power valve tuning for best fuel economy

[Havasu47]

Unless I missed someone talking about this no one has mentined the fact that the vacuum will continue to increase between the blower and carbs and unless his carbs are boost referenced his power valves will not open. If they are referenced, the vacuum/boost line to the carbs, will reference the vacuum/boost from the intake manifold. All he has to do is look at his boost gauge to tell when the pv is opening. I dont recommend power valve tuning a blown engine for economy. Tune it for safety and performance. Read your plugs at all RPM's and adjust accordingly. As info said, unless the secondaries are open the secondary pv won't deliver any fuel enrichment. Another thing, a boat is usually under a fair amount of load even a cruising speed. Jet and pv tune for the proper a/f ratio, not economy. Proper tuning will give you the best economy for that particular engine.

[Badburn]

Kieth, You mentioned that your boost referencing was disconnected. If that is so than you need to be checking the vacum under the carbs, but above the blower. Your Boost/vacum guage is irrelevant since it is connected below the blower. The power valves sense vacum through a small hole in the carb base plate.
I went through this same process on my boat a few weeks ago. First I established the highest cruising speed I could get without cracking the secondaries open. Do this by simply having your bravest buddie crawl back and check while you are cruising. In my case it is 3700 rpm, At this speed I have about 6 inches of vacum, so I put a 3.5 power valve in the primaries-so that it will be shut for better economy. I elected to remove the powervalve and upjet 7 sizes in the secondaries.
If I remember the #s right, a motor will like around 12:1 air fuel ratio for best power and around 14.5:1 for cruising. As mentioned before, there is a pretty heavy load on a boat motor, and I would venture to say that by the time you are getting into your secondaries, it would be safer to be closer to 12:1 A/F ratio. The sole function of a powervalve is to richen the A/F ratio at WOT.
You also have to be careful that your carbs aren't seeing a vacum reading higher than the rating on the power valves at WOT. If this happens, the power valves will close, and bad things could happen!
I have a portable vacum guage in my boat, you're welcome to use it on the lunch run next saturday. Hope this helps.

[Robbie Racer]

Thanks guys. Great information. I have learned a lot from this thread. I didn't know that it's possible for power valves to close at WOT. I sure wish I had an on board A/F ratio guage to better tune with. Todd, thanks for the offer on the loan of the portable vacuum gauge. I have one with a hose long enough to read from the drivers seat that I can use. For some reason, I was thinking that the readings that I had taken some time ago were the same as my boost/vac gauge was reading. But, that may have been before we disconnected the boost referencing (due to running way too fat at cruise speeds). I will hook up my portable vacuum gauge this weekend (providing it's not raining) and get some new vacuum readings under the carbs to see where I'm really at before I go changing the current PV configuration. Do you need to be cruising (under power and load) to accurately determine where the secondary butterfly's are opening? Or can you accomplish the same thing while sitting still with the motor shut off? I would think that since the secondaries are mechanically actuated, that it wouldn't make a difference (and would probably be safer for my best buddy).

[Badburn]

Another thing to consider, I have seen carbs boost referenced by blocking the passage from the power valve well to the base plate, and then drilling a vacum port into the carb. If this is the case with yours, simply unhooking the boost reference will leave the powervalves with no vacum at all, and they will always be open- resulting in poor fuel economy.
You could do it in the driveway to guage the throttle lever position and where the secondaries start to open, then you will also want a vacum reading at that position- you will need load on the motor for that part.

[Robbie Racer]

Another thing to consider, I have seen carbs boost referenced by blocking the passage from the power valve well to the base plate, and then drilling a vacum port into the carb. If this is the case with yours, simply unhooking the boost reference will leave the powervalves with no vacum at all, and they will always be open- resulting in poor fuel economy.
You could do it in the driveway to guage the throttle lever position and where the secondaries start to open, then you will also want a vacum reading at that position- you will need load on the motor for that part.
Good point. We didn't disconnect the boost referencing completely. We just replumbed the vacuum line that feeds the PV's from under the blower to under the carbs. I think they are working because as soon as we did that, the motor quit running really rich at cruise speeds.

[Robbie Racer]

Well, I tested the vacuum under the carbs and found out that it is a lot higher than I thought it was. Test was with 32 lab prop and 10 over on the blower. Numbers are approximate.
2500 rpm = 12 inches
3000 rpm = 10 inches = 50 mph. No boost yet
3500 rpm = didn't get vac reading. 60 mph. Aboout 2 lbs. boost.
4000 rpm = 9 inches = 75 mph. About 3 lbs. boost.
4500 rpm = 6 inches = 80 mph. About 4 lbs. boost.
6300 rpm = zero vacuum (at WOT) = 116 mph. About 8 lbs. boost.
Please correct me if I'm wrong but based upon these readings the power valves are still closed up to at least a 80 mph cruise speed. If that is correct, then I don't think that changing the power valves will help with cruise speed fuel economy. I meant to mark the throttle position at the point where the secondary side butterfly's start to actuate but I didn't get around to doing that. I think that is somewhere in the 3200 rpm range (based upon the feel of the throttle). I now think that the only way to conserve fuel is to stay out of the secondary side of the carbs.