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 Dual Carburetors?

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PostSubject: Dual Carburetors?   Dual Carburetors? Icon_minitimeDecember 11th 2014, 12:47 am

Has anyone ever thought of taking a few intakes from a few oppys and make a dual carb setup? Like take the original intake and cut off one side beside the carb then weld a plate to cover the hole and seal with silicon. Then do the same to another one, but cut the opposite side.
I thought about doing this, but there are some things I figured that wouldn't let it work. Like trying to synchronized the throttle and choke, getting air leaks, getting enough fuel flow from factory fuel line, and fabricating air filters that will work with each other because of being so close together.
 The whole idea behind it all would be that it would look cool and most importantly make it rev up faster, but I would have to wonder. Would it rev up so fast that it might sling a rod?
I figured I would put this out there, because I haven't seen any post with this idea and maybe someone might want to try this. I would do it, but I don't have spare intakes so far to fab one and couldn't afford to mess up my original one. I do have the stuff to do half of the setup tho.

This idea is for opposed twins if anyone didn't know I call oppys.
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PostSubject: Re: Dual Carburetors?   Dual Carburetors? Icon_minitimeDecember 11th 2014, 8:04 am

willysjeepmb wrote:
Has anyone ever thought of taking a few intakes from a few oppys and make a dual carb setup? Like take the original intake and cut off one side beside the carb then weld a plate to cover the hole and seal with silicon. Then do the same to another one, but cut the opposite side.
I thought about doing this, but there are some things I figured that wouldn't let it work. Like trying to synchronized the throttle and choke, getting air leaks, getting enough fuel flow from factory fuel line, and fabricating air filters that will work with each other because of being so close together.
 The whole idea behind it all would be that it would look cool and most importantly make it rev up faster, but I would have to wonder. Would it rev up so fast that it might sling a rod?
I figured I would put this out there, because I haven't seen any post with this idea and maybe someone might want to try this. I would do it, but I don't have spare intakes so far to fab one and couldn't afford to mess up my original one. I do have the stuff to do half of the setup tho.

This idea is for opposed twins if anyone didn't know I call oppys.

Dual carbs on an oppy (or other two-cylinder OPE engine) is something that is sometimes done in racing. Cutting down a factory intake manifold really isn't a feasible option. They are cast iron, which is a pain in the @ss to weld. Silicone is a bad idea. Any sort of leakage, and you're fighting a neverending, losing, mixture battle. The common solution is simply a custom fabricated manifold.

Fuel flow is half the problem you think it is. Literally. Since you're still feeding the same two cylinders, you are NOT using any more fuel than you did before.

Acceleration- Throttle responne may well increase slightly, but it's not like supercharging. You're NOT going to be scattering parts due to a change in carburetion alone.

Synchronization- If you were building a true high performance engine, synching is a pretty simple task with the appropriate gauge. For one of THESE engines, it is simply a matter of adjusting cable pull, such that the throttle plates sit at idle in the same pedal position, as well as beginning to open at the same time, and finally hitting WOT at approximately the same time.

It's also worth mentioning that two stock carbs aren't likely to do much by way of improving performance, in fact they may hurt it. There IS such thing as too big a carburetor for a given situation. Since ONE stock carb is generally sized to your engine's CFM demand, TWO stock carbs (one per cylinder) is therefore too big. If one were to do this, a smaller carb per side is likely the way to go. Too large a venturi size hurts performance.

Finally, if one were to go thru all this, stock type carbs is a fool's errand anyways. A stock carb has only 2 circuits, 2 tuning points. Idle (Pilot) and high-speed. This is a design that's been largely unimproved since the age of the pterodactyl. On the other hand, a good slide carb such as a Mikuni VM series, has up to five overlapping tuning points. It can be a pain to set up, but dialled in correctly is FAR superior and will offer the greatest performance over the entire range.
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PostSubject: Re: Dual Carburetors?   Dual Carburetors? Icon_minitimeDecember 11th 2014, 10:22 am

I've had an idea... But I don't think it would ever work... But how about a dual carb setup on a single cylinder
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PostSubject: Re: Dual Carburetors?   Dual Carburetors? Icon_minitimeFebruary 16th 2015, 1:44 pm

Carshowkid wrote:
I've had an idea... But I don't think it would ever work... But how about a dual carb setup on a single cylinder

it would do the same thing as a bigger carb
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PostSubject: Re: Dual Carburetors?   Dual Carburetors? Icon_minitimeFebruary 16th 2015, 2:25 pm

Dual carb opposed setup in the works for my opposed twin build off machine. I'm going to make my own manifolds though. I have two matching Tecumseh carbs. I thought of putting two stock opposed down drafts on a stock intake. That would be neat and probably easier to link together than the Tecumseh side drafts.
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PostSubject: Re: Dual Carburetors?   Dual Carburetors? Icon_minitimeFebruary 16th 2015, 4:10 pm

I like the idea of the Tec carbs, especially if they are those lovely, pre-EPA Series 1 units.

Dual Oppy carbs may be problematic. If each venturi is too large for a single cylinder, you'd lose intake velocity and she'd fall flat on her face.
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PostSubject: Re: Dual Carburetors?   Dual Carburetors? Icon_minitimeFebruary 16th 2015, 5:37 pm

Since we are on the topic. Ive kicked the idea around of duals with snowmobile carbs because of the 2 into one splitter unit, how easy it would be to rig up. But sleds are 2 stroke... How would a 2 stroke carb on a 4 stroke work out? Just tune it our different? And if it had a oil injector obviously plug it.
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PostSubject: Re: Dual Carburetors?   Dual Carburetors? Icon_minitimeFebruary 16th 2015, 6:34 pm

Doc Sprocket wrote:
I like the idea of the Tec carbs, especially if they are those lovely, pre-EPA Series 1 units.

Dual Oppy carbs may be problematic. If each venturi is too large for a single cylinder, you'd lose intake velocity and she'd fall flat on her face.

One carb I have is slightly newer, and only has an adjustable low speed jet. The other which is about the same only older, has both high and low adjustment. I have a high speed jet which will hopefully thread in to convert the newer one.

Using two opposed carbs, you might get away with two 40CI carbs on one of the larger engines...maybe. I kind of wondered why single cylinder engines have carbs maybe slightly smaller than the intake ports and the opposed engines carb is also about the same as the ports only with two cylinders. You would think two cylinders at the same speed equals twice the flow, so you would need a slightly larger carb. Or maybe they are and I didn't look very closely. I've noticed some venturis on lawn mower carburetors are really pronounced and some are only slightly smaller than the bore. You would think it would work best one way or the other and they would all look about the same.

As far as two stroke carbs go, I think the only difference would be the size of the carb vs engine size. I would guess more flow, at the same rpm with a two stoke engine as a four stroke of the same displacement. So a carb for a two stoke 250cc engine might be bigger than a carb for a four stroke 250.
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PostSubject: Re: Dual Carburetors?   Dual Carburetors? Icon_minitimeFebruary 17th 2015, 10:03 am

mr.modified wrote:
Doc Sprocket wrote:
I like the idea of the Tec carbs, especially if they are those lovely, pre-EPA Series 1 units.

Dual Oppy carbs may be problematic. If each venturi is too large for a single cylinder, you'd lose intake velocity and she'd fall flat on her face.

One carb I have is slightly newer, and only has an adjustable low speed jet. The other which is about the same only older, has both high and low adjustment. I have a high speed jet which will hopefully thread in to convert the newer one.

Using two opposed carbs, you might get away with two 40CI carbs on one of the larger engines...maybe. I kind of wondered why single cylinder engines have carbs maybe slightly smaller than the intake ports and the opposed engines carb is also about the same as the ports only with two cylinders. You would think two cylinders at the same speed equals twice the flow, so you would need a slightly larger carb. Or maybe they are and I didn't look very closely. I've noticed some venturis on lawn mower carburetors are really pronounced and some are only slightly smaller than the bore. You would think it would work best one way or the other and they would all look about the same.

As far as two stroke carbs go, I think the only difference would be the size of the carb vs engine size. I would guess more flow, at the same rpm with a two stoke engine as a four stroke of the same displacement. So a carb for a two stoke 250cc engine might be bigger than a carb for a four stroke 250.

Regarding the two-stroke carbs- at the end of the day, a carb is a carb. Size matters, not much else. There are many factors that will affect the size of the carb you need, vs the donor carb. Generally speaking (broad stroke here) if the displacement is the same, you're in the ballpark.

With regards to the venturi size variations you've noticed, you may be slightly overanalyzing things. Its not a financially sound decision for a company like Briggs (for example) to have a carb built to an exacting spec for every single different size of engine they produce. One particular carb will cover a range of displacements, and in this case, close enough is close enough. These aren't high-performance engines. Since these engines have a narrow operational speed and power band, there's a fair bit of wiggle room for them.

I have compared carbs a few times. One time in particular caught my attention. I did a direct comparison between carbs I pulled off a 16hp Gen 2 Oppy, and an 18hp Gen 2 Oppy. Not only were the carbs dimensionally identical, but they had the exact same jet orifice size too. That surprised me. I figured they'd at LEAST select a different jet!

Why, I cannot really explain. You'd certainly think the fuelling demands would differ enough between the two engines, so as to require a different jet. Briggs didn't think so. But then, there's a lot of various dynamics in play. Sure- that 18hp needs more fuel. But then again, the displacement is slightly larger.

Thinking about it- the larger displacement means that it is drawing more charge in. The difference between the 16hp and the 18hp is stroke. Therefore, its inhaling just a wee bit longer. Not too hard to imagine that a longer suck through the same straw would indeed result in more milkshake per mouthful.

Bottom line- I always tell people NEVER to make carb or jetting assumptions. Let the engine tell YOU what it needs, based on all it's variables, including air temperature and altitude.
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PostSubject: Re: Dual Carburetors?   Dual Carburetors? Icon_minitimeFebruary 17th 2015, 1:56 pm

Doc Sprocket wrote:

With regards to the venturi size variations you've noticed, you may be slightly overanalyzing things. Its not a financially sound decision for a company like Briggs (for example) to have a carb built to an exacting spec for every single different size of engine they produce. One particular carb will cover a range of displacements, and in this case, close enough is close enough. These aren't high-performance engines. Since these engines have a narrow operational speed and power band, there's a fair bit of wiggle room for them.


I agree with what your saying. But I mean I have seen carbs with about the same size throttle plate and bore, but one with almost no venturi, the other with a venturi so small, you would think it would be a serious restriction. You would think that venturi size would change proportionally to the throat and throttle plate of the carb. For example, venturi that is X% of the throttle plate works best in all cases regardless of size.
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PostSubject: Re: Dual Carburetors?   Dual Carburetors? Icon_minitimeFebruary 17th 2015, 8:34 pm

I have thought about the idea, Tecumseh v twins have dual carbs from the factory if you guys didn't know. I know Bob Morris on one of the facebook groups has a dual carb oppy. He used two mikuni slide carbs.

As for the 2 stroke carb, like you said just plug the oil barb fitting and re jet on accordance with the fuel needs.
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PostSubject: Re: Dual Carburetors?   Dual Carburetors? Icon_minitimeMarch 18th 2015, 2:39 am

We actually have a one off custom made set of intake manifolds and a custom machined adjustable linkage that connects to both carbs and then to the factory governor linkage. I have only set it up once and I did it incorrectly. I think you are suppose to leave out the spacer between the carb and manifold on the left side carb. Note the spacer on the top of the right side carb. Both carbs are suppose to sit level. But anyway heres a photo. We may try and set it up again in the near future and when we do I will post more photos and video.

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PostSubject: Re: Dual Carburetors?   Dual Carburetors? Icon_minitimeMarch 18th 2015, 9:11 am

mr.modified wrote:
Doc Sprocket wrote:

With regards to the venturi size variations you've noticed, you may be slightly overanalyzing things. Its not a financially sound decision for a company like Briggs (for example) to have a carb built to an exacting spec for every single different size of engine they produce. One particular carb will cover a range of displacements, and in this case, close enough is close enough. These aren't high-performance engines. Since these engines have a narrow operational speed and power band, there's a fair bit of wiggle room for them.


I agree with what your saying. But I mean I have seen carbs with about the same size throttle plate and bore, but one with almost no venturi, the other with a venturi so small, you would think it would be a serious restriction. You would think that venturi size would change proportionally to the throat and throttle plate of the carb. For example, venturi that is X% of the throttle plate works best in all cases regardless of size.

Unfortunately, I am no physicist, I can only guess. But you'd think so. Perhaps not.

802 Customs wrote:
I have thought about the idea, Tecumseh v twins have dual carbs from the factory if you guys didn't know. I know Bob Morris on one of the facebook groups has a dual carb oppy. He used two mikuni slide carbs.


I have not seen a true dual carb OPE engine. Every example I can think of, is a 2-barrell, not a dual carb. Got a link or further info I can look at?

98Murray wrote:
We actually have a one off custom made set of intake manifolds and a custom machined adjustable linkage that connects to both carbs and then to the factory governor linkage. I have only set it up once and I did it incorrectly. I think you are suppose to leave out the spacer between the carb and manifold on the left side carb. Note the spacer on the top of the right side carb. Both carbs are suppose to sit level. But anyway heres a photo. We may try and set it up again in the near future and when we do I will post more photos and video.


That is a really interesting setup, but I do think its very likely too much carb for the engine. I will be interested to see how it works out.
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PostSubject: Re: Dual Carburetors?   Dual Carburetors? Icon_minitimeMarch 18th 2015, 10:37 am

Redlinemotorsports has a tecumsea v twin with dual carbs, at least he used to.
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PostSubject: Re: Dual Carburetors?   Dual Carburetors? Icon_minitimeMarch 18th 2015, 10:11 pm

1997 Murray wrote:
Redlinemotorsports has a tecumsea v twin with dual carbs, at least he used to.

Yup yup. 22hp.
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PostSubject: Re: Dual Carburetors?   Dual Carburetors? Icon_minitimeMarch 19th 2015, 10:34 am

Very cool. I had entirely forgotten!
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PostSubject: Re: Dual Carburetors?   Dual Carburetors? Icon_minitimeApril 7th 2015, 5:39 pm

I've been thinking about cutting the stock manifold on my 12hp Flatty and mounting a Mikuni from one of my piles of Dirt Bike parts. Matching the inside diameter would be easy. Outside, I'd use a piece of radiator hose and hose clamps to mount it. A pod air filter would clamp right on as well. Maybe some day...

A slightly bigger carb could provide slightly better performance if jetted right. The stocker on my flatty is pretty restrictive. But I don't think the rest of the motor really can use anything bigger than 24-26mm. Bigger isn't always better unless the engine is built for it. (paraphrasing Doc here). Cool
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PostSubject: Re: Dual Carburetors?   Dual Carburetors? Icon_minitimeApril 8th 2015, 11:37 am

Dual Honda clone carbs on briggs oppy?
This thread got me to thinking about taking my smaller oppy, converting to horizontal with dual carb setup. Maybe some new internal parts to get a bit more rpms out of it...
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PostSubject: Re: Dual Carburetors?   Dual Carburetors? Icon_minitimeApril 8th 2015, 12:16 pm

T-Dub13 wrote:
I've been thinking about cutting the stock manifold on my 12hp Flatty and mounting a Mikuni from one of my piles of Dirt Bike parts.  Matching the inside diameter would be easy.  Outside, I'd use a piece of radiator hose and hose clamps to mount it.  A pod air filter would clamp right on as well.  Maybe some day...

A slightly bigger carb could provide slightly better performance if jetted right.  The stocker on my flatty is pretty restrictive.  But I don't think the rest of the motor really can use anything bigger than 24-26mm.  Bigger isn't always better unless the engine is built for it.  (paraphrasing Doc here). Cool

Absolutely. Too much carb can be just as detrimental to performance as too little carb.
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PostSubject: Re: Dual Carburetors?   Dual Carburetors? Icon_minitimeApril 8th 2015, 12:50 pm

Yeah, I think in general, our "utilitarian" motors are well carbureted for optimum performance from off-idle through 3600rpm. That's what they're built for.

What's "optimum performance"?? Well, from a manufacturer's point of view, there are 3 separate attributes to "optimum". It's a mixture of Most power output, Best fuel economy, and Lowest emissions. These three attributes are not achieved from the same cam, carb, exhaust, timing or jetting. It's all a compromise.

So, when making an engine that was designed this way into an engine built for highest power output, we sacrifice things like low RPM performance and fuel economy (as well as emissions, but nobody really cares about that here right!!! haha)

Twin carbs might help a motor with high rpm output, but low rpm throttle response would suffer (as Doc said) because there's less vacuum signal to pull fuel from the jets and because there's lower velocity in a bigger tube.

Like we're saying, A bigger carb might help a little bit, but the motor won't be able to take advantage of the increased intake capacity without swapping for a bigger, longer duration cam that's made for higher rpm.

I tend to look at the big picture when re-carbureting an engine. How much HP do you want to make? Say, for instance, my 12hp 28" Flatty might be able to push 15hp. Then, I'd look for a carb from a 15HP motorcycle. Chances are that the diameter will be close to what's going to work for the valve size and intake manifold size.

So, taking my own advice, I will look for a motorcycle carb from an older 250cc 4-stroke motor or an older 125cc 2-stroke motor (non-competiton) and I'm somewhere near the right formula. This is where I'm drawing my "educated guess" from and thinking of a 24-26mm Mikuni TM carb.

Hope this helps to clarify some things. FWIW: I wouldn't waste my time swapping to another venturi type carb from another B&S. A straight bore carb from a motorcycle is designed more for performance.
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