RumRunner

Power Valves:

A Power Valve is a vacuum operated switch. When the vacuum is higher than the rating on the Valve it remains closed, when the vacuum drops below the rating on the valve it opens. i.e. if you have a 6.5 Power Valve it will be open when the engine vacuum is less than 6.5” Hg, and will be closed when the vacuum is greater than 6.5” Hg. When the Power Valve opens it adds additional fuel to the main circuit of the carburetor. This would be equivalent to going up between 6 and 10 sizes on your main jets (depending on how the carburetor was built).

The purpose of a Power Valve: The Power Valve serves a few purposes. First, since the idle circuit gets all of its fuel from the main metering circuit running a slightly leaner main circuit will have a minor affect on the engines idle quality. Second, and main purpose is enrichment under engine load. The Power Valve system will lean the engine out under light load cruising increasing your fuel economy, but once the engine load increases and vacuum decreases the additional fuel is added.

Power Valve tuning: The number on the Power Valve controls when it opens, and closes. You can change the Power Valve to open later, and close sooner by going to a lower number Power Valve. The opposite is also true going to a higher number Power Valve will get the valve to open sooner and stay open longer richening the circuitry. i.e. If you have 12” Hg at cruise, when you accelerate the engine vacuum will decrease requiring more fuel. A 9.5 Power Valve will open sooner, and remain open longer than a lower numbered Power Valve (2.5, 3.5, 4.5, etc.). You can tune with the valve to give you optimal drivability, and performance.

Power Valve Fuel Flow: The rating or number on the Power Valve has nothing to do with the fuel flow, only the timing when the fuel is added to the engine. The fuel volume is controlled by the P.V.C.R. (Power Valve Channel Restrictors), which are the two holes behind the Power Valve in the metering block. Changing the size of these holes will control the fuel volume.


Vacuum Secondary Operation:

The Vacuum Secondary Butterflies open based on the air velocity traveling past a hole in the primary venturi, as the air passes this hole it creates a pressure differential or vacuum in the vacuum pod. Based on the amount of vacuum in the pod, and the rate of the spring it controls the opening rate of the secondary butterflies. So engine RPM, and load will vary the secondary opening rate. What does this mean? If you’re running at a specific engine RPM, but have more load on the engine (due to trim, sea conditions, weight, etc.) it will change how far your secondary butterflies are open at that point.

Changing the spring rate controls when the secondary butterflies will start to open, and at what RPM they will fully open. Having the secondary butterflies open later than optimal can increase fuel economy, but can hinder top end performance, and acceleration while having them open too soon can cause a bog or hesitation.

Pourman

What he said. The 10.5 power valve was open almost all the time and was burning a ton of fuel. Change it to a 3.5 so it opens only when needed and my milage increase accordingly. Took a while to find the right one but now it's waiting in the wings at cruise just step on it a little and opens. Good luck, Ty

CrazyCat57

WOW I am sure learning a bunch here. I went to my dealer here this morning and all he could come up with from his books was, both primary and secondary should have 2.5 power valves, his book did not list spring color or jet sizes in the factory specs. He did have the cam lift specs, intake .2343 and exhaust .2530. Thanks again for all your replies.

SB

iTrader: (1)

Hmmm.
.398", .430" lift cam ? On a big block ? Never seen that.
Not that this applies to this thread, but just in case anyone wants to know, this is what I have listed for the Merc BBC w/oval heads and hydraulic flat tappet cam:


Standard Flat Tappet (oval heads)

.004” 287*, 302
.006” 278*, 289
.050” 214*, 218*
.200” 111*, 119*
Valve lift with 1.7 .460” , .480”
ICL 110 ATDC
ECL 118 BTDC
114 LSA

CrazyCat57

I dont think that I have ever mentioned this but it is one of those dreaded omc king cobras.

CrazyCat57

I finally got the boat out. I found 2 air bleeds plugged and cleaned them. Its running 10 in of vacuum and the secondary butterflys start to open at a little over 2700 rpm, that seems way to low to me. Most all of my running is around 3000 rpm. The boat is a 1992 so I have to decide what to do, repair or replace the carb. Just to open a carb that old will probably require new gsks, power valves, secondary diaphragm, secondary opening spring set to re calibrate, accelerator pump and whatever else I might encounter. I have been thinking that instead of opening my holley to look inside this late in the Minnesota boating season I might take the money I would have spent on the holley and put some more with it and try a new #1409 600 cfm Edelbrock. Edelbrock recommends it for economy. They also say it will improve low and mid range power but probably loose 2 or 3 mph on the high end. The #1409 Edelbrock does have smaller primaries similar to the holley spread bore. I would like to hear some suggestions of what you think I should do. Thanks for your time.

SB

iTrader: (1)

10" of vacuum means you have an engine or ign timing issue. 10" (on a healthy motor) would be typical of a fairly large cam with a good amt of overlap.

Check your ignition timing and then let's look for vacuum leaks and do compression test.

2700rpm is normal range for vac secondaries to 'start' opening.

I really feel tuning a '1409' and then using it will not be worth it.

Wobble

If you feel that a new carb is in order.

RumRunner wouldn't promote his products , but I would suggest you look at a Sea Demon by Barry Grant, good quality carb and RumRunner could spec the exact model that should work best for you. USCG type approval also. You would need a spread bore adapter, but the right one may help your torque a little.

Jegs has good prices.

PatriYacht

It may be that the boat is overpropped. By putting a large load on the boat, you are required to open the throttle more to maintain speed. Large throttle load means secondaries open sooner and power valve opens sooner. I would try to prop the boat to spin 4600.

Whiteknuckle

If you want to have better fuel economy and still decent performance with a 330 I would suggest a 750 CFM Q-jet. The small primaries are very good at cruising RPM and idle. Also I think you are propped a little heavy. I would WOT prop at about 4600 minimum. I would not mess with Holleys or Edelbrocks on a stock 330, go with the Q-jet.