Bill 3

Yes, look up the Holley 4781 here - http://documents.holley.com/techlibr...al_listing.pdf

Front and rear 6.5 PV. 80 jets all in both in it's stock form.

GETTINBYE

Try 6.5 PV in Primary, block the Secondary. Jet 80 Primary and 88 Secondary and see where you are then. I think it will be real close, maybe a bit on the rich side.
Mark

Bill 3

I'm still learning here, but I don't see how removing the rear PV and dropping jet sizes would make me rich? My biggest concern right now is my lean cruise.

Bill 3

I am intrigued by the 1:1 throttle secondary linkage. May start a new thread to discuss this further.

GETTINBYE

Just suggesting a pretty common and proven combo. Used very close to that with an 800 Holley several years back. In my particular application it was a bit on the rich side. If I remember correctly I used 78-86. I fully understand your lean concerns.

Mark

Budman II

SB, the shop that dyno'd my motor had a guy in house who used to work for Quick Fuel and was supposed to be very knowledgeable on carbs. As the OP has said, for a marine engine a standard dyno session doesn't necessarily give you all the information you need. Most of us don't go from idle to WFO all the time when running our boats, and when we do, we hold it open for way longer than a few seconds. I was concerned about a number of things that might rear their heads on my motor at high speed cruising RPM's - jetting for one, but also oil control, as in making sure it was getting back to the pan to prevent any starvation (getting off tangent here). So I talked them into doing a steady-state test at around 3800 RPM with a good load on the engine, while we watched oil pressure and AFR's. After about 3 minutes, AFR's started getting into the 14's and the exhaust tubes started getting hot, especially on the back cylinders. Instead of just changing the jetting, he was concerned with even fuel distribution, so his idea was to switch from the progressive linkage to a 1:1 linkage. This still has enough slop in it to allow the primaries to open by themselves for most low speed cruising, but it starts to get the secondaries and their jets into the picture sooner so that some of the mixture is going through them while the primaries are not open as much for a given power level. When we ran the test again, it solved the problem, and didn't force us to jet way up on the primary side.

I was concerned that this would result in the boat using a lot more fuel, but I really haven't seen that.

fbc25el

You might have to play with the main air bleeds may be go just a little .001 or .002 smaller for the mid range then for the wide open take a little main jet out.

Bill 3

Update.....

I am up to 90 jets with 6.5 PV primary and 98 jets no PV secondary. I'm getting pretty close, under 13 afr at cruise.

Regarding power valves... I understand what makes them open (a vacuum lower than the PV number), but when should they be open? For instance when I am at the top of my primary (just before secondary starts to open), should the PV be open? Is it only for lower vacuum situations like accelerating or WOT? My vacuum is rarely below my PV value until closer to WOT.

ICDEDPPL

I`m tired of the powervalves. My carbs are boost referenced so pretty much anytime I`m on plane it`s open and there`s unmetered extra fuel dumping down the carb.

Too rich at 3000 , too lean at 4000.

I`m going to eliminate the power valve and jet up 10 sizes and see what happens. I`d rather pull metered fuel out of the jets than just dump it in.

After that it`s time for EFI and get out of this 1960 technology.

ezstriper

with that engine size and HP you would be much better of using dominators at this point