Dual Carbs!!

[I like karts]

Just wanted to share that I'm working on making a dual carb setup for a 3hp tecumseh flathead. Yes, I know it's not the best engine to do this to, but My friend is building a go kart and this is the only engine he has available, and with a bit of tweaking, it should work well.
As for the setup, I got the idea when I looked at the stock carb on the thing and thought "wow, that's really restrictive," first off because the intake manifold bore (and carb bore as well) is micro, and because the intake charge has to flow basically 90 degrees, straight up, then another 90 degree turn to enter the intake port. With a little relocation of the gas tank, I will weld up a Y pipe straight shot into the intake, which should help flow lots. As for the carbs, I have sort of a little collection going of all sorts of small walbro carbs off of 2-strokes. The one I plan to use for the primary carb has about a 10mm bore, and the secondary I will be using (a home modded one I bored out ) is 13mm. I plan to have wire stops on both carbs, and have a little slack in between the two, so up until about half throttle, only the primary is being used, then when you really hit the gas, it pulls out the slack and opens both for a total of 23 mm of carb! (and hopefully better performance). I also am planning to de-burr the ports, and weld up a nice slightly-muffled header for it. Should turn out well. What do you think?

 

[crazycart]

I think its will be better over stock, but wont do a lot because the intake port is small to start with. Its fun to mess around with this stuff though. Need pics

 

[jr dragster T]

Not very effective. 3 horse isnt much to start with. The reason why the venturi on stock carbs is small is for more velocity. Where the air comes in it's big. Then the venturi shrinks. Then gradualy gets larger. This is for velocity. It helps atomize the fuel better. All carbs have this effect.

 

[I like karts]

Yeah, I'm going to mildly port and de-burr it to help with that (to crazycart)

Not very effective. 3 horse isnt much to start with. The reason why the venturi on stock carbs is small is for more velocity. Where the air comes in it's big. Then the venturi shrinks. Then gradualy gets larger. This is for velocity. It helps atomize the fuel better. All carbs have this effect.

I know that. I'm not talking about just the venturi being tiny, I mean the whole carb is tiny. Its probably like 16mm max.

 

[jr dragster T]

Yeah, true. Too much volume of air flow and the engine will just drown it self with air.

 

[I like karts]

but there will be more air and gas...

(It probably will take quite a bit of tuning to get it right though :/

 

[Russ2251]

CFM of carbs cannot exceed what engine is capable of.
In other words, if engine pulls 9 CFM and carbs are rated at 15 CFM then, as Mr. Tyler suggested, engine will run lean.

 

[I like karts]

hmm....

well, you might be able to compensate for that by making the secondary open later..

I'd still really like to make it work though. It would be pretty freakin cool IMO.

 

[Russ2251]

Actually, I'm working on the same idea.
That is, once I have the math down.

 

[modelengineer]

A single good, proper sized carb will give just as good performance (probably better), will be cheaper, will be easier to tune, and you don't have to balance two carbs.

Just curious as to why you would bother?

 

[Russ2251]

Just curious as to why you would bother?

Same question was asked of a famous mountain climber.

 

[I like karts]

It would be interesting to try, and you could say, "yeah, I've put dual carbs on an edger engine before..."

 

[modelengineer]

you could say, "yeah, I've put dual carbs on an edger engine before..."

I thought that would be it. Bragging rights

But if someone told me they did that, I would be like "O...K... but why?"

 

[Jerryburger]

Doing the primary/secondary set-up might increase the quality of throttle-response, but its hard (without forced-induction) to get more into the motor than what it can draw in on its own.

There was a thing I read somewhere (Russ' favorite term!) that on a 4-stroke engine, you can figure out about what CFM an engine will require by multiplying it's cubic inches (divided by half) times max rpm (honest max rpm, not fantasy) times .75 which is a pretty honest constant for volumetric efficiency.
So, for your Tecumseh, which I THINK is 7.75 cubic inches it'd be:

7.75 x .5 = 3.875
3.875 x 5000 rpm = 19375
19375 x .75 vol eff. = 14531.25 (cubic inches pulled @ 5000 rpm)
To get CFM, divide by 1728... 14531.25 / 1728 = 8.48 CFM.
So Russ was right on with his statement of 9 CFM..... BUT! .75 volumetric efficiency is an average, and I'm sure flatheads fall towards the lower end of volumetric efficiency! (Otherwise, the latest automotive technology would still use flatheads 50 years after their demise!)
What ISN'T easy or possible to figure out is what the rated CFM of a given carb is without the manufacturer publishing the specs. Mikuni lists their carbs in mm of throat diameter, but that really doesn't give a guy squat to figure flow capacity.

If someone has a formula for that, please publish it!

 

[MDMike]

Great advice Jerry!
In order for an engine to work properly it has to be "matched" right.
On "stock" type engines not all out race engines, the primary is small, the secondaries are large.
2 reasons.
1) Fuel economy - doesnt or should I say SHOULDNT apply here.
2) It allows the engine to make torque at low end, rev up before it gets a big gulp of fuel, also making for better throttle response.

And yes 2 carbs are sometimes a bear to keep synchonized all the time BUT it would be cool to say "Hey look at This!"