Which power valve

The new carbs from PCM come with a 2.5 , they should know....

Thanks! How much work is it to change 'm out, in case I want to try both?

Does anybody know if its necessary to check the vacuum of the engine to get the right power valve? I thought I had read somewhere that that was part of the process and that there is a calculation you use with your vacuum number as part of the equation. I checked my vacuum yesterday as I was winterizing the boat, and got 15-16 at low idle. I want to get a renew kit for mine as well and had been wondering which power valve to get.

davidg wrote: Does anybody know if its necessary to check the vacuum of the engine to get the right power valve? I thought I had read somewhere that that was part of the process and that there is a calculation you use with your vacuum number as part of the equation. I checked my vacuum yesterday as I was winterizing the boat, and got 15-16 at low idle. I want to get a renew kit for mine as well and had been wondering which power valve to get.

No, The PV doesn't really matter what the vacuum at idle is.

I think you have some procedures crossed...

IF you were setting up a car or truck, and wanted to economize for steady-state cruise, one would plug the PV, set the mains till one noticed a lean surge at cruise, then richen 1 size.
Then measure the vacuum at cruise with the richer jets. then subtract one or two inches from that and that sets the PV size, so it opens when you get on it, but is closed when just cruising.
Then one fills and redrills the PVCO's to 50% of the main jet size, to deliver 25% more fuel when the PV is open. That is the optimal stoiciometric difference between lean, high vacuum cruise and max power conditions; 25% more fuel.

Most " universal" holley's have oversize PVCO's to keep hotrodders from fragging their engines and blaming holley. That is why they belch black then they punch it, and our boats do not.
The Marine and Medium duty truck holleys, like the 4150G, are setup precisely for high-load, extended duty conditions, and have properly sized PVCO's

Although we 'cruise' in the boats, its not the same. A car needs only about 20-25 hp to maintain 65 mph. A boat is in high-load almost always, and in most applications, the PV best be open by 28-30 mph or sooner, else the throttle is dull and flat to small inputs, because its too lean for the loading.


If you keep the 2.5 PV, it won't open till just before the secondaries open, at less than 2.5 inches vacuum.

My box-stock 600 4160 was dull and flat at 34-36 mph, and needed to get into the secondaries sometimes to compensate for slalom pull, just when i need precise throttle input. But secondaries responds relatively slowly.
Simply changing the 2.5 to a 6.5 or 5.5 makes the engine more responsive to precice throttle inputs, and doesn't flirt with the secondaries compensating for skier pull at 34-36 mph.

The main jet sizes stayed the same over the last 30 years, fundamentally the needs of the engine and boat didn't change. I think the 2.5 PV in the rebuild kits and new carbs was an ill-concieved attempt to reduce fuel consumtion, but it think has penalty for performance in most applications.

Pete....Yes, I plan to take it apart to see what PV I have on there now. The biggest problem I am having is that it hesitates and dies out on hard acceleration...sometimes. An intermittent thing.

Tom....I will agree with Sebastian, YOU KNOW YOUR $h*T when it comes to carbs. I had to look at your profile to see if you owned a carb shop. You have obviously dug into a few carbs. I think I did have my wires crossed. I had just remembered hearing recently that vacuum played a role in selecting the PV.

So guys, if the boat runs really good most of the time, except on hard accel, do you guys think it sounds like an accelerator pump issue? Would you bother with a whole renew kit?

I plan to take the carb apart anyway just because I want to learn more about what makes them tick, and to find that acceleration gremlin.   

Thanks,
Been dinking with them on the side for 25 years or more. Usually when someone's boat doesn't work.
However, some of the more reliable information has been from pantera user groups;
If you think our engines are finiky, team a hyper overcammed canted valve 6500 rpm smalblock with oversized intake ports to enthusiasts that want it to perform in all conditions as if enzo had made it.
I'm always learning, and corrected on some things here too.