what size carb to run? stock vs slide carb?

M

MasonIsGreat35

Member
ive not learnt much about carbs and wondering if anyone could supply me with some info

ive just built a 196cc clone and im still running the stock carb but with some larger jets
motor specs
196cc block 14cc head
stainless valves 24/25
18lb springs
28degree pvl flywheel 4degree offset flywheel key (32degrees combined)
10.8:1 compression
stock cam/pushrods/lifters/rocker
current carb is a stock carb with 140 e tube, .038 main .020 pilot

how do i calculate what size carb would be ideal for the motor ive built
whats the difference in larger mm carbs like 32mm vs a 28mm carb, im assuming it allows more fuel?
what is the benefits of a aftermarket slide carb versus stock carb, is it more tuneability or better fueling?
thanks
 
Smaller carb has less total volume at a given RPM, but has more velocity. Higher air velocities improve both throttle response and fuel atomization. But, depending on the build, it might not have enough total volume, so at higher RPM it just won't be getting enough charge to make it's full potential. Not that it will be lean, as the fuel can still be the proper ratio with the air. Just that the engine could use more charge. IE: fill the cylinder more at those higher RPMs.

Large carb has higher total volume at a given RPM, but generally has less velocity. The trade-off will be the loss of some low end to midrange power as well as throttle response because of the lower velocity. Now, there are some trick ways to get a larger carb to keep velocity up while still maintaining that higher volume. Which helps alleviate those problems that come with a larger bore. Which generally gives you moderate to good mid range while really shining on the top or higher RPMS. There are extremes here and more of a general broad view.

The ticket is matching that to your build, most experienced builders flow heads and carbs together to get the numbers they're looking for. It all depends on the work you've done to the head and how much total volume the engine needs to perform. There are calculations that can be done to determine the volume needed at a given RPM.
 
1. A good place to start is by measuring the vertical or horizontal cross-section of the inlet of your port. Most will measure around 22-23mm unless they have been ported. The bore of your carburetor should be about 100-110% of that measurement. Since you have a stock cam and a 25mm intake valve, a 22mm carburetor is what I would recommend. You'll run out of camshaft and cylinder head flow before you run out of carburetor.

2. 32mm and 28mm is just airflow. The fuel circuits of the same brand/model are often identical or use the same mechanisms and can be adjusted for tuning—no reason to use either with your build.

3. There are a lot of differences between stock and slide carburetors, but one of the most crucial is the EPA regulations of stock carburetors that restrict tuning. A slide of equal size will flow better, and you have more options that better fit your application to make more power.
 
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