Baja Rooster

So then my next question would be, and it seems like what the OP may be dealing with, is figuring out when the secondaries PV starts influencing things?

SB

AT 6.5" intake vacuum. Same time as the primaries.
He has 6.5 pv's frt and rear.

Baja Rooster

But doesn't that event happen later for the secondaries after the butterflies open, or is it more off of the manifold vacuum then the individual venturies? Sorry for the daq's. Carbs have come a long ways since I've played with Q-Jets on small blocks.

MILD THUNDER

Yes. The power valves, when opened, feed extra fuel to the main well, same place the fuel from the jets goes to.

Power valves are somehow looked at as bad things. They are nothing more than a fuel switch in a way, that is adjustable in two ways. First, vacuum, second, power valve restriction size.

Having power valves, and knowing how to tune them, can be very beneficial, if you are really looking for a good fuel curve.

Power valve opening points, have nothing to do with idle "richness" , as so often said. If a power valve was opened at idle, it has no effect on the mixture at idle. Same way installing bigger jets, has nothing to do with idle. That recommendation of choosing a PV opening point, of half the engines idle vacuum, is better suited for cars, than boats. I think the term "cruise" is very vague. A chevelle "cruising" down the freeway at 55mph, can do so almost entirely on the idle circuitry, as it takes very little horsepower to keep the chevelle going steady 55mph. Now, a boat, to "cruise" at 55mph, might take 300, 400, or more HP to "Cruise" at 55mph. Hence the fact the throttle blades are opened way more than they would be in the chevelle. This is also why most car carb's, need to go to exactly what the OP is finding. In that, the typical jetting, isn't working. I have seen quite a few boats, that need to up the primary jets substantially, and lean the secondary jets substantially, to get the fuel ratios in line.

I myself, would like to play with 1:1 linkage, and square jetting and see how that works in boats. In theory, it should work better, and provide a more even fuel distribution while underway. Problem with the OP's setup, is that while it may provide the right AFR on his meter, his distribution to the rear cylinders could suck. Why? Because he's gonna fuel the primarys with a PV and 90 jets lets say, and in the rear, say no PV, and 86 jets. So, most of his fuel is coming from the primary side at wide open throttle.

vintage chromoly

As has already been pretty much covered, "cruise" in a boat is way different than "cruise" in a car.
There really is no light load/cruise in a boat. This is why you won't see vacuum advance on a boat engine and why boat carbs are generally jetted richer than car carbs.

I've been playing with an old carter AVS on my old car and the power enrichment system is pretty cool on those as its really tunable with different "step up" springs and metering rods. Kinda off topic as you don't genrally see carter/Edelbrock carbs on hotrod boats, but the enrichment requirements under heavy load apply to both.

MILD THUNDER

I remember playing with quadrajets back in the day. Different metering rods, hangers, stuff like that. They were actually really good carbs in their day, and quite a bit more sophisticated than your typical holley. I had them in several vehicles. One was a 76 Cadillac Coupe deville with the 500ci. That thing ran like efi.

My 78 Trans Am, I had built the stock 400 up a bit. Thought the holley 750 was the hot ticket on it. After trying to get it dialed in, I gave up and put the stock quadrajet back on. Car picked up 2/10ths, and simply ran better all around. That was before o2 sensor days though

Bill 3

Mild Thunder helped me get my carbs to where my idle circuit was set up correctly. THANK YOU!. My primaries were so far open to get it to idle that it was in the main circuit, when put in gear it would flood out because the power valve would open. When I got it they has put in 2.5 PVs to kep this from happening.

I have thought about this a little. A lot of fuel in the front vs the rear (or even the other way around) doesn't sound like the best way to go. It's not like we want to be lean anywhere.

MILD THUNDER

You wanna be the guinea pig?

https://static.summitracing.com/glob...HLY-20-122.pdf

https://www.summitracing.com/parts/h...a/instructions

Baja Rooster

That's why you have to go faster - to push the mixture back into the latter cylinders.

Is fuel distribution as much of an issue on a SC setup? More critical for sure, but does the positive airflow help distribute the mix around better?

SB

Joe has a lot of great carb info, however I feel 1:1 will be horrible . Throttles will be too sensitive (oops, didn't want that much power - think docking at etc) and air speed will be down when starting to accelerate....will take a bit for the boosters to get going.

And if I'm wrong, which is very possible, what do you do about the accelerator pumps ? Every touch of the throttle will be a ton of fuel.

IMHO - let's get this thing to work as a normal carb before trying other more unconventional things.