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04-13-2009, 09:07 PM
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#1 |
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"the other guy"
Join Date: Oct 2008
Location: Olivehurst California 95961
Posts: 901
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250 ported or unported vacuum, timing
im about to start tuning my 250, i need to know if the vac advance for the hei needs to be ported or manifold vacuum? also need to know the same for the vac modulator. i rebuilt the carb and i still get the same stumble even the occasional pop when i snap the throttle. my timing tab is there but the digits are unreadable, does anyone have a readable pic of there timing tab? thanks
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God Said,"Building A 10 Second Import Is Like Coming Out Of The Closet, In The End Ur Still....."EXPLETIVE" 
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04-14-2009, 12:27 AM
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#2 |
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"the other guy"
Join Date: Oct 2008
Location: Olivehurst California 95961
Posts: 901
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Re: 250 ported or unported vacuum, timing
any1?
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God Said,"Building A 10 Second Import Is Like Coming Out Of The Closet, In The End Ur Still....."EXPLETIVE"
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04-14-2009, 06:39 AM
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#3 |
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Senior Member
 
Join Date: Feb 2002
Location: Tucson, AZ USA
Posts: 7,215
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Re: 250 ported or unported vacuum, timing
What Carb?
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Every 25 years I like to rebuild that 292, whether it needs it or not. |
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04-14-2009, 09:38 AM
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#4 |
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"the other guy"
Join Date: Oct 2008
Location: Olivehurst California 95961
Posts: 901
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Re: 250 ported or unported vacuum, timing
Rochester 1 barrel model b
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God Said,"Building A 10 Second Import Is Like Coming Out Of The Closet, In The End Ur Still....."EXPLETIVE"
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04-14-2009, 09:57 AM
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#5 |
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Active Member
Join Date: Jul 2007
Location: Centrally located between Houston, Austin and Waco. BCS area.
Posts: 7,947
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Re: 250 ported or unported vacuum, timing
I've used both.
I used ported with my points distributor and I'm using non ported with my HEI (I tried it with both). I would just play with it and see which one you like. Adjust the carb for best vacuum at idle and advance the distributor (counter clockwise) so that it accelerates smoothly without hesitation. Make sure it doesn't ping/rattle under load. Mostly it's just trial and error to see what the best settings are for your vehicle. |
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04-14-2009, 11:48 AM
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#6 |
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Its a Truck Thing......
Join Date: Dec 2004
Location: Sierra Vista, Arizona
Posts: 3,159
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Re: 250 ported or unported vacuum, timing
Vac advance should use ported vac.
Modulator should be unported. Smitty
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71 C10 283/3SPD Full Resto 71 GMC 1500 Sierra Grande http://67-72chevytrucks.com/vboard/s...d.php?t=518599 70 C10 Suburban Former military GSA truck. 72 Chevy Blazer 4X4, Sloppy Jo, Mountain Climber. Wife says no more trucks. http://67-72chevytrucks.com/vboard/s...d.php?t=707378 72 GMC 1 Ton Motor Home, wife said no more trucks until she saw this one. Gen 3 6.0/4L80E 4.10 http://67-72chevytrucks.com/vboard/s...d.php?t=761110 68 GMC Suburban was 3/4 ton, now 1/2.Wife shook her head 71 C30 Wrecker 71 C20 Scott-Bilt As weird as it gets..BB Cheyenne AC Truck 68 GMC Long Stepside. They keep following me home 71 C20 Longhorn/Ole Yellor 69 C30 Former Motor Home, Flat Bed time |
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04-14-2009, 12:02 PM
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#7 |
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Registered User
Join Date: Nov 2006
Location: Alabama
Posts: 120
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Re: 250 ported or unported vacuum, timing
Here's a rough sketch of the 250 timing tab .
The factory points were set @ 4degrees BTDC . With my HEI I'm set @ 10 degrees BTDC.
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Working on my first 1,000 post. I'm sure I'll get there before this project is over  .
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04-14-2009, 03:44 PM
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#8 |
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"the other guy"
Join Date: Oct 2008
Location: Olivehurst California 95961
Posts: 901
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Re: 250 ported or unported vacuum, timing
blue71, it may be my computer screen but i cant make out the numbers?
__________________
God Said,"Building A 10 Second Import Is Like Coming Out Of The Closet, In The End Ur Still....."EXPLETIVE"
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04-14-2009, 04:21 PM
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#9 |
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Registered User
Join Date: Nov 2006
Location: Alabama
Posts: 120
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Re: 250 ported or unported vacuum, timing
12 - 8 - 4 - 0 - 4 - 8
\/ \/ \/ \/ \/ \/
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Working on my first 1,000 post. I'm sure I'll get there before this project is over .
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04-14-2009, 04:26 PM
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#10 |
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"the other guy"
Join Date: Oct 2008
Location: Olivehurst California 95961
Posts: 901
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Re: 250 ported or unported vacuum, timing
???
Last edited by vasquezmrysvll; 04-14-2009 at 04:28 PM. |
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04-14-2009, 04:27 PM
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#11 |
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"the other guy"
Join Date: Oct 2008
Location: Olivehurst California 95961
Posts: 901
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Re: 250 ported or unported vacuum, timing
12-8-4-0-4-8
\/ \/\/ \/ \/\/ is it like this?
__________________
God Said,"Building A 10 Second Import Is Like Coming Out Of The Closet, In The End Ur Still....."EXPLETIVE"
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04-14-2009, 09:06 PM
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#12 |
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Active Member
Join Date: Jul 2007
Location: Centrally located between Houston, Austin and Waco. BCS area.
Posts: 7,947
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Re: 250 ported or unported vacuum, timing
Here's my 250 timing mark... Graduated in 2 degree increment. If you look real close you can see where the "0" is on the third mark from the bottom. Barely see the "A" on top but it's the 8th increment from the bottom.
Ported vs. non ported has been discussed and argued but it really is up to where the truck runs the best. The following site has some real good info re: vacuum advance theory. http://www.hotrodders.com/forum/dist...1-a-59033.html |
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04-14-2009, 09:23 PM
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#13 |
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"the other guy"
Join Date: Oct 2008
Location: Olivehurst California 95961
Posts: 901
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Re: 250 ported or unported vacuum, timing
68gmsee, thanks! on your dampener, is there a groove for the timing mark? mine just has a nick in the backside of the pulley. is that it?
__________________
God Said,"Building A 10 Second Import Is Like Coming Out Of The Closet, In The End Ur Still....."EXPLETIVE"
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04-14-2009, 10:43 PM
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#14 |
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Active Member
Join Date: Jul 2007
Location: Centrally located between Houston, Austin and Waco. BCS area.
Posts: 7,947
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Re: 250 ported or unported vacuum, timing
Re: timing mark... nick is all that's on mine..
What I've done. Best way to make sure is to remove the spark plug from #1 cylinder and rotate the engine so that it's at TDC on the compression stroke. Plug the hole with your finger or thumb and you feel air pressure coming out of the hole. If you have a bolt on the crankshaft it's easier to turn if you remove all the plugs. Visually check or feel with dowel or screwdriver to insure piston is at the very top. Rotor should be pointing to #1 plug wire and the harmonic balancer should be at TDC or "0" if you have a marker like mine. If you don't see the notch on the harmonic balancer, scribe one with a small file. Last edited by 68gmsee; 04-14-2009 at 10:45 PM. |
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04-14-2009, 10:58 PM
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#15 |
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"the other guy"
Join Date: Oct 2008
Location: Olivehurst California 95961
Posts: 901
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Re: 250 ported or unported vacuum, timing
hey thanks alot!
__________________
God Said,"Building A 10 Second Import Is Like Coming Out Of The Closet, In The End Ur Still....."EXPLETIVE"
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