help interpret vacuum readings, please
Thread Starter
Burning Brakes
Joined: Jan 2006
Posts: 957
Likes: 6
From: Burlington wisconsin
help interpret vacuum readings, please
I am chasing my tail trying to get my 383 to idle. I know it's going to be somewhat poor, but it konks out when I put it in gear. WHY??
Here is what I have:
383 stroker (69 L-46 block)
CC XE274 cam
KB 10.2:1 pistons
Dart eagle SS heads
Long tube hookers
duals w/ cross over pipe
Holley St. Avenger 83-670 carb
TCI Super St. Fighter TH-350 W/ Hughes GM2500 T-Converter
3:55 gears
MSD Pro billet dizzy
Black 18 deg bushing
1 hvy & 1 light silver springs
MSD 6AL BOX
Plugs gapped @ 0.035" (AC R45TS)
DIAGNOSIS
In park I can get it somewhat smooth or at least idle @ 1000-1100 rpm
If I try to get 900 in park she wants to die.
At the 1000 in park idle, when put in drive she drops to aprox. 400 rpm and dies
If I get a good "in drive" idle (lumpy) but 6-700 rpm, then put it in park, she climbs to aprox. 1500 rpm
Idle vacuum reading off full manifold vac fitting on carb = 12"-13"
when throttle is snapped it drops to aprox. 3"-5" and on deceleration it climbs to aprox. 18"
Initial timing = 16 deg
Any ideas???? Or is this a Holley P-Valve issue due to low vacuum?
Any discussions would be appreciated.
Here is what I have:
383 stroker (69 L-46 block)
CC XE274 cam
KB 10.2:1 pistons
Dart eagle SS heads
Long tube hookers
duals w/ cross over pipe
Holley St. Avenger 83-670 carb
TCI Super St. Fighter TH-350 W/ Hughes GM2500 T-Converter
3:55 gears
MSD Pro billet dizzy
Black 18 deg bushing
1 hvy & 1 light silver springs
MSD 6AL BOX
Plugs gapped @ 0.035" (AC R45TS)
DIAGNOSIS
In park I can get it somewhat smooth or at least idle @ 1000-1100 rpm
If I try to get 900 in park she wants to die.
At the 1000 in park idle, when put in drive she drops to aprox. 400 rpm and dies
If I get a good "in drive" idle (lumpy) but 6-700 rpm, then put it in park, she climbs to aprox. 1500 rpm
Idle vacuum reading off full manifold vac fitting on carb = 12"-13"
when throttle is snapped it drops to aprox. 3"-5" and on deceleration it climbs to aprox. 18"
Initial timing = 16 deg
Any ideas???? Or is this a Holley P-Valve issue due to low vacuum?
Any discussions would be appreciated.
Le Mans Master
Joined: Jun 2005
Posts: 5,447
Likes: 3
From: Syracuse NY
I wish I could give the member that posted this credit for doing so but this was posted earlier today. You might find it helpful.
http://www.secondchancegarage.com/public/186.cfm
http://www.secondchancegarage.com/public/186.cfm
Thread Starter
Burning Brakes
Joined: Jan 2006
Posts: 957
Likes: 6
From: Burlington wisconsin
I wish I could give the member that posted this credit for doing so but this was posted earlier today. You might find it helpful.
http://www.secondchancegarage.com/public/186.cfm
http://www.secondchancegarage.com/public/186.cfm
Race Director
Joined: Dec 1999
Posts: 19,610
Likes: 778
From: Forked River NJ
That cam is not that radical for a 383. Your idle vacuum is normal. I run at about 8 to 9 inches of vacuum but I also have a stick in my car. I also had to run 18 to 20 degrees initial advance. Are you running vacuum advance?
What PV are you running?
What PV are you running?
Last edited by Gordonm; Jul 15, 2009 at 09:28 PM.
Thread Starter
Burning Brakes
Joined: Jan 2006
Posts: 957
Likes: 6
From: Burlington wisconsin
Yes, I do run a vacuum advance, I actually switched the MSD B-26 can to a B-28 now. The PV is the stock one that came in the St. Avenger # 6.5
Last edited by cherrybombc3; Jul 15, 2009 at 09:58 PM.
Le Mans Master
Joined: Jun 2008
Posts: 5,284
Likes: 235
From: Rockledge FL
I wish I could give the member that posted this credit for doing so but this was posted earlier today. You might find it helpful.
http://www.secondchancegarage.com/public/186.cfm
http://www.secondchancegarage.com/public/186.cfm
you can give me the credit, i dont mind.
with a performance cam does the general rule of thumb to diving vacuum in gear and get a PV a few points under that.
chances are the standard holley 6.5 will work.
also what is the secondary throttle plate set at?
Last edited by baxsom; Jul 15, 2009 at 09:36 PM.
Le Mans Master
Joined: Jun 2005
Posts: 5,447
Likes: 3
From: Syracuse NY
I did save the link.
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Race Director
Joined: Dec 1999
Posts: 19,610
Likes: 778
From: Forked River NJ
I hada problem running a vacuum can on mine. At a stop light when my idle was set at 850 to 900 rpm the vacuum was around 8 inches. It would idle for a little bit but it was on the brink of dropping out the vacuum can and just a little would drop the can out and the idle would drop and the motor would stall out. I then ran no vacuum and it ran fine. Then I put it on a ported vacuum signal and it would only pull in the vacuum at a cruise rpm. It would idle OK and I was getting the advance at cruise. I have since switched out to FI and the timing is controlled by the computer. You might try to block off the vacuum and set the initial at about 18 to 20 and see what happens.
The 6.5 PV should work fine at the vacuum you are running.
The 6.5 PV should work fine at the vacuum you are running.
Thread Starter
Burning Brakes
Joined: Jan 2006
Posts: 957
Likes: 6
From: Burlington wisconsin
I hada problem running a vacuum can on mine. At a stop light when my idle was set at 850 to 900 rpm the vacuum was around 8 inches. It would idle for a little bit but it was on the brink of dropping out the vacuum can and just a little would drop the can out and the idle would drop and the motor would stall out. I then ran no vacuum and it ran fine. Then I put it on a ported vacuum signal and it would only pull in the vacuum at a cruise rpm. It would idle OK and I was getting the advance at cruise. I have since switched out to FI and the timing is controlled by the computer. You might try to block off the vacuum and set the initial at about 18 to 20 and see what happens.
The 6.5 PV should work fine at the vacuum you are running.
The 6.5 PV should work fine at the vacuum you are running.
Last edited by cherrybombc3; Jul 15, 2009 at 10:03 PM.
Thread Starter
Burning Brakes
Joined: Jan 2006
Posts: 957
Likes: 6
From: Burlington wisconsin
I will & report back tomorrow. As for Baxom's earlier question regarding what my seconday throttle plate is set at. I don't know, didnt mess with that?
Thread Starter
Burning Brakes
Joined: Jan 2006
Posts: 957
Likes: 6
From: Burlington wisconsin
Melting Slicks
Joined: Apr 1999
Posts: 2,010
Likes: 3
From: Moon Twp. PA USA
No idle issues.
12 cc dished pistons
'0' decked block
Performer RPM intake with modified/rebuilt stock Q-Jet
TF 23* heads - 64 cc - 0.041 head gaskets
Dynomax ceramic 1 5/8" headers - chambered exhaust
And, of course CC XE274H cam
HEI distributor - 35* timing, all in by 2600 RPM +
16* vacuum advance from manifold
I had to change vacuum cans to get the advance to pull max. advance @10 - 11" vacuum
I don't care what initial timing is, only max!
4-speed idles @ +/- 900 RPM.
Static CR is calculated @ 10.30, dynamic is 8.3. Running 89 octane BP for 3 years without problems.
Any more questions?
Thread Starter
Burning Brakes
Joined: Jan 2006
Posts: 957
Likes: 6
From: Burlington wisconsin
No idle issues.
12 cc dished pistons
'0' decked block
Performer RPM intake with modified/rebuilt stock Q-Jet
TF 23* heads - 64 cc - 0.041 head gaskets
Dynomax ceramic 1 5/8" headers - chambered exhaust
And, of course CC XE274H cam
HEI distributor - 35* timing, all in by 2600 RPM +
16* vacuum advance from manifold
I had to change vacuum cans to get the advance to pull max. advance @10 - 11" vacuum
I don't care what initial timing is, only max!
4-speed idles @ +/- 900 RPM.
Static CR is calculated @ 10.30, dynamic is 8.3. Running 89 octane BP for 3 years without problems.
Any more questions?
12 cc dished pistons
'0' decked block
Performer RPM intake with modified/rebuilt stock Q-Jet
TF 23* heads - 64 cc - 0.041 head gaskets
Dynomax ceramic 1 5/8" headers - chambered exhaust
And, of course CC XE274H cam
HEI distributor - 35* timing, all in by 2600 RPM +
16* vacuum advance from manifold
I had to change vacuum cans to get the advance to pull max. advance @10 - 11" vacuum
I don't care what initial timing is, only max!
4-speed idles @ +/- 900 RPM.
Static CR is calculated @ 10.30, dynamic is 8.3. Running 89 octane BP for 3 years without problems.
Any more questions?
What plugs & gap are you running?
You know I am on my 3rd carb now, an EB 750 performer, a Ho;;ey HP 750 & now a Holley St. Avenger 670 and I've tried no vac can, a B26 can & now a B-28 can & this thing wont seem to run right. I had the best luck with no vac can and a Holley HP 750, it idled but seemed like it only had 350 crank HP. I even now have a burp in the mufflers when I shut it off hot. Not a backfire or pop, just a burp per say, like a double boom boom, kinda muffled. Not a bang or a solid clank or pop, but a small double detonation almost. Funny thing is it does this aprox. 2-3 seconds after turning the key off!! I now beleive I'm chasing the wrong problem. WTF MSD Ignition issues or valve timing??????
Last edited by cherrybombc3; Jul 16, 2009 at 11:05 PM.
