Vacuum & Timing Setting...Help!!
Thread Starter
Instructor
Joined: Dec 2006
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From: Keller TX
Vacuum & Timing Setting...Help!!
Trouble with my timing.
First, I rebuilt my 77, 350 auto. Mild cam, vortec heads, stock bore and stock cylinders, eldelbrock intake, Q-jet (rebuilt by Lars), 2500-3000 stall.
Car idles good at about 800.
Here are my problems.
1. I put a vacuum gauge on the carb where the distributor vacuum hose would go. I only get about 12 on the gauge. Is that the right place to check for vacuum? I was thinking I needed about 20? I unplugged all other vacuum hoses from the carb and plugged the carb port. I also unplugged the two lines from the fitting on the manifold and plugged the fitting. No change in the vacuum. Do you think I have a leak? Does a cam cause lower vacuum?
2. I go to set my timing. Started out at 12*. Then brought it up to about 3000 and adjusted to 36*. It is all in at 3000. But.....when I go back to check my base, it is 22* (yikes) I drove the car around the block and it ran fine as far as I could tell. Got any ideas what I am doing wrong?
Doug
First, I rebuilt my 77, 350 auto. Mild cam, vortec heads, stock bore and stock cylinders, eldelbrock intake, Q-jet (rebuilt by Lars), 2500-3000 stall.
Car idles good at about 800.
Here are my problems.
1. I put a vacuum gauge on the carb where the distributor vacuum hose would go. I only get about 12 on the gauge. Is that the right place to check for vacuum? I was thinking I needed about 20? I unplugged all other vacuum hoses from the carb and plugged the carb port. I also unplugged the two lines from the fitting on the manifold and plugged the fitting. No change in the vacuum. Do you think I have a leak? Does a cam cause lower vacuum?
2. I go to set my timing. Started out at 12*. Then brought it up to about 3000 and adjusted to 36*. It is all in at 3000. But.....when I go back to check my base, it is 22* (yikes) I drove the car around the block and it ran fine as far as I could tell. Got any ideas what I am doing wrong?
Doug
Thread Starter
Instructor
Joined: Dec 2006
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From: Keller TX
Update on Timing.
I have 15* at idle and 36* at 3,000 rpm (all in). I didn't realize that my idle had gone up signficantly when trying to adjust (wife was in the car and didn't tell me).
But...when I hook up the distributor vacuum advance, it takes it to 55*...which I know is too high.
I have an AS-20 advance. I'm thinking it is an adjustable but I can't find any info on it Heading to Napa to see if they have a 15 or 16 * advance unit.
Hoping that does it. Any feedback? Am I way off on this?
Doug
I have 15* at idle and 36* at 3,000 rpm (all in). I didn't realize that my idle had gone up signficantly when trying to adjust (wife was in the car and didn't tell me).
But...when I hook up the distributor vacuum advance, it takes it to 55*...which I know is too high.
I have an AS-20 advance. I'm thinking it is an adjustable but I can't find any info on it Heading to Napa to see if they have a 15 or 16 * advance unit.
Hoping that does it. Any feedback? Am I way off on this?
Doug
Drifting
Joined: Mar 2006
Posts: 1,526
Likes: 2
From: Oak Ridge NC
Update on Timing.
I have 15* at idle and 36* at 3,000 rpm (all in). I didn't realize that my idle had gone up signficantly when trying to adjust (wife was in the car and didn't tell me).
But...when I hook up the distributor vacuum advance, it takes it to 55*...which I know is too high.
I have an AS-20 advance. I'm thinking it is an adjustable but I can't find any info on it Heading to Napa to see if they have a 15 or 16 * advance unit.
Hoping that does it. Any feedback? Am I way off on this?
Doug
I have 15* at idle and 36* at 3,000 rpm (all in). I didn't realize that my idle had gone up signficantly when trying to adjust (wife was in the car and didn't tell me).
But...when I hook up the distributor vacuum advance, it takes it to 55*...which I know is too high.
I have an AS-20 advance. I'm thinking it is an adjustable but I can't find any info on it Heading to Napa to see if they have a 15 or 16 * advance unit.
Hoping that does it. Any feedback? Am I way off on this?
Doug
I don't know what an AS20 is but if the 20 is the amount of advance, then it does look like too much. I think for your year the stock lower HP cars (165HP?) had an 18 can and the higher HP car(205HP?) had a 15 can. You not want to go higher than 52 total. There is a sticky post at the top of the tech forum by BarryK that has really good info.
Here it is:
http://forums.corvetteforum.com/show....php?t=1430339
Last edited by SmokinBBC; Jul 28, 2007 at 04:43 PM.
Thread Starter
Instructor
Joined: Dec 2006
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From: Keller TX
Thanks for the reply.
Here's another update. I found out the AS-20 is an 'adjustable' can. I adjusted it so I now have 52* total combined (yippe finally).
Now....can anyone help me understand my vacuum question. I connected it to the port where the distributor vacuum advance goes (one the carb). Reading only about 12-13 at idle. Am I supposed to have more vacuum at idle?
When I hit the gas, it drops to near zero and then up around 20 or so....then back to 12-13.
The car idles fine. Starts fine. Seems responsive.
Who can educate me on my vacuum?
Doug
Here's another update. I found out the AS-20 is an 'adjustable' can. I adjusted it so I now have 52* total combined (yippe finally).
Now....can anyone help me understand my vacuum question. I connected it to the port where the distributor vacuum advance goes (one the carb). Reading only about 12-13 at idle. Am I supposed to have more vacuum at idle?
When I hit the gas, it drops to near zero and then up around 20 or so....then back to 12-13.
The car idles fine. Starts fine. Seems responsive.
Who can educate me on my vacuum?
Doug
Melting Slicks
Joined: Oct 2005
Posts: 2,056
Likes: 8
From: Redruth Cornwall
A cam will change your vacuum. A 'bigger' cam will reduce your vacuum readings. I dont know the whys but it will. Some guys who have really big cams install vacuum pumps.
Some more knowledgeable peeps will chime in.
If you have the timing set and your brakes work well, you will probably be ok.
Some more knowledgeable peeps will chime in.
If you have the timing set and your brakes work well, you will probably be ok.
Thread Starter
Instructor
Joined: Dec 2006
Posts: 207
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From: Keller TX
Thanks for the reply. I'm no expert at reading a cam card. My machine shop recommended this cam. Comp Cam Part # 12-246-3
At the bottom of the card, it says,
DURATION @ .050 INTAKE 230, EXHAUST 236
LOBE LIFT INTAKE .3270, EXHAUST .3270
The car certainly sounds good with it. Hope its a decent cam.
Doug
At the bottom of the card, it says,
DURATION @ .050 INTAKE 230, EXHAUST 236
LOBE LIFT INTAKE .3270, EXHAUST .3270
The car certainly sounds good with it. Hope its a decent cam.
Doug
Instructor
Joined: Mar 2007
Posts: 223
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From: Henryville Indiana
Thats a real healthy cam,.490 lift and 230/236 duration.Try adjusting your carb mixture for highest vacuum reading at idle and you'l be surprised at what it'll feel like then reset your timing,and idle speed.You might get 15" vacuum out of it.
That cam will pull to near 6000 with a good intake and good exhaust.You got a HOLLEY?
I hope you got a 4 speed or a looser converter.....It'll want to creep on you at lights.
That cam will pull to near 6000 with a good intake and good exhaust.You got a HOLLEY?
I hope you got a 4 speed or a looser converter.....It'll want to creep on you at lights.
Thread Starter
Instructor
Joined: Dec 2006
Posts: 207
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From: Keller TX
Thanks for the reply.
Yes, I have a looser converter. ...Hughes 2,500
I'm running my Q-Jet (rebuild by Lars). I realize there is a love/hate thing with Q-Jets.
I'll try to adjust the idle mixture tomorrow to see if I can get any more vacuum. Thanks for the advise.
Thanks,...Doug
Yes, I have a looser converter. ...Hughes 2,500
I'm running my Q-Jet (rebuild by Lars). I realize there is a love/hate thing with Q-Jets.
I'll try to adjust the idle mixture tomorrow to see if I can get any more vacuum. Thanks for the advise.
Thanks,...Doug
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Verdad Gallardo
Instructor
Joined: Jun 2006
Posts: 146
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From: North Brunswick New Jersey
Someone please correct me if I'm wrong but don't you check the vacuum below the throttle plates(manifold vacuum)? The place you describe your vac hookup and the question you posed may be above the throttle plates at a ported vacuum nipple. I don't think you're going to get the results you're looking for on a ported vacuum supply.
Racer
Joined: Jul 2003
Posts: 455
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From: Buffalo New York
If it runs well with 36, keep it there, however I have more power retarding it back to 32 all in by 2800. That leaves me with 18 at idle and 14 mechanical advance. With vaccum advance hooked up I am at 48 total timing, which is nice and safe for cruising.
On your vaccum, 12 is pretty good. I have 14 at idle. Tune it for highest idle, but as long as everything works, ie headlights and such you are in good shape.
Good Luck!!!
Chris
On your vaccum, 12 is pretty good. I have 14 at idle. Tune it for highest idle, but as long as everything works, ie headlights and such you are in good shape.
Good Luck!!!
Chris
Thread Starter
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Joined: Dec 2006
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From: Keller TX
Thanks for the advise guys.
My carb right now is set to 5 & 1/2 turns out. Lars told me today that it was very likely too much....so Ill try again for the highest vacuum or idle in the next day or so and see if it makes much difference after a test drive.
May play with the distributor springs and timing to see if I can get all in a 'little' sooner than 3K....I think?
Doug
My carb right now is set to 5 & 1/2 turns out. Lars told me today that it was very likely too much....so Ill try again for the highest vacuum or idle in the next day or so and see if it makes much difference after a test drive.
May play with the distributor springs and timing to see if I can get all in a 'little' sooner than 3K....I think?
Doug