power valve - Hot Rod Forum : Hotrodders Bulletin Board
Hotrodders.com -- Hot Rod Forum



Register FAQ Search Today's Posts Unanswered Posts Auto Escrow Insurance Auto Loans
Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Engine
User Name
Password
lost password?   |   register now

Reply
 
LinkBack Thread Tools
  #1 (permalink)  
Old 04-29-2010, 11:22 PM
Registered User
 
Join Date: Apr 2010
Location: Temple City
Posts: 53
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
power valve

what power valve should I be using I have a 6.5 in a Holley 750 vacuum secondaries. i am concerned that it is opening too soon but I am not sure. How should I go about choosing what size. I know that if it is too small it will not let fuel into the circuit but I am wondering if mine is staying open because the car smells really rich and I have a radical camshaft and I think it is making about 10 inches of vacuum at idle. would i divide that in half and use a 4.5 power valve?
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 04-30-2010, 12:16 AM
ericnova72's Avatar
More for Less Racer
 
Join Date: Jan 2007
Location: S.W. Lower Michigan
Age: 47
Posts: 7,642
Wiki Edits: 0

Thanks: 10
Thanked 143 Times in 132 Posts
That is one accepted way to find a size. Another is to have the PV opening point 1.5" below the vacuum reading at cruise, this will give a little sooner enrichment. Choice is yours and what your engine responds to the best.

Rich at idle has nothing to do with the Power Valve used, the Power Valve channels feed the main well in the metering block up high at the booster, fuel flow is not pulled that high in the block at idle because of the idle syphon break(idle air bleed) kills the signal up high in the block preventing the fuel from the PV being pulled over the top of the main well into the idle well. The only fuel being fed to the idle screws is coming from just the idle well and the power valve doesn't feed this.

You likely have too large an idle fuel feed restriction in the metering block for your combo, too small an idle air bleed, or have the initial timing set to low to produce a good burn. Or some combination of these and possibly a couple other issues.

Detail the cam, compression, intake manifold, exhaust, initial and total mechanical advance timing, and whether you are using vacuum advance to get more info on fixing the problem.
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 04-30-2010, 11:51 AM
Registered User
 
Last photo:
Join Date: Apr 2007
Location: Seattle, Wa
Posts: 6,436
Wiki Edits: 0

Thanks: 3
Thanked 331 Times in 288 Posts
Quote:
Originally Posted by 406MOUSE
what power valve should I be using I have a 6.5 in a Holley 750 vacuum secondaries. i am concerned that it is opening too soon but I am not sure. How should I go about choosing what size. I know that if it is too small it will not let fuel into the circuit but I am wondering if mine is staying open because the car smells really rich and I have a radical camshaft and I think it is making about 10 inches of vacuum at idle. would i divide that in half and use a 4.5 power valve?
How is the idle air being accommodated, radical cams require a lot of throttle opening. If you get the blades too high they expose the transfer slots, then both the idle and transfer circuits start feeding fuel which will make a rich idle and a power dip just off idle. If you meed more idle air the secondaries can be cracked open if this isn't a four corner idle circuit. There is an adjustable screw stop on the secondaries accessible from under the carb. This can be screwed in toe open the blades a little, but the same rule applies here as on the primaries which is the top of the blade must be below the bottom of the transfer slot to keep that system from suppling fuel in addition to the idle circuit. The process of last resort for more air is to drill a small hole in the throttle blades for additional air. If you must start small like 1/32 and work up. Usually a 1/16 hole in each primary blade is sufficient. Another trick is to get an aquarium air supply valve from the pet store and hook it into the not timed vacuum source nipple of the carb. Then use the needle valve of the aquarium air adjuster to dial in enough air leak to satisfy the idle requirement. Keep in mind this leak will always be there which might push you to a jet size bigger on main metering.

The power circuit has two elements the vacuum at which the enrichment valve opens, that's the number you see stamped on the valve. There are also jets that control the amount of fuel from this circuit in the metering plate on the back side of the valve cavity. But I certainly wouldn't start by messing with them.

If cruise comes up too rich you can do as suggested and go to a valve that opens at a lower vacuum (more throttle opening). You can also swap back and forth with main metering jet sizes. That's to say if the power valve comes in early, you can dial the main meter back a bit to lean it out. But the purpose of the power valve is to allow a leanish cruise and richen the top end, which is why its also called the economizer valve.

Radical cams and rich jetting can also upset the air bleeds, typically a larger size is needed. If the bleeds are too small the mixture goes rich and if the mixture is lean the bleeds can be too big. But exhaust the fuel jet adjustments and power valve cut and metering before messing with these. An error here is expensive to fix unless your carb has replaceable bleeds.

Bogie
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 04-30-2010, 12:41 PM
302 Z28's Avatar  
Last photo:
Join Date: May 2003
Location: North Texas
Posts: 10,810
Wiki Edits: 0

Thanks: 1
Thanked 81 Times in 68 Posts
I try at all costs to avoid drilling holes in the throttle blades, but they can always be soldered closed if too big initially.

Vince
Reply With Quote Quick reply to this message

Recent Engine posts with photos

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the Hot Rod Forum : Hotrodders Bulletin Board forums, you must first register.
Please enter your desired user name (usually not your first and last name), your email address and other required details in the form below.
User Name:
If you do not want to register, fill this field only and the name will be used as user name for your post.
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.




Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

Posting Rules
You may post new threads
You may 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
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Correct way to set valves on sbc w/ solid flat tappit camshaft? Joshua Lee Engine 17 02-09-2011 05:55 PM
master cylinder interchange runn141 Suspension - Brakes - Steering 7 05-03-2009 05:15 AM
700R4 Why are the TCI manual valve body instructions so lousy? Bayley Transmission - Rearend 24 02-16-2009 12:49 PM
Power Valve Blowout novatech Engine 8 09-27-2005 10:18 AM
Changed jetting, power valve...still runs terrible BrackneyC Engine 3 04-05-2003 04:52 AM


All times are GMT -6. The time now is 03:11 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
Copyright Hotrodders.com 1999 - 2012. All Rights Reserved.