6066 (1960-1966) GMC Truck Club Bitcoin now accepted here! 
Pay Dues
Pay Dues or become a Site Supporter
 



Go Back   6066 (1960-1966) GMC Truck Club > 6066 GMC Truck Club Forum > GMC V6 and V12 Engines


GMC V6 and V12 Engines Engine repair and rebuilding

Reply
 
Thread Tools
  #11  
Old June 16th, 2022, 06:45 AM
BobBray BobBray is offline
-= Dues Paid =-
 
Join Date: Feb 2013
Location: Los Angeles, CA.
Truck: 1967 CM 2500
Posts: 193
Rep Power: 110
BobBray is on a distinguished road
Default Re: Terrible milage

Quote:
Originally Posted by Jim A View Post
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
Reply With Quote
  #12  
Old June 16th, 2022, 05:18 PM
Jim A Jim A is offline
Member
 
Join Date: Jan 2013
Location: Potter Valley, CA
Truck: 1960 3/4, 4 spd, stepside
Posts: 93
Rep Power: 83
Jim A is on a distinguished road
Default 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.
Reply With Quote
  #13  
Old June 16th, 2022, 05:26 PM
Jim A Jim A is offline
Member
 
Join Date: Jan 2013
Location: Potter Valley, CA
Truck: 1960 3/4, 4 spd, stepside
Posts: 93
Rep Power: 83
Jim A is on a distinguished road
Default 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.
Reply With Quote
  #14  
Old June 16th, 2022, 05:37 PM
ilvracn's Avatar
ilvracn ilvracn is offline
-= Dues Paid =-
 
Join Date: Nov 2013
Location: atlantic iowa
Truck: 1966 gmc 1500
Age: 57
Posts: 194
Rep Power: 103
ilvracn is on a distinguished road
Default 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.
Reply With Quote
  #15  
Old June 16th, 2022, 06:33 PM
jagarra jagarra is offline
-= Dues Paid =-
 
Join Date: Apr 2014
Location: Reno, NV
Truck: 1962 Custom Wideside SB
Age: 76
Posts: 249
Rep Power: 115
jagarra is on a distinguished road
Default 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

Last edited by jagarra; June 16th, 2022 at 07:01 PM.
Reply With Quote
  #16  
Old June 16th, 2022, 08:47 PM
ilvracn's Avatar
ilvracn ilvracn is offline
-= Dues Paid =-
 
Join Date: Nov 2013
Location: atlantic iowa
Truck: 1966 gmc 1500
Age: 57
Posts: 194
Rep Power: 103
ilvracn is on a distinguished road
Default 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
Reply With Quote
  #17  
Old June 20th, 2022, 03:21 PM
jagarra jagarra is offline
-= Dues Paid =-
 
Join Date: Apr 2014
Location: Reno, NV
Truck: 1962 Custom Wideside SB
Age: 76
Posts: 249
Rep Power: 115
jagarra is on a distinguished road
Default 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
Reply With Quote
  #18  
Old June 20th, 2022, 04:40 PM
James James is offline
-= Extreme Supporter =-
 
Join Date: Jun 2013
Location: Greer, SC
Truck: 1964 GMC 1500 2wd
Age: 67
Posts: 357
Rep Power: 151
James will become famous soon enough
Default Re: Terrible milage

Quote:
Originally Posted by jagarra View Post
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.
Reply With Quote
  #19  
Old June 24th, 2022, 03:48 AM
BillT BillT is offline
Senior Member
 
Join Date: Jan 2013
Location: Moneta, VA
Truck: Drove and Owned Many
Posts: 294
Rep Power: 133
BillT is on a distinguished road
Default 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.
__________________
Drove and owned many, but some of the more interesting ones were:

-'60 GMC 2500 Dump, 305 V-6
-'62 GMC 4000 Flatbed, 305 V-6
-'62 GMC 6500 Flatbed Dump, 478 V-6
-'67 GMC 7500 Box, 6V-53 Detroit
-'71 GMC 9500 Flatbed Dump, 637 V-8
Reply With Quote
  #20  
Old July 1st, 2022, 03:58 AM
jagarra jagarra is offline
-= Dues Paid =-
 
Join Date: Apr 2014
Location: Reno, NV
Truck: 1962 Custom Wideside SB
Age: 76
Posts: 249
Rep Power: 115
jagarra is on a distinguished road
Default 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,
Reply With Quote
Reply

Bookmarks

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Terrible gas mileage, jagarra General Discussion 24 January 30th, 2019 12:24 AM
351 gas milage? Archiver Previous Forum Posts 0 January 4th, 2005 12:21 AM
Re: most milage on GMV V6 Archiver Previous Forum Posts 0 September 27th, 2002 01:20 AM
Re: Re: Re: most milage on GMC V6 Archiver Previous Forum Posts 0 September 23rd, 2002 01:30 AM
most milage on GMV V6 Archiver Previous Forum Posts 0 September 21st, 2002 02:16 PM


All times are GMT +1. The time now is 01:28 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2013, Jelsoft Enterprises Ltd 931656686078|1655771282|0
Feedback Buttons provided by Advanced Post Thanks / Like (Lite) - vBulletin Mods & Addons Copyright © 2022 DragonByte Technologies Ltd.