Terrible milage
 

I have complained about this before. I haven't driven the truck much the past year, maybe a couple times not putting much milage on it. I decided to check out why it was running bad and refused to idle. The vacuum port at the base of the Holley 2300 had cracked. I replaced it with one I had on hand. I have been going through these caps and they seem to deteriorate at an alarming rate even when the vehicle is not used. I finally went to Summit and bought Dorman brand of caps hoping that they would last longer. I did find another cap that was cracked, all were replaced with the Dorman brand. Then I went to top off my tank, it took 9.3 gallons for milage of 36 miles!!! After I picked myself off the ground I did the vacuum leak check by spraying everything with carb cleaner, around the base, all the caps, the vacuum line going to distributor. Everything was good, no change in idle. I had changed the jets from a #74 to a 71 and then to either a 70 or 69 as that is what is in the truck now. I checked the dwell and it was about 32, fiddled with the mixture screws for max RPM, they were close to where I ended up. Any ideas what I should do next, should I go down even lower in jet size???
chuck this carb and get a new one, I really hate going back to the stocker with the oil bath.

Re: Terrible milage
 

I'm running a 78 main jet size with a 45 power valve in my 305 V6. I am getting 14 mpg.

My thought:
Have you replaced your power valve? If the diaphragm is leaking and/or its gasket is split more fuel is added to the engine.

Re: Terrible milage
 

check your float level, bad needle and seat?

Re: Terrible milage
 

I don't think that much gas can actually be going through the carb without several other symptoms.
Leakage (internally or externally) or theft come to mind.
While money is important, right now I think fire prevention might more so.

Re: Terrible milage
 

My thoughts is that a 305 V6 isn't physically capable of burning that much fuel without some serious issues - like a ruptured power valve- or gas tank leak. Does it blow black smoke?

Re: Terrible milage
 

I never thought about the power valve. Engine runs clean, milage is at 68500 when I took last reading. since there is a Summit in my area will see what they have in stock.

thanks for the info,but keep them ideas coming as this carburetor which was a remanufactured unit has always given me terrible milage compared to the stock unit.

Re: Terrible milage
 

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?

Re: Terrible milage
 

also check the fuel pump, it can leak externaly as well as internally, check oil level level, if it is overfull you may be leaking fuel into the crank case from a ruptured fuel pump diaphram

Re: Terrible milage
 

Got my new power valve, will install tomorrow and drive her around. The fuel pump is relatively new, changed it last year, but will check sump just in case. Just to address leaks, this truck doesn't leave any marks on the ground, it is pretty tight. Fuel evaporation is a thought, but I have been keeping track of milage since I have had vehicle, it has always been lousy with the Holley.

checked my milage figures, best with stocker was 14mpg, Holley ran about 11 mpg when installed and has gone steadily downhill since.

Re: Terrible milage
 

I believe according to the Holley bible that your power valve rating should be half of what your idle manifold pressure is - so if your manifold vacuum of your engine at idle is 18 - then you would use a 9 power valve. My engine a 351 has a custom grind cam and idles at about 10 vacuum - so I run 4.5 rating power valve. You want your power valve to open only when you're under acceleration or pulling an load like going up a long hill. I have a in-dash vacuum gauge and to me is the key to see how your engine is running and a wise investment. I set the engine timing and carb idle mixture to achieve the highest reading possible. A reading of 21 is the very best you can ever hope for - I have achieved that by going down a big hill with your foot off the gas pedal causing your engine to act as a compressor.

Re: Terrible milage
 

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

Re: Terrible milage
 

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.

Re: Terrible milage
 

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.

Re: Terrible milage
 

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.

Re: Terrible milage
 

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

Re: Terrible milage
 

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

Re: Terrible milage
 

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

Re: Terrible milage
 

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.

Re: Terrible milage
 

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.

Re: Terrible milage
 

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,

Re: Terrible milage
 

holley carbs are great, but they are very sensitive to changes in barometric pressure and temperature. to keep your truck running properly, you should check your float level and idle mixture screws on a regular basis. now that you have the truck running better, i would suggest putting a new set of spark plugs in and running the engine for a week or so and getting a plug reading. you want the porcelin on the plug to be light brown or gray. if it is darker or black, drop the jet size by 2, clean the plugs and take another reading. i think your needle and seat is fine, if it were bad you would not be able to adjust your float level, fuel would be running out of the sight hole at a rapid rate.

Re: Terrible milage
 

I have the float bowl off and pulled the float to check its condition, also pulled the needle and seat to see if the rubber tip was OK. Both items seem fine, but I found this in the bowl under the float, the holes don't line up to anything in chamber, any idea where this goes as it is not in the illustrated parts breakdown.

thanks

Re: Terrible milage
 

That is a bowl vent baffle. See attached picture for location. Can be left out of a street vehicle, is suppose to prevent fuel from entering the vent. A lot of Holley carburetor do not have this. If I remember correctly my 2300 does not have one.