CC247

Ok- I've read all the threads. Here's my question. When would be the best time for the power valve to open? Right now my vacuum says 12" idle, and at 3000 before the secondarys are starting to open I see 8".At this point I'm cruising on plane at a speed of 28-32mph If I push the throttle a little more the vacuum drops to 4.5-5.5, and goes lower the more throttle I give it up to wide open whick is 0-1" So?? a 5.5 valve would be open as soon as the secondarys open,would that be the best time or later? Maybe a 4.5 in case I'm running with the secondarys cracked opened. 35-45mph. Would this be the lean part of the fuel cruve I need to watch out for?It's got a 6.5 in there now and seems fine, but.....

Griff

You want the primary PV to open when it starts getting lean on the primaries, I would say about 400rpms before the secondaries open while just cruising. Find your vacuum there and chose the closest PV.

The secondary PV's are pretty much open as soon as the secondaries open since vacuum is already low. You can just block the secondary PV and jet it higher or run the same PV as the primaries.

CC247

With the 6.5 that's about what's happening now. I'm getting a surge just before the secondarys open.

bcarpman

Spent a lot of time researching this. Got a lot of different answers than if I was tuning a car. Ideally you want to decide where you like to cruise. Find a boat speed and vacumn that is fast enough to get you were you want to go on a regular basis. Set the Primary a bit below that. Make sure that doesn't give you any holes in the powerband. It should be low enough cylinder pressures at that point that you won't hurt anything even if you go a little lean. It will just misfire if there's a hole, but won't blow a piston or anything (but don't quote me on that, hah)

Hopefully the secondaries will kick in shortly thereafter. It's nice if there's a little separation.

The secondary PV isn't essential, but it is a nice tunning aid. On my engine (using a Wide band O2), I have to run REALLY rich as soon as the secondaries open in order to have the correct A/F at WOT unless I have a secondary PV. Not a big deal on a race boat, but if I can save 20% on fuel when I'm running fast, I'll take it. (without the secondary PV you'll actually lean out a little as you get into the secondaries more)

I've got my primary at 6.5 and secondary at 3.5. Secondaries start to really open around 5. Gives me three nice steps, and allows me to keep fuel usage under control yet still go rich at WOT. I even enlarged the secondary powervalve orifices a tad.

BenPerfected

I agree that if you really want to know the PV timing and the exact A/F ratio at every RPM, you need a wide band O2. It is worth the $300 (plus cost of exh bung). If you only use it once, sell it on OSO for $200 and now you know...

CC247

Thanks for the info, I'll stick with what I've got and watch the plugs.

CC247

One other question, while testing, I had #75 jets in the pri. and the temp would jump up to 180* as soon as I got on plane. I went up to #76 without any other changes and now the temp stays right at 140* no matter how fast I go. Would it make that much of a difference one step up?

BUIZILLA

this man knows what he's talking about right here..

bcarpman

If I remember the difference in flow is 3% plus or minus 1% (sorry if I'm wrong). I go four sizes at time to really see any difference. Regardless, even with a large fuel change I wouldn't expect that much difference unless you were just super rich with the #76. If you were anywhere near stoic, operating temps shouldn't change much with small fuel changes.

Does this experiment repeat? Can you change back and see the temps go up again?

CC247

I'll try going back to the #75 and see.