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GMC V6 and V12 Engines Engine repair and rebuilding

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  #1  
Old June 15th, 2022, 02:33 AM
jagarra jagarra is offline
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Default 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.
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  #2  
Old June 15th, 2022, 01:43 PM
James James is offline
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Default 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.
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Old June 15th, 2022, 02:32 PM
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ilvracn ilvracn is offline
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Default Re: Terrible milage

check your float level, bad needle and seat?
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Old June 15th, 2022, 06:47 PM
Jim A Jim A is offline
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Default 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.
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Old June 15th, 2022, 07:50 PM
lizziemeister'sV6 lizziemeister'sV6 is offline
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Default 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?

Last edited by lizziemeister'sV6; June 15th, 2022 at 07:51 PM. Reason: left out something.
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Old June 15th, 2022, 08:04 PM
jagarra jagarra is offline
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Default 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.
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Old June 15th, 2022, 08:38 PM
Jim A Jim A is offline
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Default 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?
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Old June 15th, 2022, 08:40 PM
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Default 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
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Old June 15th, 2022, 08:58 PM
jagarra jagarra is offline
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Default 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.

Last edited by jagarra; June 15th, 2022 at 09:05 PM.
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Old June 15th, 2022, 11:22 PM
lizziemeister'sV6 lizziemeister'sV6 is offline
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Default 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.
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