zero compression
Race Director
Joined: Apr 2007
Posts: 11,150
Likes: 890
From: South Western Ontario
It could be tuning. It could possibly be the valve adjustment needs more work. I would have expected more idle vacuum from your engine.
In my opinion, the set screw adjustment in the nipple on an adjustable vacuum advance can isn't for adjusting the advance limit. You use it to adjust the vacuum required to fully pull the can and then use a stop to limit the total vacuum advance.
In my opinion, the set screw adjustment in the nipple on an adjustable vacuum advance can isn't for adjusting the advance limit. You use it to adjust the vacuum required to fully pull the can and then use a stop to limit the total vacuum advance.
Thread Starter
Pro
Joined: Jul 2020
Posts: 642
Likes: 104
From: Miami florida
NOW WITH VAC CONNECTED:
I set MY GUN to zero and my damper is 15* advanced. SOMEONE SAY CHECK.
I'm now at 20* advanced. better? Do we want 25*. All I did was open up the vac can 2 turns clockwise.
I set MY GUN to zero and my damper is 15* advanced. SOMEONE SAY CHECK.
I'm now at 20* advanced. better? Do we want 25*. All I did was open up the vac can 2 turns clockwise.
Race Director
Joined: Apr 2007
Posts: 11,150
Likes: 890
From: South Western Ontario
I don't understand, are you at 15* or 20* at idle?
Double check the mechanical advance at idle again each time you adjust it to ensure you stay at 10* without the vacuum advance.
Double check the mechanical advance at idle again each time you adjust it to ensure you stay at 10* without the vacuum advance.
Race Director
Joined: Mar 2017
Posts: 11,807
Likes: 4,429
From: Virginia
You can also pull vacuum with your Mity-Vac, to make sure your total vacuum advance is 10-12 degrees. You might find some more vacuum when you tune your carb a bit.
Team Owner
Joined: Oct 2004
Posts: 81,241
Likes: 3,062
From: altered state
St. Jude Donor '05
havent gone through 33 pages. .did you try the EOIC method one valve at a time when adjusting?
Not spinning the pushrod but check for up/down play then add preload. Foolproof..one thats too tight can cause all kinds of issues. Hope you get it ironed out.
Not spinning the pushrod but check for up/down play then add preload. Foolproof..one thats too tight can cause all kinds of issues. Hope you get it ironed out.
Thread Starter
Pro
Joined: Jul 2020
Posts: 642
Likes: 104
From: Miami florida
will check mechanical to ensure accuracy. But basically with vac instead of 15 on damper I’m at 20. That should be better. Would 25 be better
Le Mans Master
Joined: May 2007
Posts: 5,007
Likes: 1,114
From: North tonawanda NY
Again it sounds like you have 8 about degrees of vac advance right now when between 10and 12 is ideal.
Last edited by augiedoggy; May 18, 2021 at 09:10 AM.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
2027 Corvette vs The World: Every Model vs Closest Competitor
Joe Kucinski
10 Most Common Corvette Problems of the Last 20 Years!
Joe Kucinski
5 MOST and 5 LEAST Popular Corvette Model Years in History!
Joe Kucinski
2027 Corvette Buyer's Guide: Everything You Need to Know!
Joe Kucinski
10 Things C8 Corvette Owners Hate (But Won't Tell You)
Joe Kucinski
10 Best Corvettes Coming to Barrett-Jackson Palm Beach 2026!
Brett Foote
Every Corvette Grand Sport Explained! (C2, C4, C6, C7, & C8)
Joe Kucinski
Grand Sport & Grand Sport X Launch Alongside All-New 535hp LS6 V8!
Michael S. Palmer
5 Reasons Bad Drivers Crash & 5 Ways to Avoid a Costly Mistake!
Joe Kucinski
Race Director
Joined: Mar 2017
Posts: 11,807
Likes: 4,429
From: Virginia
Initial timing (no vacuum advance) 15 degrees BTDC
Idle with initial and vacuum advance 20 degrees BTDC
So, with whatever vacuum you are pulling at idle, you get 5 degrees of vacuum advance.
If you connect your Mity-Vac to the vac advance can, and pull 20 in-hg, how much vacuum advance to you get then?
You want 10-12 degrees of vacuum advance at idle, so 25 total at idle would be better, but don't get there the wrong way. Your mechanical is already set.
Had a 1976 L-82, 4-sp
Joined: Mar 2011
Posts: 5,338
Likes: 1,209
From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
Michael, it appears you are trying to set your final timing by adjusting your vacuum advance. Thats a no, no. You are way too focused on vacuum advance for some unknown reason. I can't stress enough the importance of setting up your total timing to 36* @ 3000 rpm with the vacuum advance disconnected. You must do it this way first!!! Then connect your vacuum advance and chech for any engine pinging when you are out cruising around (you can't check for ping sitting in the driveway), otherwise, your done! Go for a beer and burger and pat yourself on the back.
Thread Starter
Pro
Joined: Jul 2020
Posts: 642
Likes: 104
From: Miami florida
lol. I did it like that! I'm good with mechanical and at 3k at 36. Been done. I'm addressing the fact that with vac i'm low at 15 out so I increased it to 20. Zero pinging by the way when I went for a drive fyi.
Thread Starter
Pro
Joined: Jul 2020
Posts: 642
Likes: 104
From: Miami florida
This was what I was getting at! If we scroll up a bit my 5th point which most agree was too low states:
NOW WITH VAC CONNECTED:
I set MY GUN to zero and my damper is 15* advanced. SOMEONE SAY CHECK.
See responses to that. So I went from 15 to 20 on the gun WITH VAC.
NOW WITH VAC CONNECTED:
I set MY GUN to zero and my damper is 15* advanced. SOMEONE SAY CHECK.
See responses to that. So I went from 15 to 20 on the gun WITH VAC.
Had a 1976 L-82, 4-sp
Joined: Mar 2011
Posts: 5,338
Likes: 1,209
From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
Exactly. There is very little gain trying to max out your vacuum advance and it could actually put you in the detonation zone and you may not hear it but damage could still be done. Leave it alone, Your done! 
Thread Starter
Pro
Joined: Jul 2020
Posts: 642
Likes: 104
From: Miami florida
excellent. thanks.
NOW, the next issue or actually related: idles at 850ish rpm. When I throw it in gear stopped it shakes pretty bad. idle drops to 550rpm.
what I thought is perhaps carb adjust seems to me may be an issue with vac advance, the thing I'm infatuated with. Some say it's advance, some say idle mixture screw. I tuned the screws counterclockwise 6 spins and the fast idle 3 counterclockwise as suggested throughout forum. I put it back as it made no difference. What say you?
idle in park/neutral - 850ish rpm nice idle.
put it in gear: 500rpm ish.