Maybe try a 2.5 or 4.5 PV to delay the PV opening point. It may be opening too early, hence the rich low speed cruise. Should not affect your WOT afr.

My afr's, can differ a bunch based on driver settings. If its rough, and my drives are tucked cruising, dragging some tab, I'm at a much larger throttle opening at 3500, than say, light load, drives up, tabs up, at 3500. The PV opening point comes into play here.

I have my port engine still struggling to see good A/F, starboard is not at 11.8-12.0 ish all the time, port still high 10/'s to low 11's, nothing I am doing seems to change that, Gonna jet down a little more and see if anything happens... swapped O2's just to be sure its accurate also...

Sounds reasonable enough.


I also noticed a significant difference as the air temp dropped into the evening. From what I understand the PVs open more under load and accelerating but pretty much close off once wide open or under (relatively) light load? Correct? It seems that currently with the 6.5s they're pretty much open off idle fattening up the whole midrange.

Power valves are full open at wide open throttle and under heavy load.( if the proper size ) I think you have to high of a power valve. Try a 4.5 and then play with the jetting. What I have done is remove the linkage to the secondaries and lock them closed. adjust the idle first then with the proper power valves now start playing with the primary jets for the AFR all the way open with the primaries. So now you have the primaries jetted properly. Now re connect the secondary linkage and figure out what your secondary jetting needs to be. The procedure always worked for me.

I just changed to EFI and love it.

I plan on switching to EFI in the winter if the O2 sensor survives this season. What system are you using?

Only problem I see with that, is most carbs are progressive linkage, to where the primaries aren't "fully" opened, when the secondaries "begin" to open. Standard quick fuel carbs are 40% primary open when secondaries open, or if you switch it to "super progressive", they are 60% primary open when secondaries open.

I would stick a vacuum gauge on the engine, and make note of where the vacuum readings are , as well as AFR's, at a specified RPM. For example

3000 RPM= 7" vacuum 12.5 afr

3500 RPM= 5" vacuum 11.5 afr

4000 RPM= 3" vacuum 12.0 afr

and so on.

The idle vacuum divided by 2 for power valve selection in a boat, imo, is useless. Vacuum reading higher than PV stamping, means PV is closed. Vacuum reading lower than PV stamping, means PV open. Although, keep in mind, they aren't perfectly precise.

IF you're happy with your WOT AFR numbers at this point, you have the total amount of fuel sorted out. Now, you need to work on WHEN.

They stay open once your intake vacuum drops below the PV set point (6.5, 4.5, 2.5, etc).

In a car, most of the time, the PV is closed, unless, your are heavily accelerating. In a boat, they can be open, at a cruise rpm. Mainly, because boats almost never run in "high" vacuum conditions.

This is one of the reasons, I don't care for boost referenced PV's in roots blower engines in boats. Pretty much, you're PV is simply ALWAYS open, whether you have a 8.5, or 2.5. Unless you're running a really light cat or something like that. I just bought new "standard" 850 marine quick fuels for my twin carb roots setups. The carbs are basically identical in every way as the "SC" versions, except for the boost referencing. Also 200 per carb cheaper, and when you're buying 4 of them, it adds up.

With a roots, at 3500RPM, my boat is already at 0-2psi of positive pressure in the manifold. So any PV is gonna be open if it sees that, which it would if its "boost" referenced. However, being NON boost referenced, the PV sees the signal on top of the blower, which is different than what is going on in the intake. So, while I may have 2psi in the intake, the carbs are seeing 6-8 inches of vacuum lets say at that point. Now I am in PV tuning territory. Meaning, I at least have a window of fuel tuning, that you do not have, with boost referenced carbs in boats.

BUT - IF you do as I said you will have the primaries and the secondaries at the mixture that you want. IF you try to adjust with the linkages hooked up you might be leaning out the primaries when it is the secondaries that are causing it to be rich. This method has always worked well for me.

I always ran the primaries just perfect and the secondaries just a little rich. That way when I was just cruising I was running best efficiency and when I got on it it would fatten up to a safe level.

People have been testing PV's for some years now and you'd schit yourself how inaccurate they are.