such thoughts I just feel this is a better idea on a parallel twin, than opposed, and such.
Be sure to post some pictures of your projects!!!
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Old 03-28-2011, 09:35 PM   #8
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So, in the opposite mindset, should I put multiple carbs on my BMVW? It has one carb and four cylinders.
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Old 03-28-2011, 10:57 PM   #9
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Originally Posted by Houseoffubar View Post
BTW I agree with the "out of the box" thinking. This is a bad idea, but only because the design of the engine does not lend well to such a modification, however, I'm planning to use a single carb on a XS650 twin, and hardly ever call myself an idiot for such thoughts I just feel this is a better idea on a parallel twin, than opposed, and such.
Be sure to post some pictures of your projects!!!
That's an entirely different mechanical situation, and for vertical twins with a 360 crank like your XS, single carbs been used since before WWII. Mr. Turner's 650 twin was originally equipped with one Amal, as the Thunderbird. Bonnevilles with two carbs soon followed, and those had more top end but lost a bit of low-end tractability.

But those two bikes had a carb(s) that was only a few inches upstream of the intake valves. Imagine how much fun it would be if the T-Bird's single carb were parked behind the battery and connected by a long pipe to the intake ports.

Just because a concept or a piece of hardware works well in one situation doesn't mean it should blindly be applied everywhere. That's not being creative...it's showing a lack of originality and a failure to understand how things work.

Successfully thinking "out of the box" also means that you're able to see when the path you've chosen is leading into a corner. Being different just to be different is puerile... difference being actually better is what matters. Best of all is a change/modification that's so subtle a casual inspection won't see it, but the riding gets better.

Try that sometime.
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Old 03-28-2011, 11:30 PM   #10
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So, in the opposite mindset, should I put multiple carbs on my BMVW? It has one carb and four cylinders.
If you want to: sure, go for it. I've used dual Webers on Type I engines, and as long as you don't get silly and use carbs that are too big, it works pretty well.

No more carb icing, either.
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Old 03-28-2011, 11:32 PM   #11
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Ouch!
I understand the mechanics, I just did not want to rain on his proverbial parade!
I agree this is a terrible idea, in this circumstance. But I have had some terrible ideas, that until I talked about it, did not know they were so bad. I've made a career out of bad ideas, I did not want to crush his! Thinking out of the box, does not mean good ideas are found out there, but at least they are ideas.
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Old 03-28-2011, 11:36 PM   #12
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I had a VW powered Ural,and fitted a single SU carb,as others have mentioned,carb icing was an issue.I sold it before I had it sorted,but in hindsight larger diameter runners would have slowed airspeed...but that would have brought other issues.I think the idea has merit,motorcycle carbs are spaced too close to the inlet valve,they ideally need to be further back....but in the centre of an airhead crankcase is too far.

Fitting a single carb on anXS650 is not easy either - there is the seat frame tube in the way,it has to come in from the side which would give mixture bias.On a custom frame on the other hand it would be no problem.My custom XS650 frame still has a frame tube in the way.
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Old 03-28-2011, 11:41 PM   #13
Tosh Togo
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Ouch!
I understand the mechanics, I just did not want to rain on his proverbial parade!
I agree this is a terrible idea, in this circumstance. But I have had some terrible ideas, that until I talked about it, did not know they were so bad. I've made a career out of bad ideas, I did not want to crush his! Thinking out of the box, does not mean good ideas are found out there, but at least they are ideas.
True dat.

Fwiw, Mr. Honda always saw failures as better learning opportunities than winning.

But...I just don't see much point in going to a lot of work that will, at best, de-tune an already slow bike. On the upside, it would be a great way to learn ugly things about how to size an inlet tract and the downsides of strong intake resonances. Cold-day starts would probably be a bitch.

And you'd get a lot of questions beginning with "WTF were you thinking?."

If the OP wanted to truly get creative, how about major improvements to the brakes and suspension that don't show on the outside, but give the bike both better handling and better ride quality?.
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Old 03-29-2011, 06:28 AM   #14
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BMW has already done it with the R12.

http://www.bmwr12.com/
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Old 03-29-2011, 09:57 AM   #15
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Hasn't the decision to use a single carb been an economic choice? I think it has been a given for a long time that having a throttle close to the port and one per port is best.
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