dual carbs GX200

#41
IMG_20181224_142208185.jpg


You guys are funny and inspiring at the same time..

I put my manifold 'under the knife' yesterday, getting rid of the choking
aluminum hose connector that measured just 20mm ID.... I spliced a piece
of 32mm into the T. I surprised myself with a solid fit and now full 32mm flow.

That made a difference. Started fine but reved up a bunch until I screw both idle speed
screws out maximum, then it idles 21-2200 rpm. I didn't run it much after that but thinking
that re-jetting is in the cards, I hope to road test it today.
Have a fine day, thanks for all the feedback.
 
#42
View attachment 237556


You guys are funny and inspiring at the same time..

I put my manifold 'under the knife' yesterday, getting rid of the choking
aluminum hose connector that measured just 20mm ID.... I spliced a piece
of 32mm into the T. I surprised myself with a solid fit and now full 32mm flow.

That made a difference. Started fine but reved up a bunch until I screw both idle speed
screws out maximum, then it idles 21-2200 rpm. I didn't run it much after that but thinking
that re-jetting is in the cards, I hope to road test it today.
Have a fine day, thanks for all the feedback.
Sounds like an intake manifold leak. Sucking air from somewhere other than the carbs and the idle circuits have to supply excess fuel to compensate. I'll guarantee you will get more performance from what you did by eliminating that choke point.
 
#43
If you guys hadn't pointed out choke points,etc. I would have probably retired this project already,
but now it seems more hot-to-trot than ever after adding a 1/2", a lot, to the manifold.
It will be fun to try it today.
 
#46
My dual carb experiment is over after testing today, here are a couple observations.
1. I think the T-shaped manifold is the worst shape, because once one of the carbs is opened up,
the air flow is so established that the other carb stays "starved" for air when activated.
2. I think opening both at the same time distributes the intake air between them and to me
gives a short, minor improvement in power in a narrow range of lower rpms.* Purely a subjective opinion.*
3. All in all, like you guys said, it's not worth doing, it doesn't work, at least not with a T manifold and 24 flatsides.
Maybe a different configuration would work out like you guys noted.
It's been an interesting try and I'm grateful for your input and encouragement, it was fun. RIDE ON.
 
#49
I've managed to get 10" from intake valve to the carb mouth,
but I ordered some stuff to extend it on out a few more inches.
I'll send pics.
I'm gonna predict you'll have mixed results with this. This tuned intake might give you a slightly higher peak in the powerband, I'd say 0.5HP at best. But the thing is, you'll start to run into a problem that automakers ran into doing the same exact thing you plan to do, especially at idle and low-end power. Atomized fuel will not want to travel that long before it falls out of suspension with the air and just flows like a river on the bottom of the intake. Think of it like this: You have the mist setting on your garden hose. You can spray it 6" and it will stay a mist very easily. Now try to spray it 12 feet. By the time the water reaches the 12 foot mark, it's not mist anymore but a small creek in your driveway. This problem leads to inaccurate air/fuel ratios and misfires due to the spark not reaching the fuel.

Now modern automotive engines have huge plastic intake manifolds with long runners. Some even have multiple runners that are switched by a butterfly valve to achieve two or more runner lengths for intake tuning at different rpms. But how do those engines keep the fuel atomized? They spray the fuel out of fuel injectors directly behind the intake valve (in the case of multi-point injection) or directly into the combustion chamber much like a diesel (in the case of direct injection). This allows them to have a tuned intake runner length AND keep the fuel atomized. This is one of the reasons why carburetors and THE reason why throttle body fuel injection is now considered obsolete in the automotive world (don't get me wrong, I love carburetors, it's just that designers don't).

Now if you're gonna take on this task, my advice is to keep the intake runner diameter/area very consistent throughout the system to avoid drops in intake velocity that pose a danger of de-atomizing the suspended fuel. Also, talk to @ole4 as he is very well-read on engine physics.
 
#50
Cool, the analogies are clear and advice helpful my friend.
I remember reading on Mikuni's website that their carbs liked
straight intakes but nothing on length. When I use one formula
it says the length should be around 15"!
 

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