Terrible milage

[BobBray]

Originally Posted by Jim A

I have had my GMC for a while and rebuilt the one barrel Holley a few times.
That doesn't seem to have saved me from the stupid question: What is a power valve?

Not a stupid question!

The power valve enriches the mixture off idle at low manifold vacuum, as when the engine is under load with the throttle open. Part #26 in this diagram:


https://www.carburetorfactory.com/expvw11.html

[Jim A]

OK, I'll ask again.
What is a power valve?
I have had and done all the work on my '60 305A for over 50 years and thought I knew it pretty well.

[Jim A]

Sorry, I missed BobBray's post before I made that one.
26 looks like the accelerator pump to me.
In my 60 plus years of wrenching, I've never heard of its function as you describe and I 've never heard of any huge mileage problem it can cause with failure.

[ilvracn]

this is a holly 2300 carb we are talking about. the power valve is located on the the plate that the jets screw into. it is basically a needle valve with a vacuum diaphram on the end of it. the numbers on the valve refer to the number of inches of vacuum it takes to move the diaphram and open the needle which allows more fuel to flow into the engine, for enrichment when accelerating. the lowest number i have seen is 2 and the highest number i have seen is 7. the # 7 valve will open sooner than the 2 valve, vacuum drops when the throttle plate is opened. high vacuum at idle, no vacuum at wide open throttle.

[jagarra]

I installed the 45 power valve in the truck, the one that was in there was a 50, but it had two gaskets on it, don't know if that screwed up anything. I will check vacuum to see where it sits after I warm it up, have grandson working for me today so I won't have a chance to test out any changes. Will top off tank before I make a run so I get an accurate reading. If did find a good tutorial on tuning these carbs on line, plus I bought a book on the 2300. You guys seem to be chiming in with the same issues as the tutorial brought up.
hopefully we can get this problem sorted.


UPDATE;;;; took vacuum reading at idle, ran about 13 inches so that means I need a 65 correct. Does it mean that the power valve opens at a higher RPM as the higher the valve number goes??

thanks again,

gg

[ilvracn]

the higher the number on the power valve, the sooner it opens, takes less throttle to let the valve open. two gaskets are for different style carbs, only use the one like the one you took out. could cause vacuum leak into fuel supply with two gaskets, causing same overly rich condition as you are trying to fix

[jagarra]

From observing my gas level gauge it seems like my issue is still around. Used 1/4 of a tank for a distance of 11 miles. I was thinking about buying a new metering block and replacing the needle and seat just to see if that affects the milage. buying a new carburetor for about $500 isn't out of the question, the metering block is $200 plus another $30 for a new needle and seat. Maybe a new needle and seat first and then revaluate the problem. any ideas?

thanks

[James]

Originally Posted by jagarra

From observing my gas level gauge it seems like my issue is still around. Used 1/4 of a tank for a distance of 11 miles. I was thinking about buying a new metering block and replacing the needle and seat just to see if that affects the milage. buying a new carburetor for about $500 isn't out of the question, the metering block is $200 plus another $30 for a new needle and seat. Maybe a new needle and seat first and then revaluate the problem. any ideas?

thanks

If the needle and seat was the problem you won't be able to set the fuel level. With the engine idling, remove the plug on the right side of the fuel bowl. The fuel level should be at the bottom of the hole.

Another easy thing you could do, you can look down the carburetor (while the engine is at idle) to see if fuel is dripping out of the booster nozzles, if it is the fuel level is too high. This will make idle very hard to set (too rich all the time).

If you can't adjust the fuel level either the fuel pressure too high or the needle and seat is bad.

Also I wouldn't relies on the fuel gauge, not accurate. Topped up the tank and do the math. This is the only way to now for sure.

On a side note: you could add an O2 sensor and indicator to the exhaust pipe to measure your mixture. This will tell you if it is really rich or lean.

[BillT]

As mentioned a time or two before, after one of my super tunes, my '66 305 got 15 1/2 Hi-way with my original Stromberg.

[jagarra]

Hey guys thought I would give you an update on my milage issues. went and filled tank a day or so ago it was still getting about 4 miles per gallon. Today I adjusted the float down fully and started to bring it back up looking at the sight hole with a flashlight. Idle improved nicely as the level got close to level with the sight hole. Took it for a test drive of about 50 miles plus and my milage improved to 10.3 MPG, yahoo. So we are on the right track. I will pull the float chamber next to see if the needle and seat is loose.
I had purchased a new 2300 carburetor from Summit a week or so ago, guess I will return it since this carb is starting to come around a bit. Still looking for improvements.

thanks again,