Ignition Timing Questions - Page 2 - 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
  #16 (permalink)  
Old 10-11-2010, 09:26 AM
Greg T's Avatar
www.krusinklassics.net
 
Last photo:
Join Date: Jul 2007
Location: Escanaba, MI.
Age: 60
Posts: 1,151
Wiki Edits: 0

Thanks: 20
Thanked 41 Times in 32 Posts
Full manifold vacuum is a must for long duration cams. The engine runs freer, cooler and idles much better. My 388 is set up with 18* initial, 36* total mechanical all in by 2800 and 12* of vac at idle. My total at idle is 30*. The engine runs smoother, has much better throttle response and street manners. With ported vacuum, I would have to richen the idle mix and open the trottle blades more to maintain my 900 rpm idle. With full manifold vac the 4 corner idle adjustments work just fine, the idle is cleaner, cooler, and I don't have to worry about run-on from having the throttle open.

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #17 (permalink)  
Old 10-11-2010, 09:52 AM
cobalt327's Avatar
WFO
 
Last wiki edit: Intake manifold
Last journal entry: 1980 Malibu Wagon
Last photo:
Join Date: Mar 2009
Location: Atlanta
Age: 60
Posts: 5,037
Wiki Edits: 1616

Thanks: 128
Thanked 597 Times in 546 Posts
Quote:
Originally Posted by skizot
I should be hooking the distributor vacuum advance up to the full manifold vacuum port on the carburetor, right?
Right. You have enough cam to need more advance than a stock cam needs, at idle.

The advantage to using a manifold vacuum source instead of a ported source is that the manifold vacuum source to the vacuum advance increases the idle speed- this is to your benefit because this allows you to close the throttle blades via the curb idle adjustment screw.

By doing so, the transfer slot is now less exposed. Having the transfer slot exposure correct is KEY to having good idle control using the idle mixture screws (which is lost if the throttle blades are open too far).

Having the throttle blades set properly will allow the engine to idle w/o being too rich and unresponsive as you transition from idle to the main circuit- an all too common problem w/large cams/low idle vacuum conditions.

Quote:
After more reading, I decided to do this, and noticed that the distributor was now advancing the timing above what I'd set it to with the vacuum disconnected. But that brings me to another question. I set the initial timing to 12 with the vacuum advance disconnected. With it hooked back up, I looked at the timing again, and it was only 15.5. Is that all a stock GM vacuum advance can gives at 12" vacuum? Shouldn't it be providing at least 10? It is not adjustable.
There are ways around this common problem.

First, you need to know how much mechanical advance is being supplied by the distributor as it now is. The HEI distributors are all capable of a lot of advance, it's the weights and weight cam that determines how much of this potential advance is actually seen by the engine.

Once you know that, you will also know whether the mechanical advance combined w/the initial timing needs to be limited or increased to have the 32 of total timing you need w/Vortec heads (maximum of 34, but try 32 first).

This is where limiting or adjusting the vacuum advance comes into play. The vacuum advance hooked to manifold vacuum will allow leeway in what the initial is set at. Obviously, you will need to know how much the vacuum advance is supplying and at what in/Hg the advance begins.

Just as an example:

Say your mechanical advance supplies 22. You need a total of 32, so you need to add 10 of initial timing. You will find that's not enough initial timing, so you'd need to either modify the distributor to limit the timing allowed (in order to dial in more initial), or add timing by using the vacuum advance supplied by manifold vacuum to give you the additional timing.

Usually, 10 to 12 of vacuum advance is sufficient, so you may also find a need to adjust how much vacuum advance is supplied and at what vacuum it is supplied at.

You should be seeing more than you are from your vacuum advance at 12 in/Hg of vacuum. You're using a used OEM distributor, so be sure the advance plate is free to turn easily- they will have dry bushings after a long life, and can need cleaned and relubricated. Also be sure the OEM can is holding vacuum. Even if it is, it may have a dried out, still diaphragm keeping it from working as it should.

HERE'S an exploded view of an HEI distributor.

THIS is a description of an HEI rebuild.

The ACCEL #31035 is an adjustable vacuum advance can for GM HEI that allows infinite adjustment to BOTH the amount and rate of advance. Comes w/instructions and tool.

To limit the total amount of vacuum advance w/an HEI distributor, you will need to physically limit the vacuum advance can's travel w/a vacuum advance limiter plate like the Crane #99619-1, #99619-1 Instructions. Or you can easily make one.

DETERMINING TDC will allow you to be sure the timing tab and damper are correct for TDC.

MAKE A TIMING TAPE to see what the total timing is, w/o needing to use a dial back timing light. Or buy one, they cost about $5.00, but be sure it's for the right diameter damper or it'll not be as accurate.

Last edited by cobalt327; 10-11-2010 at 09:59 AM.
Reply With Quote Quick reply to this message
  #18 (permalink)  
Old 10-11-2010, 03:34 PM
Member
 
Join Date: Mar 2004
Location: Midwest
Posts: 31
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Thanks for the responses, guys.

cobalt327, do you think I'd be better off getting a new aftermarket distributor? I know it's going to cost more, but maybe it would be worth it. If so, what would you suggest?
Reply With Quote Quick reply to this message
  #19 (permalink)  
Old 10-11-2010, 09:29 PM
cobalt327's Avatar
WFO
 
Last wiki edit: Intake manifold
Last journal entry: 1980 Malibu Wagon
Last photo:
Join Date: Mar 2009
Location: Atlanta
Age: 60
Posts: 5,037
Wiki Edits: 1616

Thanks: 128
Thanked 597 Times in 546 Posts
There was a thread awhile ago that compared an internet "cheap" distributor to a name brand, and they appeared to be very nearly the same part. I am not recommending either distributor- because I have not used either of them. But the comparison is very interesting.

That said, if the GM HEI has a tight shaft, no bushing wear, there's no reason it cannot be cleaned, have the end play set to ~ 0.015", an advance kit bought for the springs (I always use OEM weights), maybe an adjustable vacuum advance can and/or limiter plate, and maybe a better module installed and it would be a good unit. I have done this MANY times, w/o a single failure.

The thing is, the cheap distributor comes w/much of this already- including a cap and rotor IIRC, so would be hard to beat.
Reply With Quote Quick reply to this message
  #20 (permalink)  
Old 10-18-2010, 09:27 PM
Member
 
Join Date: Mar 2004
Location: Midwest
Posts: 31
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Thanks, guys. The vacuum can on the distributor had a ruptured diaphragm; it wasn't holding vacuum.

I went ahead and got a new Skip White 6500-R distributor, and a Crane Cams adjustable vacuum canister kit. Got them both installed this weekend, and it's running much better. The vacuum can is now hooked up to the full manifold vacuum port as well.

I've got the MSD 8984 Starter Saver on its way, and am in the process of calculating mechanical advance limit using circumference calculations and the "screw limiter" method. The goal is to get my initial set at 20 degrees, and the mechanical limited to 12 degrees.
Reply With Quote Quick reply to this message
  #21 (permalink)  
Old 10-18-2010, 10:12 PM
aintmisbehavinn's Avatar
Jesus loves you I don't...
 
Last photo:
Join Date: Sep 2010
Location: Lawrenceville GA
Posts: 31
Wiki Edits: 0

Thanks: 1
Thanked 0 Times in 0 Posts
Manifold vac all the way, unless you're stock and need the government regulations or pollution controls after 73. Holley placed the ported vac on the carb to meet EPA government specs on pollution controlled vehicles nationwide, you'd be lucky to find one on older holleys. Mine runs 80% better with manifold vac at idle, the cam loves it too.
Reply With Quote Quick reply to this message
  #22 (permalink)  
Old 10-19-2010, 07:09 AM
Registered User
 
Join Date: Oct 2010
Location: Calgary
Posts: 11
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
hi,
I am new here, but i have learn many things herein short duration of time, and will keep on learning about automobiles. It happens for many time that we may facing same kind of problem and got solution by reading these replies, I really like to thank you all for letting us know your problem and heartly wants to thanks those who give there ideas and views about the same problem

Thanks
Reply With Quote Quick reply to this message
  #23 (permalink)  
Old 10-24-2010, 02:29 PM
Registered User
 

Last journal entry: ok im on to my next big dream beast
Last photo:
Join Date: Jul 2005
Location: fort riley kasas
Posts: 82
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
ported vac?

so how exactly does a vacuum secondary work because my understanding is that it takes vacuum to open the secondaries and my knowledge leads me to believe that i only want them open at wot which according so some of these posts suggests that no vacuum opens the secondaries and if thats the case why are they closed when the engine is not running at all?

so either my limited carb research has decieved me or their is a port on the carb that produces more vacuum as you increase the throttle opening
Reply With Quote Quick reply to this message
  #24 (permalink)  
Old 10-24-2010, 02:51 PM
Custom10's Avatar
my KARMA ran over my DOGMA
 

Last journal entry: SS
Last photo:
Join Date: May 2009
Location: Saskatchewan, Canada
Age: 50
Posts: 1,101
Wiki Edits: 0

Thanks: 18
Thanked 31 Times in 27 Posts
Quote:
Originally Posted by machinestheman
so how exactly does a vacuum secondary work because my understanding is that it takes vacuum to open the secondaries and my knowledge leads me to believe that i only want them open at wot which according so some of these posts suggests that no vacuum opens the secondaries and if thats the case why are they closed when the engine is not running at all?

so either my limited carb research has decieved me or their is a port on the carb that produces more vacuum as you increase the throttle opening
If we are talking about a typical holley vacumm secondary carb,,,there is a vacuum secondary diaphram that acts against a spring, the spring pressure wants to open the secondaries, when under light load the engine high vacuum acts upon the diaphram and overcomes the spring force so the secondaries are pulled or held closed. As engine load increases the vacuum drops and the spring moves the secondary diaphram connecting rod to open up the secondaries, however:

the secondaries are prevented from opening due to the position of the primary throttle plates, if the primaries are closed the secondaries are prevented from opening due to a mechanical linkage connection, so no matter what the engine vacuum is the secondaries won't open unless the primaries are open

summary; the primarys must be open AND the engine vac force on the diaphram needs to be lower than the effective force of the secondary diaphram spring before you see the secondaries come on, thats my understanding of it.
Reply With Quote Quick reply to this message
  #25 (permalink)  
Old 10-24-2010, 11:05 PM
ericnova72's Avatar
More for Less Racer
 
Join Date: Jan 2007
Location: S.W. Lower Michigan
Age: 47
Posts: 9,637
Wiki Edits: 0

Thanks: 41
Thanked 542 Times in 491 Posts
Vacuum secondary Holley carbs use the venturi vacuum to open the secondarys, not engine manifold vacuum. The port that runs the vacuum diaphragm can be seen as a 1/8" diameter tube , slash cut on the end, sticking out about 1/16" into the smallest diameter(the venturi) section of the passenger side front carb barrel. If you look down into the carb from above you will see the end of this tube at about the 10 o'clock position in that front pass side carb throat. It is high air speed though the front barrels that creates vacuum in this tube, pulling open the secondarys against the spring pressure. Spring tension controls how fast it opens(Holley sells a kit with various tension springs).

Totally unrelated to engine vacuum.
Reply With Quote Quick reply to this message
  #26 (permalink)  
Old 10-25-2010, 03:44 AM
F-BIRD'88's Avatar
Yada Yada Yada
 
Last photo:
Join Date: Oct 2005
Location: Ontario, Canada
Posts: 9,202
Wiki Edits: 0

Thanks: 5
Thanked 388 Times in 382 Posts
With that 268 cam you will find that the car runs the best with the best idle quality, most stable idle in and out of gear and overall throttle response if you recurve the distributor.

You want: 20 up to 24 inital-base timing at idle (700rpm). 34 to 36 max mechanical advance peaking around 3000-3200rpm
(shorten the mechanical advance curve limit from stock 20-22deg to a short 10-12deg.)

You must modify the distributor advance stop limit. You cannot get this curve by just swapping advance springs.

Once you have modified the mechanical stop/limit, use 1 medium and 1 light tension advance spring. This gives a smooth stable mechanical advance from just off idle to around 3000rpm +/-.

Search my posts for how.

Then for vacuum advance you want 10-12 deg of additional vacuum advance to come in when hiway driving at steady hiway speed when manifold vacuum is high and engine load is light. adjust the vac advance as required and limit it to around 15deg max at highest vacuum.

You do not want all the vac advance at idle.

This is the curve that works the best with a auto trans and a high perf cam.

Use ported vacuum.

Last edited by F-BIRD'88; 10-25-2010 at 03:52 AM.
Reply With Quote Quick reply to this message
  #27 (permalink)  
Old 10-25-2010, 09:38 AM
Custom10's Avatar
my KARMA ran over my DOGMA
 

Last journal entry: SS
Last photo:
Join Date: May 2009
Location: Saskatchewan, Canada
Age: 50
Posts: 1,101
Wiki Edits: 0

Thanks: 18
Thanked 31 Times in 27 Posts
Quote:
Totally unrelated to engine vacuum
thanks eric, my mistake on the source of vacuum/pressure to the diaphragm.
Reply With Quote Quick reply to this message
  #28 (permalink)  
Old 10-27-2010, 12:53 PM
Registered User
 

Last journal entry: ok im on to my next big dream beast
Last photo:
Join Date: Jul 2005
Location: fort riley kasas
Posts: 82
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
so is this not the same type of vacuum that normally operates the vacuum advance on a stock distributor/carb setup?

more rpms=more vacuum=more advance

unless your running a long duration cam where you need the advance at low engine speeds and it is hooked directly to manifold vacuum as stated before?

sorry trying to nail this down

Quote:
Originally Posted by ericnova72
Vacuum secondary Holley carbs use the venturi vacuum to open the secondarys, not engine manifold vacuum. The port that runs the vacuum diaphragm can be seen as a 1/8" diameter tube , slash cut on the end, sticking out about 1/16" into the smallest diameter(the venturi) section of the passenger side front carb barrel. If you look down into the carb from above you will see the end of this tube at about the 10 o'clock position in that front pass side carb throat. It is high air speed though the front barrels that creates vacuum in this tube, pulling open the secondarys against the spring pressure. Spring tension controls how fast it opens(Holley sells a kit with various tension springs).

Totally unrelated to engine vacuum.
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
Symptoms of Late Ignition Timing Argess Electrical 6 10-11-2010 07:38 AM
installing distributor/timing on sbc 350 075turbo Hotrodding Basics 38 05-19-2010 08:33 AM
Should ignition timing be locked out for drag racing? chevyv8_power Engine 4 05-04-2008 05:55 PM
SBC Ignition Timing MarcoG Engine 15 04-09-2008 05:52 PM
ignition timing amblessed4life Engine 1 05-21-2006 05:53 PM


All times are GMT -6. The time now is 09:57 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.