I agree. It should be the same pressure at both carbs.


There isn't enough line there to cause a pressure drop from primary to secondary, and unless you max out the pump volume wise, there isn't going to be an issue.

The main reason for a regulator is to bring the fuel pressure down to and acceptable rate so you don't push fuel past the needle and seat and flood the bowls.

I agree that a lot of people think that bigger is better in carburetion, but some engines like CFM (air flow) and with proper jetting (fuel flow) can get good results with a large carburetor.

My Hondo has a relatively mild BBC with two 600 Holley vacuum secondary carbs and my plug readings look good.  But with the vacuum secondaries working properly, I probably rarely get full throttle. 

CFM is not adjustable without modifications to the carb, but jetting is fairly easy to adjust for tuning.  I'd bet that many jet boats could stand to be jetted down a little without the chance of running lean, and doing damage.

there was a thermostat housing on e-bay last night

But, shouldn't you regulate the amount of fuel from the fuel pump to the carbs so as to not pump more fuel through the carbs than needed and therefore have unused fuel dumped out through the exhaust?   I have read that most people seem to think big CFM numbers means more power and in reality it just means waisted gas.  Most people over size the carb, be it car or boat.

There is a needle and seat on the carbs - think about the toilet in your house. Same thing. You have 60-100 PSI going to the inlet of the toilet. As long as you dont overpressurize the house to 200 psi, water will not flow unless the bowl gets low (like when you flush).

A carb is the same thing. The fuel is pressurized to the float bowl, which acts as a mini gas tank. There is a needle and seat which shuts off the inlet flow of gas when the float bowls are full. As long as the fuel pressure doesn't push the needle open, it will work as planned. Above 9-10 PSI this can get pushed open. A mechanical pump and some of the lower flow electric pumps are preset to pump a maximum of 7 psi.

For the most part, changing PSI will not alter your jetting (there are some exceptions I'm not going to get into now). Jetting is where you are either being rich (dumping fuel out the exhaust), correct, or lean (melting a hole in your piston from the lack of fuel and the heat it creates).


There is more to carb CFM and requirements than just RPM. Intake manifold, head cfm, valve diameter, cam duration, all play a part. I have a 454 that needs 450 cfm max - while one of my 440's needs around 900 and the other 440 needs 1700. Factory 440's with a 5500 rpm shift point came from the factory with 1300 CFM. The same engine also came with a single 625 CFM 4bbl.

If you run a single plane you should run a smaller carb than if you have a true dual plane. This is because the engine only sees half of the carb. If you run a tunnel ram, the manifold makes the head flow better, so you need a larger carb.

If you are running a dual plane and want to experiment, try different open carb spacers (1/2, 1 and 2 inch). This will allow the engine to "see" a bigger carb, and make your intake look bigger. If you gain RPM, then you know you needed more CFM (or a different runner length).

I'm leaving some stuff out so this post isn't too long....but you guys get my drift..