Help.....6-71 blower sbc 350 problems - 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 09-19-2008, 10:38 PM
Member
 
Last photo:
Join Date: Jan 2005
Location:
Posts: 3,707
Wiki Edits: 0

Thanks: 0
Thanked 3 Times in 3 Posts
Help.....6-71 blower sbc 350 problems

I just picked up an old school Nova with a 6 3/4-1 compression 350 chev. It has a Dyer's 6-71 blower that's 6% overdriven and an old Comp solid lifter cam 12-405-5 grind # CS 290AS-14
255@.050 .540 lift intake 265@.050 .563 lift exhaust on a 114 LSA
Ignition is a dual point mechanical advance only distributor.
Two Holley 1850 600VS carbs

This is an old build that ran good for many years but now she has a low rpm backfire, but the midrange and top end seem fair. It blows the left side pop-off valve open. The valve lash seems stable so I think the cam is OK. I just bought it this way so I need to dig in and find out what's wrong with her and hope you guys can give me some thoughts on what to look for. I've got a new unilite distributor that I plan to install and will freshen the carbs. In the Dyer's paperwork I got with the car it says possible causes for a low rpm backfire to be:
Vacume leak/blown blower gaskets
lean idle mixture
vacume leak on blower between end plates and case
carb base gaskets
ruptured power valves
leaking intake gaskets

Can you think of anything else to look for?
Blowers are new to me, any reason why I should keep the powervalves?
If I pull the powervalves and jet it up would that be a good move?
This is in a 72 Nova, th400, 4.10 gears--A real old school car with chrome ladder bars, ET mags, candy red lacquer, the whole 70's vibe...

Thanks for your help, Bob

One more question The blower front gear case has an oil line plumbed from the front of the block and a drain tube that goes to the right valve cover. In the feed line there is an adjustable valve that I assume is to regulate oil flow- any idea how this should be adjusted?

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 09-19-2008, 10:57 PM
carsavvycook's Avatar
My 2 cents worth
 
Last photo:
Join Date: Sep 2007
Location: Lakeside, CA
Age: 58
Posts: 2,855
Wiki Edits: 3

Thanks: 2
Thanked 2 Times in 2 Posts
badbob,

You say it is an "old school build with a lot of miles on her". This makes me think of carb problems/tuneup problems. They make kit's that block off the port that pop's the power valves, stopping this problem. Another factor could be the jetting for the fuel being blended now, compared to back then. Not knowing how long it has been sitting, it could be just a plugged jet, or sinking float.

I also know from experience that bolts will vibrate loose occasionally on blown engines.

As far as the oil line, I have never had to adjust one.

Stephen
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 09-19-2008, 11:21 PM
Member
 
Last photo:
Join Date: Jan 2005
Location:
Posts: 3,707
Wiki Edits: 0

Thanks: 0
Thanked 3 Times in 3 Posts
So you're leaning toward's a lean fuel mixture or vacume leak being the probable cause? I think I'll order up a gasket kit and pull the blower and intake off to get a look at everything when I go through the carbs. Anyone else?
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 09-19-2008, 11:28 PM
carsavvycook's Avatar
My 2 cents worth
 
Last photo:
Join Date: Sep 2007
Location: Lakeside, CA
Age: 58
Posts: 2,855
Wiki Edits: 3

Thanks: 2
Thanked 2 Times in 2 Posts
When you get the carb kit's, get the power valve saver kit's also. They make a power valve tester, which might be wise to also get, because I have seen some brand new ones off by up to 4" hg.
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 09-20-2008, 07:36 AM
Member
 
Last photo:
Join Date: Jan 2005
Location:
Posts: 3,707
Wiki Edits: 0

Thanks: 0
Thanked 3 Times in 3 Posts
I usually do my own powervalve protection by drilling the baseplate, notching the well, and installing a check ball. I've wanted a pv tester and if you've seen 4" of difference, I can definately see the need. It's too bad they aren't adjustable. Where's the best place to by gaskets for the 6-71 base and top?
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 09-20-2008, 07:56 AM
dawg's Avatar
NAVY VETERAN
 
Last photo:
Join Date: Aug 2004
Location: Colebrook Connecticut
Age: 55
Posts: 912
Wiki Edits: 0

Thanks: 0
Thanked 7 Times in 5 Posts
time for a carb rebuild
also I would get rid of the dual point dist.(points wear and can cause that issue aswell) and go with a good electronic ignition with a boost retard (good insurance)
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 09-20-2008, 09:42 AM
carsavvycook's Avatar
My 2 cents worth
 
Last photo:
Join Date: Sep 2007
Location: Lakeside, CA
Age: 58
Posts: 2,855
Wiki Edits: 3

Thanks: 2
Thanked 2 Times in 2 Posts
Quote:
Originally Posted by baddbob
I usually do my own powervalve protection by drilling the baseplate, notching the well, and installing a check ball. I've wanted a pv tester and if you've seen 4" of difference, I can definately see the need. It's too bad they aren't adjustable. Where's the best place to by gaskets for the 6-71 base and top?
This is where I have ordered parts.
http://www.hamptonblowers.com/Order.htm
Reply With Quote Quick reply to this message
  #8 (permalink)  
Old 09-20-2008, 10:19 AM
Registered User
 
Join Date: Apr 2008
Location: Raleigh, NC
Posts: 30
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Maybe a rounded off exhaust cam lobe? I know years ago Chevy went through this problem on the 350 (I realize all your parts are probably aftermarket parts). I had it myself. Would back fire real easy at low RPM on stock motors.

Real easy to tell just by pulling valve cover and watching it while you crank it over.
Reply With Quote Quick reply to this message
  #9 (permalink)  
Old 09-20-2008, 01:57 PM
T-bucket23's Avatar
Hotrodders.com Moderator
 
Last wiki edit: Engine basic condition - how to check Last photo:
Join Date: Aug 2005
Location: Massachusetts
Age: 57
Posts: 5,336
Wiki Edits: 26

Thanks: 7
Thanked 143 Times in 127 Posts
See if you can find someone with an Air/Fuel ratio meter and see what it looks like. You need to be fat with a blower or you might melt the pistons. A vacuum leak would likely also show up at cruise. Do the idle mixture screws have any effect. If the pop is consistent you can partially block the air intake and see if the issue goes away.
What is the idle vacuum at the base of the carb and at the intake. These readings may give you some clue.

If the carbs are not specially setup for the blower the power valves are likely not working at all as even at WOT with a blower you will still see vacuum at the base of the carb because of the suction of the blower. You wil see negative vacuum (boost) at the manifold at wot.
Generally a blown power valve will give you a rich condition not a lean one. I also agree with loosing the dual point. A decent electronic will make a big difference just make sure if it requires full battery voltage as most of them do, you bypass the resistor wire.


Chet
Reply With Quote Quick reply to this message
  #10 (permalink)  
Old 09-21-2008, 08:27 AM
Member
 
Last photo:
Join Date: Jan 2005
Location:
Posts: 3,707
Wiki Edits: 0

Thanks: 0
Thanked 3 Times in 3 Posts
The carbs were reworked many years ago and they drilled the secondary plates to Dyer's recomendations-it's kinda cool to have all the old recipts from 20+ years ago I'll pull the valve covers and check the valve lifts. Also was thinking of pulling the blower belt off and starting the car so I can check for vacume leaks with some carb cleaner or WD40-think that will work? I'll let you all know how it goes and what I find.
Reply With Quote Quick reply to this message
  #11 (permalink)  
Old 09-21-2008, 04:09 PM
41willys's Avatar
Blowin' it
 
Last wiki edit: Choosing a trailer
Last journal entry: Running again...
Last photo:
Join Date: Aug 2002
Location: Katy, Texas
Age: 39
Posts: 417
Wiki Edits: 11

Thanks: 0
Thanked 0 Times in 0 Posts
[QUOTE It blows the left side pop-off valve open. [/QUOTE]


Are you stating that it is blowing the manifold pop off valve, or are you meaning to say the power valves on the carb(s)?

Jason
Reply With Quote Quick reply to this message
  #12 (permalink)  
Old 09-21-2008, 10:45 PM
Member
 
Last photo:
Join Date: Jan 2005
Location:
Posts: 3,707
Wiki Edits: 0

Thanks: 0
Thanked 3 Times in 3 Posts
I tore into it today and found some problems, two burnt plug wires, 4 fouled spark plugs to start. Then pulled the distributor and checked the dual points. Points looked fine so I gapped them at .019 and dropped the distributor back in. Valve lash was good and cam looks OK with no short lobes. I then noticed the manifold pop-off valve looked strange with only one bolt and spring so I pulled the blower off and found the valve was indeed missing one of the bolts and springs and the other bolt was loose and bent with the valve left partially open. Hard to believe it even ran like that! Also when I pulled the blower off there was a lot of gas in the manifold so I'm sure the power valves in the carbs must be shot and with the electric fuel pumps on the power valve leak just emptys into the blower.
So, the plans are to order up a new blower base gasket, plug wires, power valves or power valve plugs and jet up? Also need to order a new spring and bolt kit for the manifold pop-off valve. AC R43T plugs. And do an oil change. I think it should run good with the ignition system working properly and the carb and pop-off valve issues taken care of.
This is a really old Dyer's blower kit, it uses a modified Offenhouser 360 intake and a 3/4" thick adapter plate to mount the blower.
Do you guys have any good sources for the blower gasket and pop-off valve parts? Your thoughts on if I should block off the powervalves and jet higher or maybe just get some power valve protectors and just put in new valves... These are old carbs with no powervalve protection.
Reply With Quote Quick reply to this message
  #13 (permalink)  
Old 09-22-2008, 12:40 AM
F-BIRD'88's Avatar
Yada Yada Yada
 
Last photo:
Join Date: Oct 2005
Location: Ontario, Canada
Posts: 9,211
Wiki Edits: 0

Thanks: 5
Thanked 389 Times in 383 Posts
For a street driven engine keep the power valves.
Reply With Quote Quick reply to this message
  #14 (permalink)  
Old 09-22-2008, 12:53 AM
Member
 
Last photo:
Join Date: Jan 2005
Location:
Posts: 3,707
Wiki Edits: 0

Thanks: 0
Thanked 3 Times in 3 Posts
F-bird, will the powervalves work as is? or do I need to modify the carbs so they get vacume from the intake. I notice blower specific carbs have a modified power valve vacume circuit so they get signal from below the blower. This is an old Dyer's kit, I have the original paperwork and it's interesting how they specify for the 1850 carbs to drill the main jet and rear plate to a spec'd size but there's no mention of power valve modifications.
Reply With Quote Quick reply to this message
  #15 (permalink)  
Old 09-22-2008, 01:41 AM
carsavvycook's Avatar
My 2 cents worth
 
Last photo:
Join Date: Sep 2007
Location: Lakeside, CA
Age: 58
Posts: 2,855
Wiki Edits: 3

Thanks: 2
Thanked 2 Times in 2 Posts
Keep the power valves, but get the kit's that plug the port that damages them. The installation instructions are in the kit.

A 8"hg power valve is very commonly used on this type of application.

This is also why I purchased a tester for them. I had one that had a flat spot mid range, and it turned out to be a defective 8"hg power valve, that popped off at 12"hg. It was frustrating, to say the least, to find it had this running problem after carb rebuilds.

You cannot run 'blown' motors lean, they need to run a little rich.

Like posted earlier, get a MSD Boost timing retard box, along with a Good electronic ignition distributor.

I prefer to use all MSD components.

Last edited by carsavvycook; 09-22-2008 at 01:54 AM. Reason: addition
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:
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
SBC 350 superiority rebuttle...was:POWERED BY CHEVY, WRITTEN ON A FORD, (from 'Engine Oldsmolac911 Hotrodders' Lounge 50 01-12-2011 11:00 PM
350 SBC doesn't idle, other problems too. ajax1 Engine 17 02-25-2008 08:33 PM
newer sbc 350 to older turbo 350 yragat Transmission - Rearend 8 10-20-2007 08:52 PM
Timing woes on SBC 350 with bigger cam. Brawler Engine 5 09-07-2007 05:33 PM
SBC 350 Crank Balancer Problems bjam2 Engine 8 05-04-2004 08:36 PM


All times are GMT -6. The time now is 05:13 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.