PVCRs are .070.

I'm using J-tubes to make there, err, USCG compliant.

Reading up on these carbs in particular, they do seem to want a lot of jet.

Going to methodically work through the configuration... starting at the IFRs to fix the super-rich transition circuit and go from there. May even consider a PV in the secondary. Will have to test at each point.

Fuel pressure was good. I was standing at the back of the boat with the hatch off watching.

Might have to give this a try - easy to fab up for a test.

From my experience, IF a typical holley carb with regular vents needs to have say a 82 main jet to reach target afr, with J hooks that same carb may need a 88 or so from negative pressure in bowl screwing up the jetting and IF has the crossover tube like I just pictured, will be the opposite, that 82 would go down to about a 76.
Just wrapped up a dyno job for a member off here larose481, a pair of rebuilt HP500 mercs. he switched to promaxx 290 ovals, edelbrock performer rpm intakes, stock carbs off the HP 500s. We were at our target afr on tailpipe and had reasonable enough cylinder balance with 79/80 jetting in front and a 88/91 jetting on rear. After putting the vent on (j hooks were removed before dynoing), our afrs dropped a point or so. Had to go to 74/75 front and 84/90 rear to get back on target and get power back at same air denisty

How/Where are you measuring your AFR numbers. Only on one side of each engine? Or both sides of each engine. Have you done steady state cruise at a given rpm for several minutes, killed the ignition and checked the plugs. ( I do realize for many set ups this is no t always possible.)This will give you an idea of how equal the distribution is in each cylinder. Also if your checking port engine in one manifold, and starboard in the opposite manifold could explain your differences in readings between the two engines.

Okay, looks like most everyone selling those tubes now.
Referred to as

Holley Bowl Vent Tube Crossover Off Road Truck

AFR readings taken on port side of each engine. O2 sensors are installed in the tailpipes a few inches down stream from from the flange where it connects to the header elbow.

I have not done any real steady state runs for given rpm though have let it settle before going up to the next rpm point. We're testing in a small area and tend to run out of room. On the days we've gone out, the weather wasn't cooperating so this has been the compromise. But, even if I was out in the Chesapeake, anything over 3500 under load is getting to the point where meltdown could happen. It's just too lean above 3000 rpm. So I need to understand why everything begins to go so lean when increasing RPM.

I agree about checking plugs - need a fresh set to test with as these are a bit on the done side.

When I first talked about using these carburetors, there were some comments about it being pig rich if I increased the jetting over stock (stock with BGs emulsion was 86 PMJ/93 SMJ, .030 emulsion in 1-3-5, and .070 IAB, .028 MAB). It was lean from the get-go and needed 88s and 96s to run but still produced a lean pop at peak torque. And now, here we are.

So J-tubes would increase the jet requirement or reduce? At first the explanation was that it would dictate a need for more jetting, but then it looks like you had to take jet out of larose481's once the J-tubes went back on.

I have the original vent tubes to try to se what we get. I can definitely understand the effects the moving air would have on the J-tubes (and its effect on the float bowls). My tubes have breaks mid-way that should negate the effect but by how much is an unknown.

In your testing over the years, have you come across a fairly mild build, 500 cube, 600-ish HP setup needing so much jet? 91s with a 070 power valve on the primary side (and still be lean into the secondaries) seems seriously excessive, don't you think?

On laroses, the J tubes were removed before ever dynoing and never put back on. My carb expert /dyno partner Norm has said for years the J tubes impart a vacuum on the bowl, disturb the fuel in bowls and to the jets and cause the need for carbs WITH them to have jets approx 5 numbers larger to yield the same AFR, especially at higher rpms. Since they are prone to cracking and falling off, he always removes them. So simply removing them and putting regular straight tubes in carb body richens the tune on same engine. In Laroses case, he also removed them BUT felt the need to have something on the carbs for his customer. So he brought me both engines to dyno and ordered those "truck" tubes pictured BUT we started dynoing without them. I had tune pretty much dialed in on engine one and the truck tubes were scheduled to be delivered the following day so we left engine one on the dyno to see what happened when they went on. Well it was a good thing we did because the afrs plunged pretty much a full point, we were still debating how to get #6 cylinder out of the high 13s too as it was the leanest BUT paired to 7 which was the richest. We had afrs ranging from 10.5 to almost 14-1 and tailpipes were 1.7 points apart side to side from all the lean cylinders on even side. So surprisingly the truck tube not only richened the tune overall even at part throttle, it also closed the gap some between leanest to richest which was a plus. so I ended up pulling 5 jets sizes out of the primary's just to get back to the averages we previously had ( from 79/80 to 74/75), I also pulled secondary jetting down 4 sizes from 88/91 to 84/ 87 BUT between pulling fuel from front and rear, it drove # 6 back into high 13s, almost 14s. So I went back to a 90 in THAT corner, in end it had 74/75 in front and 84/90 in rear. I had a post 3 or 4 years ago on a 502 with a different brand 850 that needed all kinds of jet to get to a safe afr, I ll see if i can find it, Smitty

Heres a old post on a dyno job i did WITH a QF marine carb that was way lean:
https://www.offshoreonly.com/forums/...carb-lean.html