Just a (silly?) thought re carburettors

[faffi]

I have a Daytona donk with the dreaded Mikuni CV carbs. They suck air through a Thunderbird airbox that has been handed a bunch of holes for better breathing. Exhaust is through a Thunderbird system with TORs.

Getting optimum jetting is difficult. I knew that going in, but then again the original plan was to use a stock Thunderbird engine with stock carbs and airfilter system. The plan changed when that engine proved damaged.

We know that FCRs from Keihin will improve performance, but they are costly. I also suspect that you need to modulate them a bit at lower revs to prevent bogging, although I do not know this for sure.

However, it wouldn't be terribly difficult to fab up a 1-3 inlet manifold that would allow the use of a single carb. Stock carbs for Harleys are cheap and plentiful since so many upgrade their Ablesons.

But what would it take to make such a carb perform well? What is the major deciding factor for jetting - horsepower per cylinder, volume per cylinder, cam timing or It would be good to get a decent starting point instead of starting out in the dark completely.

There would be benefits with a single carb; no synchronizing ever, easy to tune, easy to remove, strong bottom end performance. I can only see two real issues against; having to fab up the manifold and jet the carb properly. Of less concern (for me) would be a slight loss in top end power. I'd happily swap 20 horses at 9000 rpm for a 10 hp gain at 4000 rpm

Anybody hear of such a thing done to a triple?

[KitNYC]

Never heard of this being done, but you've certainly piqued my interest!

Cheers,
-Kit

[Coroja]

Faffi,

A single carb (or throttle body) is not a bad idea, but designing an intake manifold is a tricky business. IMHO, most motorcycle manifolds lack the sophistication found in automotive high-performance manifolds, so studying automotive manifold design would be useful. This article provides some insight into the variables and their interrelationships:

http://www.team-integra.net/sections...?ArticleID=466

There is also some useful information in Kevin Cameron's Sportbike Performance Handbook and in Jim McFarland's Enginology column that has been running for most of this year in Circle Track Magazine.

Most motorcycle manifolds that I'm familiar with (i.e. Harley and Classic British Twins), lack a plenum and have runner lengths that appear to be chosen mainly for packaging convenience. Packaging may be the biggest consideration with a carburetor, because of wet flow issues resulting in mixture differences between cylinders. I suspect that is why most manufacturers of relatively high performance engines (i.e. non-Harleys) have opted for multiple carbs, rather than trying to design manifolds. It would be an interesting project though, and possibly not too hard to make work, if you are satisfied with 70 - 80 percent of maximum potential.

Just my $0.02.

[faffi]

Coroja, my head hurts But in a good way Although I do not understand all of the details mathematically, it assured me that a simple design can work well. To get an ideal match, it will need a lot of trial and error, probably. But I just want smooth, predictable throttle response from idle onwards and I'm less concerned with maximum power at high rpm.

I used the calculator and came up with a 1.2 square inch inlet size for a 6000 rpm torque peak.

At first, I thought I would have to weld up a system of 3 curved tracts ending up in a single tube for the carb, not unlike a 3-1 exhaust system. But those articles shows that I can take 3 straight tubes as long as possible out of the inlet rubbets and then weld a tube goind 90 degrees over their backs (after drilling appropriate holes, of course) and have the carb sticking out to either side under the tank. I just need to make sure the air filter and carb doesn't interfere with my knee. There are kits that will allow the air filter to mount forward on a 90 degree bend, so it should be quite possible.

Hm, more things to ponder.

[Coroja]

Faffi - yes, you probably can get straight tubes to work fine. I think I would get some plastic drain pipe in various sizes to make a mock-up for flow and fitment testing. Once I had the manifold mocked up, I would want to try some wet flow tests to determine how much "fuel" was coming out of suspension at the runner farthest from the carb.

From a design standpoint, you need to be careful about getting your runners too long and introducing excessive throttle lag. Practically, I think using a Harley-style carb end on the manifold would give you the widest choice of carbs. Also, you might investigate having your "final" design cast in aluminum, rather than trying to weld it up out of tubing. There was an article in one of the racing journals recently about using castings for one-off parts, I'll try to find you a reference if you are interested.

[faffi]

The article would be of interest, but it does sound out of my abilities - but it's always fun to learn about new stuff

Several people have made more or less crude 2-1 manifolds for the old Kawasaki 400 twin with pretty good results. I'm fully aware that I will likely lose a significant amount of top end power, but that wouldn't be so bad as I tend to ride faster than my abilities - and guardian angel - allows. And I have the scars to prove it

What I want is linear power from idle onwards. If I can get a strong pull up to 6000 rpm, it wouldn't matter much what happened afterwards. As it is now, it goes like crazy from 8000 rpm and pulls OK under light throttle from 1000 rpm, but there isn't real power under 8 grand. On the gnarly, winding backroads I love, this powerband is quite tedious.

I still have to try the Factory needles and springs after the latest air box mods, and if I can get the bike to run well with the stock carbs, I'll probably leave it as that. I'm not a great fan of wrenching just for the heck of it. (Lazy is probably a better word ) However, and this is important, maintaining a single, easily accessible carb is far easer and more satisfying than the stock #¤"/£$ setup. Add the rapid wear of the Mikuni units, and there's another argument for trying out the single carb conversion.

Again thanks for the input - I may do a mock-up over winter just to see how things will fit.

[sailfish]

Have you considered sourcing a set of FCRs from a Japanese sport bike and building a rack of 3 from a set of 4? Compared to new a used set is a pretty good deal. You could put the extra carb on your snow machine.

[faffi]

You mean something like these? http://cgi.ebay.com/ebaymotors/Kawas...item4ceba6fa40

There will be issues with spacing, I presume. And I will still have 3 carbs that I will have to mess with until I get the jetting right etc. I have always liked single carbs - they make life easier - although a pre-jetted FCR kit or similar wouldn't be out of the question. But trying to adapt one from another bike doesn't tempt me much.

[Roden]

Interesting Faffi, if you can create a working set up, you could then hand it to a pattern maker, hand the resulting pattern to a foundry and presto, cast aluminium inlet manifolds!

Cheers,

Roden

[Flying Custard]

I considered a single carb for my chopped Wing, but then it thrives on torque not hp; it already has very long intake runners; the carbs are a funking bar steward to set up, so maintenance ease is a big draw.

Advantange of the banked lineup vs a plenum or manifold is a very straight run into the inlet, no fuel/air distribution issues, no fuel drop-out, and good signal strength from inlet to carb. Personally I wouldn't want it on a performance bike, but would be fine for the right purpose.