Dual Carbs

Has anybody tried to line up Carbs yet? Like, putting them in a straight line to the engine? I'm guessing you'd have to split the fuel line with a "Y" or a "T" fitting, and run adjustable rods to the choke/ throttle. Just an idea, I may end up trying this with a T twin. (I have it, and two 20hp carbs laying around) Anybody see any major complications?

why with you ever wont to do that for jest asking whut do u think your get out of it and i dont think it will work do to the 2ent carb jest dumping gas in the one behind it and giving you jest way to much gas

i may be rong i dont know thats jest my thinking on it

My tecumseh v twin has dual carbs. Has a t fitting and dual linkage, ect. Pretty neat

redlinemotorsportts wrote:

My tehcunseh v twin has dual carbs. Has a t fitting and dual linkage, ect. Pretty neat

so it dont over gas or is that stock onit for yours jest anyway that cool any pics of it somewhere

ericsp0f wrote:

redlinemotorsportts wrote:

My tecumseh v twin has dual carbs. Has a t fitting and dual linkage, ect. Pretty neat

so it dont over gas or is that stock onit for yours jest anyway that cool any pics of it somewhere

No its stock. not sure how much they put out though. not to bad i imagine, its 640+cc's so i think two single cyl carbs would work.

redlinemotorsportts wrote:

ericsp0f wrote:

redlinemotorsportts wrote:

My tecumseh v twin has dual carbs. Has a t fitting and dual linkage, ect. Pretty neat

so it dont over gas or is that stock onit for yours jest anyway that cool any pics of it somewhere

No its stock. not sure how much they put out though. not to bad i imagine, its 640+cc's so i think two single cyl carbs would work.

wow thats cool looking i need to keep a eye out for that now

redlinemotorsportts wrote:

ericsp0f wrote:

redlinemotorsportts wrote:

My tecumseh v twin has dual carbs. Has a t fitting and dual linkage, ect. Pretty neat

so it dont over gas or is that stock onit for yours jest anyway that cool any pics of it somewhere

No its stock. not sure how much they put out though. not to bad i imagine, its 640+cc's so i think two single cyl carbs would work.

This may even better idea than what I imagined. I was thinking about like, putting them from one air intake to the fuel side of another. I'll see if I can find a pic.

If you mean connecting them together as in a straight line, I don't see how you would have any advantage. But, dual carbs on a single cylinder engine is an idea. The way I understand it is that you want a decent sized carb for more flow at higher rpms. Obviously if you go too big, there isn't enough vaccum at lower engine speeds for it to run good with a large venturi. But if you hook up two (smaller) carbs with a progressive linkage, then you have good bottom and top end. At idle, throttle plates on both carbs are closed. As you step on the gas, the throttle in the first carb begins to open. Then at some point in the range, adjustable by you if you build the thing of course, the second carb begins to open. At wide open throttle you have maximum flow (both full open) and at lower engine speeds, you still have good throttle response and smooth running. At least that's how I understand it more or less. In this picture, it's on a two cylinder two stroke, but that's pretty much the same idea. Same thing going on with automotive 4 barrel carbs with a progressive linkage. I guess it's also better for fuel economy.

mr.modified wrote:

If you mean connecting them together as in a straight line, I don't see how you would have any advantage. But, dual carbs on a single cylinder engine is an idea. The way I understand it is that you want a decent sized carb for more flow at higher rpms. Obviously if you go too big, there isn't enough vaccum at lower engine speeds for it to run good with a large venturi. But if you hook up two (smaller) carbs with a progressive linkage, then you have good bottom and top end. At idle, throttle plates on both carbs are closed. As you step on the gas, the throttle in the first carb begins to open. Then at some point in the range, adjustable by you if you build the thing of course, the second carb begins to open. At wide open throttle you have maximum flow (both full open) and at lower engine speeds, you still have good throttle response and smooth running. At least that's how I understand it more or less. In this picture, it's on a two cylinder two stroke, but that's pretty much the same idea. Same thing going on with automotive 4 barrel carbs with a progressive linkage. I guess it's also better for fuel economy.

That about covers it. One needs only to have a look at how a spread-bore 4 barrel automotive carb works. The 2 smaller primaries open first, and then when you really get into the pedal, the larger secondaries open. A square-bore is similar, but all four are the same size. If I was going to attempt this on one of our small engines, I think I'd try the spread-bore style.

A rather small carb for the primary would be my choice- maybe one off a 5hp or smaller engine. Then a much larger one. These would be in parallel, NOT in series. Putting 2 carbs in series (one feeding the other) would probably be a total nightmare to try to set up. Think airflow, vacuum, venturis, etc.

I was always going to try doing that on something. Didn't quite get to it yet. I was kind of thinking you could use a fairly small primary carb too. Good thing is if you welded up a nice manifold, it would be easy to experiment with different carbs. Instead of only high and low speed mixture adjustments, now you've got 4 total for the whole range. If you have the patience to play with it for a while, it should make a really nice running setup. If nothing else it would look cool and you could be all mysterious about it when people ask what the deal is. "Oh yeah, doubles the power of any engine, I designed it myself."

Duel carbs are pretty cool. I do all my carb work myself, understand all aspects of it. I think I can give a little insite to some questions. This apply's to these lawnmower engines pretty much. So here goes-

Every engine is designed to work at a certain rpm, then math wiz people do their thing to find out how much airflow is needed for targeted rpm's (yes airflow, not fuel) which come up with the size of the barrel of the carb. The barrel is the part where the ventury and butterfly are. That's how designers determine carb size for rpm before engine is made. When everyone gets an engine to modify or just fix, this is already done which gives you a base line to go off of.
Now you have your engine with carbs or carb on it. If it's a single carb, the barrel is determined to be a certain diameter by surface area, for targeted rpm, say 3000 rpm for baseline. If it's a duel carb on the same engine with same target rpm, the same diameter is needed for the duel carbs ( this is where it gets misunderstood)and this is done by dividing by half. The single barrel is mathmatically divided by 2 which will give you two smaller barrels for the duel carb setup. At this point there are minor benefits to a duel carb setup like better idling of engine and a little quicker throttle responce. Thinking just adding the same size carb to the single carb engine for this engine, the engine would not run right because there would not be enough air moving through the two carbs to run right, especially at idle. Most single barrel carbs on engines, usually bigger then 3hp, have idle circuits on the carbs. The carbs are not designed to be completely closed, which make it almost impossible to idle, thinking just closing one carb just for idle. So this is where the size of the barrels come into play for a duel carb setup. Since they are smaller, the air will move through them faster or at the speed needed for the idle curcuits to work right.
OK lets talk about the engine in question in the pic. Looks like maybe a V-twin Briggs, lets say 20hp. Instead of having a big diameter single barrel carb on it, the designer decided to go with something more compact which in this case is duel carbs. Me mentioning the diameter is cut in half mathmatically comes into play and for sake of this discusion, two 10hp engine single barrel carbs are used for this 20hp engine.
This way of thinking also works on single cylinder engines also. Lets use a 10hp single cylinder engine for this example. So a single cylinder 10hp engine with a single barrel carb, and wanting to put duel carbs on it. The best choice for duel carbs would be two carbs from a 5hp engine. You would have the same amount of air moving through the single carb at idle or this air would split in half for the two 5hp carbs to work at idle.
Carb linkages and carb designs are determined by intake design and performance requirements but what is talked about here is the starting point of switching from a single barrel to a two barrel carb setup or how the engine designer made the engine a duel carb setup. The linkage would be the two barrels opening up at the same time for these engine examples for starting out. Not a good idea to have one barrel closed all the way while the other barrel is open half or wide open because all the air would be moving through the carb that is open and with the other carb still set for idle, well there wouldn't be enough air moving through the closed carb for the idle curcuit to work which means you have air moving through a carb and not picking up the needed fuel for the engine to run right. Unless you have one of the carbs on a duel carb setup modified or made to completely close the barrel at any rpm, the carb linkage must be that both carbs open at the same time.

I hope this helps answer any questions.

Some good info there, my man. I'm sure some folks will learn something.

So if I understand it all, a person could get away with using 2 5.5 or 6.5 clone carbs on an 12-16hp twin?
Is there any real performance benefit on an otherwise stockish engine? (thinking briggs oppy here)

if it is a better carb (as in it mixes the air/fuel better and more accurately) then it would. you should probably go by engine displacement instead of hp, 6.5 hp hondas are like 200cc and a briggs oppy is like 650cc. so two 350cc carbs would be best.

I would go by HP rating on engine, that number is after the fact of it getting built and tested. If you go by displacment, that is not taking in account the inefficiency of the whole motor, like intake, exhaust runners, carb selection, valve size, and so on. Engine HP is after all this is selected and running to test engine to get HP rating.
All the small engine carbs are pretty much the same in performance by basic design. The thing that makes a good carb is adjustability!!!. A good carb, would have adjusting screws on everything. From idle speed, and idle mixture, to main jet adjusting screw.Techumsa makes a great carb with all this on the 5 to 6 HP engines. Plus depending where the engine will be spinning will help carb selection. The example of a 12 to 16 HP oppy, if you are going to keep gov on and just hitting the gov limit now and then, then go with 2 smaller carbs like off a 5 to 6 HP Techumsa, this would idle real good and easy to tune in carbs. An engine with no gov and high revvin then for same engine use 2 carbs off 8 HP Briggs motors so you get more air for higher revvin but will be harder to idle because of the big carbs so to idle the idle would be a little faster then the engine with the 2 smaller carbs because of the air flowing through to slow for the bigger carbs to work right at idle. So to solve this you speed up idle a little.
Performance for duel carbs is mentioned in above post of mine. More performance gains and better duel carb setup would be depending on intake design, and that would be another topic on intake manifolds for duel carbs.

I as considering clone carbs simply because they are cheap, and plentiful.
Maybe the Tec are too? I havnt looked into it.

How about the manifolds? Whats the best way to go about building?
I have some ideas, just curious to what others are thinking.

TheBeal wrote:

I as considering clone carbs simply because they are cheap, and plentiful.
Maybe the Tec are too? I havnt looked into it.

How about the manifolds? Whats the best way to go about building?
I have some ideas, just curious to what others are thinking.

I think there pretty good engines too, as far as I can tell. Everybody seems to be buying them and those Honda originals too. I'm sure the carbs are pretty reliable too because of all the tech behind them and people using them to race with. I think I even have one of those carbs too, bought it at BMI for like 4 bucks on sale. probable won't ever buy a whole motor though, them being from over seas and all. The carbs will require tuning but no adjusting screws I think, which means you would need to buy jets or modify the curcuits or pathways for the fuel. My 4 buck carb will probably cost another 50 bucks for the jets. Something to think about when buying or doing research.

Manifold design and building can get pretty specific. When designing parts you first need to know what you want the engine to do, like rpm range, use and gearing. There's no one design for everything. Please be specific on use for motor as wrong intake can hurt performance rather then increase it. For some ideas on rpm ranges- idle to 1500 rpm would be low end power, -1500 to 3500 rpm or gov limit would be mid range, - and 3500 rpm and up would be high end, but this rpm level would need billet piston rods.

As for material I would use piping conduit for the runners or pathways, and 1/16 inch flat stock for flanges and braze it together for smoothness. welding can get pretty messy and usually leaves little holes for leaks. Size of piping matters too but that depends on use and rpm range.

Im not looking for crazy high rpms, and not wanting to put loads of $$$ into it. This is all just thoughts for now, but id like to turn it on its side. Figured since I would be needing to modify, or build a manifold might aswell see about putting 2 carbs on it.

As for use... maybe a gokart or mower. Or something. lol.


I would Tig it together. Maybe a bit thicker for the flanges aswell.

Here's an idea for duel carbs on a horizontal shaft oppey. Make the manifold so each carb and runner is completely seperate from the other. The manifold runners would be identical in length and bend going up and curving into the main flange. The spacing of the engine cylinders would seperate the two. Where the carbs bolt onto is the main flange where the runners attach to and as you see one carb on each side of the flange also.

I would go with smaller carbs. The barrel of the carbs will dictate the size of the tubing being used for the manifold runners. Don't use really big tubing because the bigger the tubing the slower the air moves through them which cuts torque. If you build for torque, the horsepower will follow. Each carb should run independently from the other which means the manifold runners don't have a crossover tube connecting the runners. Tuning the motor will be a little harder but will get the best performance from stock parts. The curve instead of a 90 degree bend on the runners will help move the air through faster into the motor. The carb linkage can run between the 2 carbs and ofcourse open and close the carbs at the same time.

This can also be used for a vertical oppey motor. Never made one yet but should work. This is all in theory so far. I have an 18 hp oppey vertical shaft I will be building up in a few years and will be testing out my theory on it but my manifold will be different but with duel carbs on it.

I was thinking something kind of similar. But the carbs being level with each other, with a piece of steel bent to fit over top of the carb body. Sort of a "throttle plate"?
Then with the air filter(s) coming up on either side of the plate. Trick being keeping the manifolds the same length, though I don't think that would be too terribly difficult.

I think the clone carbs are somewhere around 1in. Pretty close to what the stock manifold is I believe.
How long? Is my understanding correct that some length helps build up some speed of the air/fuel into the engine?

I have a 12hp and a 16hp I could work with. A couple 5.5 or 6.5hp clone are cheap and should work.. Maybe drill out the jets a size or 2 for the 16hp?

[quote="TheBeal"]I was thinking something kind of similar.  But the carbs being level with each other, with a piece of steel bent to fit over top of the carb body. Sort of a "throttle plate"?
Then with the air filter(s) coming up on either side of the plate. Trick being keeping the manifolds the same length, though I don't think that would be too terribl difficult.

I think the clone carbs are somewhere around 1in. Pretty close to what the stock manifold is I believe.
How long? Is my understanding correct that some length helps build up some speed of the air/fuel into the engine?

I have a 12hp and a 16hp I could work with. A couple 5.5 or 6.5hp clone are cheap and should work.. Maybe drill out the jets a size or 2 for the 16hp?

[/quote
I think you got the right idea on the design. I would use the 5.5 HP carbs for the 12 HP motor and 6.5 carbs for the 16 HP motor. Drill out the jets always when the carbs are on motor and running. To know what size to drill depends on reading the spark plugs. Plus using small carbs, be prepared to modify the idle circuit also, probably will not put out enough fuel and air. Might have to drill out the hole for fuel in the ventury that supplies fuel for idle and file down the butterfly next to the hole to let more air in also to keep the idle in the idle circuit range. The correct order to do the tunning for the carbs is, the engine has to idle first to tune in the main jets. So work with the idle first by adjusting fuel first then air. With the engine idling then you tune in the main jets for fuel. And you should be reading the spark plugs every step of the way.

As for manifold length, this doesn't have to be perfect, a couple inches longer on one side won't make a difference at all. As for length, how it works is, the longer the distance between carb and valve, the better performance is, because air has weight which carries momentum, which means the more force the air has to get into the cylinder. This is called the tunnel ram effect. Knowing this keep in mind sharp bends slows the air down which is why the picture I drawn up has curves instead of sharp bends, curve will slow down the air way less. Plus another bad thing is the further away you put the carb, the less throttle response you have. Throttle response is the time it takes the engine to respond to throttle input. An example is you goose the throttle and there is a noticeable delay when the motor actually reves up. Manifold design helps some which is where the curves instead of bends come into play also.

I think you got the right idea on the design and should work out great for you. Happy building

The problem I have with the Honda GX and clone carbs, is the same problem I have with the factory carbs on all our OPE engines- they're archaic. As I'm sure Pranc can attest- they are simply... Simple. These are not at all designed for performance. At best, they have two point of adjustment. Idle, and high speed. Between idle and full bore, is a whole lot of whatever-it-feels-like doing.

A slide carb like a Mikuni VM series has FIVE overlapping points of adjustment. Throttle response, power, and even efficiency cannot HELP but be improved.

Now- This DOES mean a more expansive carburetor, and this DOES mean it can be a complete and total PITA to set up in the first place, but once done, will outperform the factory carbs every time.

Where I'm going with this, is that I don't at all see the point in going thru the trouble to replace one inferior carb with another.

Yah, The Doc's right. on these carbs. 2 points of adjustments only. These carbs are not even close to even a simple automotive 1 bbl carb. The funny thing about all of this is you still got the same kind of carb, just 2 of them, and the only thing your acomplicing is a fancy intake. More show then go. You would gain maybe 1 to 2 foot pounds of torque and maybe 1/2 horse power. Just because it revs quicker while standing still don't really mean it's stronger. More power gains can easily be gained by just tearing down the motor and hopping up the insides.

But still Doc, you got to admit, It still looks COOL, LOL.

It do look cool. But y'all know me. I'm function over form, hands down. I got a line on three Mikunis off an old motorcycle- if at least two of them are salvageable, I might do something cool with a twin.

Heh, Im looking for show and some go. lol.
Heck, Id bet a half dozen or so little weed whacker carbs would look pretty cool. All done up nice and shiny with cute little velocity stacks stickin up.