Re: 40 hp to 50 hp
Actually, I mis-spoke when I wrote venting because the venturi dip tube is air bled from various points in various carbs to balance fuel flow, and on some carbs, you can see a tiny air bleed hole in the side of the manifold side of the venturi. Very small and you need to know it's there to see it. Some have an air bleed in the front portion of the venturi. Some of the newer carbs have an air bleed JET in the side of the casting. Not having the carbs in front of me, I can only state that in the mid-sized engines, The venturi is either 1 3/16, or 1 5/16. The fuel Jet only functions to limit fuel flow at or near full throttle, since as I said before, one of the properties of a venturi is to get richer as airflow increases through it. Thus, at full throttle, without a jet, the engine would run too rich when set up to deliver the correct amount of fuel at mid throttle.
For example, the 75 Chrysler uses three 1 3/16 carbs while the 85 and 90 with the exact same bore and stroke uses three 1 5/16 carbs. HOWEVER, The 75 has a lower compression ratio, and the ports are not only cut lower in the cylinder but smaller in size. Changing to three 1 5/16 carbs will not yeild much more horsepower at all--maybe a couple, but that's it. Remember, as I said, these engines are carbureted to almost their limit. (and it never fails to amaze me that they will accelerate without an acceleration pump like auto carbs.) Remember also, with these carbs, you are not talking a great difference in CFM. It's not like you are going from 350 to 650. If you look closely at the different carbs, in this case, you will see that the butterflies are slightly larger for the 85 carbs. They actually bore out the back of the carb right up to the venturi. However, with the smaller exhaust ports, I believe that airflow through the 75 is limited by the engine, and not the carbs. Thus the larger carbs will give a little more power due to less air restriction, but will not deliver significantly greater volumes of air because the engine is not capable of pumping it. So, you may see better low end acceleration but top speed will not change much at all. BTW: While the 90 is ported the same as the 85, in addition to higher compression, the 90 also has a lower gear ratio to let the engine rev a little higher and get the extra 5 HP. So: putting 90 carbs on an 85 gives you 85 Horsepower. If I remember correctly, the 75 produces its rated power at 4750, the 85 at 5000or 5250, and the 90 at 5500.
Now, I have ported an 85 block, installed a 90 HP head, 90 lower unit, and three carbs off the 100 HP engines. Originally the 85 pushed the boat at 34-36 MPH. after the work, it pushed the boat at 42-45. There, I am possibly reaching the potential of the larger carbs. And, even on this engine, when I go from 3/4 to full throttle, the engine makes a lot more noise, uses a lot more fuel, and only goes about 1-2 MPH faster. So, that tells me that the engine work was more important than the carb change in making horsepower.
And don't ask how they vary the compression ratio because I don't know: the heads all look alike. However if you see the charts. Generally (with a few exceptions) as the engines increase in horsepower, the compression ratio goes up.
I think the thing you are missing is that you are talking about basically a stock engine that was designed to deliver a rated horsepower, and thinking about engines set up for racing. With just a carb change, you will get an increase in Horsepower, but it will in most cases be so small that you will not notice it during operation. Remember: drag goes up by the cube while horsepower increases linearly; it takes roughly 4 times the horsepower to double speed. Yeah, play with your 40 a bit and you will get 50 out of it, but it's going to take more than a carb. Let's not forget reeds, but I don't want to get into them because they are most likely the same on the 40 and 50.
Also think about this: On the old Chrysler 318 engine with a 2 barrel, it delivered (I think) 180 horsepower. Put on a holley 4 barrel and you only got about 10 or so ponies more. And this was on a 5 liter engine. Might make 1/10 second less at the track--16.9 versus 17 --Not significant at that level.
If you really wanted to get the potential of the 4 barrel, you needed to install 340 heads with the bigger valves (ports?)
