Post by humanshield on May 20, 2015 12:29:13 GMT -5
Silar, we may not be thinking about the same thing. The only carburetors I know of with no venturi are called "pumper" carburetors. But as far as I know, all scooters utilize carburetors with venturis. Well, there's orifice plates but that's irrelevant here.
On a 18mm carb I think the minimum venturi restriction is 16mm or so. On a true 20mm carb, the smallest venturi restriction I believe is 18mm. Either way, machining the inlet and outlet tube sizes is not the correct way to upsize a carburetor. It may actually decrease performance.
When carburetors are designed, a LOT of science goes into it.
If scooters to go has such flow equipment to accurately measure the changes to carbs then I would say their results are backed by proof.
I wish it was this easy. No matter what you do to an 18mm carburetor, you will still have an 16mm restriction in the carburetor. It is not going to flow to the potential of a 20mm carburetor.
You might increase or decrease the vacuum slightly by modifying it, but I say these carbs were designed under manufacturing processes and with testing equipment noone on this forum can touch.
They probably flow as good as they can (specific to the venturi) right out of the box. In fact, without adequate (expensive) flow benching equipment, it's guess work and you have no real clue what the changes will do.
So whether or not a modified 18mm carburetor is sufficient is the question. Below I will illustrate why it probably cannot be...again...depending on your expectations.
When you get into performance stuff, it's not easy to say what will or will not work based on someone elses results because professional tuners will tell you that every engine is different (even if it's the exact same build and motor) and that tuning is all about adjusting for a particular engine's needs.
I look at it this way.....
The engineers who designed the QMB139 calculated the best carburetor size for the compromise they intended. Fuel econmy / power / rideability and they settled on an 18mm carb for the 49cc.
For the 150cc carb, 24mm seems to be the chosen size. 30mm for 250cc
The 150cc engine is 80% larger than the 83cc engine and the 83cc engine is 60% larger than the 49cc engine.
The important thing to note is that the engineers did not choose 18mm for all those engines for a very good reason. While the 18mm carb "might" work on a 150cc engine, it would not be ideal.
Just going off the engineers math, we'd expect to need a proportionately larger carburetor for a 60% larger engine....to keep the flow within the ideal range.
Just based on the design engineering for these engines...here's a pattern...
The bold figures are typical specs chosen by the design engineers. The rest are interpolations.
Notice that a 1mm increase in venturi size for every 20cc's (give or take) seems to be a very clear pattern.
18mm for 49cc
19mm for 72cc
20mm for 90cc
21mm for 110cc
22mm for 120cc (give or take)
23mm for 130cc
24mm for 150cc
25mm for 170cc
26mm for 190cc
27mm for 210cc
28mm for 220cc (give or take)
29mm for 230cc
30mm for 250cc
So wouldn't it be reasonable to interpolate that for 83cc a true 19mm or 20mm carb would logically be the best choice?
But wait, then you throw in a Big Valve Head and Cam ....and those changes put the ideal carb for a 83cc engine with BVH and cam right dab at 20mm as I see it. Not 18. If I'm wrong, by all means explain.
This is why I have sought a true 20mm carb so hard. For my build and goals it seems to be the right choice. Again, YMMV.
If the demand for 20mm carbs ever gets high enough, I suppose someone will then make one.
These are just my thoughts and observations. Please note that I do have some engineering background if I seem anal about it. Sorry. Obviously you may get exactly what you are looking for with any carb you get. That's up to each individual. I'm personally willing to sacrifice a bit of low end for a better top end since cruising at 45-50 is my goal with my scooter. With that goal, the true 20mm carb or even a 21mm carb makes more sense than an 18mm carb (albeit modified)
Your goals may vary of course.
No matter what, please share your results.
On a 18mm carb I think the minimum venturi restriction is 16mm or so. On a true 20mm carb, the smallest venturi restriction I believe is 18mm. Either way, machining the inlet and outlet tube sizes is not the correct way to upsize a carburetor. It may actually decrease performance.
When carburetors are designed, a LOT of science goes into it.
If scooters to go has such flow equipment to accurately measure the changes to carbs then I would say their results are backed by proof.
I wish it was this easy. No matter what you do to an 18mm carburetor, you will still have an 16mm restriction in the carburetor. It is not going to flow to the potential of a 20mm carburetor.
You might increase or decrease the vacuum slightly by modifying it, but I say these carbs were designed under manufacturing processes and with testing equipment noone on this forum can touch.
They probably flow as good as they can (specific to the venturi) right out of the box. In fact, without adequate (expensive) flow benching equipment, it's guess work and you have no real clue what the changes will do.
So whether or not a modified 18mm carburetor is sufficient is the question. Below I will illustrate why it probably cannot be...again...depending on your expectations.
When you get into performance stuff, it's not easy to say what will or will not work based on someone elses results because professional tuners will tell you that every engine is different (even if it's the exact same build and motor) and that tuning is all about adjusting for a particular engine's needs.
I look at it this way.....
The engineers who designed the QMB139 calculated the best carburetor size for the compromise they intended. Fuel econmy / power / rideability and they settled on an 18mm carb for the 49cc.
For the 150cc carb, 24mm seems to be the chosen size. 30mm for 250cc
The 150cc engine is 80% larger than the 83cc engine and the 83cc engine is 60% larger than the 49cc engine.
The important thing to note is that the engineers did not choose 18mm for all those engines for a very good reason. While the 18mm carb "might" work on a 150cc engine, it would not be ideal.
Just going off the engineers math, we'd expect to need a proportionately larger carburetor for a 60% larger engine....to keep the flow within the ideal range.
Just based on the design engineering for these engines...here's a pattern...
The bold figures are typical specs chosen by the design engineers. The rest are interpolations.
Notice that a 1mm increase in venturi size for every 20cc's (give or take) seems to be a very clear pattern.
18mm for 49cc
19mm for 72cc
20mm for 90cc
21mm for 110cc
22mm for 120cc (give or take)
23mm for 130cc
24mm for 150cc
25mm for 170cc
26mm for 190cc
27mm for 210cc
28mm for 220cc (give or take)
29mm for 230cc
30mm for 250cc
So wouldn't it be reasonable to interpolate that for 83cc a true 19mm or 20mm carb would logically be the best choice?
But wait, then you throw in a Big Valve Head and Cam ....and those changes put the ideal carb for a 83cc engine with BVH and cam right dab at 20mm as I see it. Not 18. If I'm wrong, by all means explain.
This is why I have sought a true 20mm carb so hard. For my build and goals it seems to be the right choice. Again, YMMV.
If the demand for 20mm carbs ever gets high enough, I suppose someone will then make one.
These are just my thoughts and observations. Please note that I do have some engineering background if I seem anal about it. Sorry. Obviously you may get exactly what you are looking for with any carb you get. That's up to each individual. I'm personally willing to sacrifice a bit of low end for a better top end since cruising at 45-50 is my goal with my scooter. With that goal, the true 20mm carb or even a 21mm carb makes more sense than an 18mm carb (albeit modified)
Your goals may vary of course.
No matter what, please share your results.
