Late valve timing?
Thread Starter
Advanced
Joined: Jan 2007
Posts: 96
Likes: 0
From: Sandnes Norway
Late valve timing?
My 1974 BB draws only about 8 inches of vacum at 6oo rpm. I have learned from different places that it could be late valve timing and that it takes a lot of work to correct. What exactly is "late valve timing" and how do I check it and how to fix it?
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
By late valve timing, that refers to the intake valve closing event occuring after the optimum point. Generally, the later in the combustion stroke it closes the less vacuum "signal" you'll have in the intake at idle. Another adverse effect of late timing is the loss of dynamic compression, as the engine cannot begin to build compression until the intake valve is shut.
Sounds like you may need to advance the cam, as it should help not only idle vacuum, but could wake up some hidden performance. However, I wouldn't do it arbitrarily without doing some homework first.
Sounds like you may need to advance the cam, as it should help not only idle vacuum, but could wake up some hidden performance. However, I wouldn't do it arbitrarily without doing some homework first.
Last edited by TheSkunkWorks; Jul 21, 2007 at 02:38 PM.
Thread Starter
Advanced
Joined: Jan 2007
Posts: 96
Likes: 0
From: Sandnes Norway
Thanks for your reply
Yes I will try to do my homework before starting anything, but my problem is that I have no ide how to check my valve timing and nor do I know how to eventually correct it. Any help would be appreciated.
Yes I will try to do my homework before starting anything, but my problem is that I have no ide how to check my valve timing and nor do I know how to eventually correct it. Any help would be appreciated.
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
Assuming your engine has the stock cam, it's relatively easy to find out your valve timing specs. Of particular interest are lobe seperation angle and intake centerline angle.
Next, look for information on how to degree a cam. This will help you determine where your cam is indexed and will reveal the timing of the intake valve closing event. (If your cam isn't stock, and you can't find the cam card to your stick, understanding this can help you graph your specs.) You'll need a piston stop, degree wheel and dial indicator, all available thru any good speed shop, to do this.
Should you not wish to go thru the trouble to remove the front cover before you commit to adjusting the cam, it may be possible to improvise with a fully degreed timing tape on your balancer, but it won't be as accurate as using a degree wheel.
Once that's established you'll be able to judge, with a little good advice, whether or not to change things. If so, you'll need a timing chain set with provisions to adjust timing. They are also available at any good speed shop, Cloyes being one of the brands to look for.
Hope that gives you some insight. If you know which end of a screwdriver to bang on, you can do it.
Next, look for information on how to degree a cam. This will help you determine where your cam is indexed and will reveal the timing of the intake valve closing event. (If your cam isn't stock, and you can't find the cam card to your stick, understanding this can help you graph your specs.) You'll need a piston stop, degree wheel and dial indicator, all available thru any good speed shop, to do this.
Should you not wish to go thru the trouble to remove the front cover before you commit to adjusting the cam, it may be possible to improvise with a fully degreed timing tape on your balancer, but it won't be as accurate as using a degree wheel.
Once that's established you'll be able to judge, with a little good advice, whether or not to change things. If so, you'll need a timing chain set with provisions to adjust timing. They are also available at any good speed shop, Cloyes being one of the brands to look for.
Hope that gives you some insight. If you know which end of a screwdriver to bang on, you can do it.
Last edited by TheSkunkWorks; Jul 21, 2007 at 03:27 PM.
Thread Starter
Advanced
Joined: Jan 2007
Posts: 96
Likes: 0
From: Sandnes Norway
Is it so to understand that the cam has come out of adjustment? How can this happen?
I have uploaded a videorecording of my vacumreading on youtube, this is done with all the vacumlines disconnected wich gave me a little higher reading, about 10 inches at 600-700 rpm. If you bother to take a look at it it would be nice since it don't tell me more than it's not perfect.
You can see it here : http://www.youtube.com/watch?v=ugZuA8ymxXI
The quality is not so good, was difficult to get a good footage of the gauge in the sun, but the red lines on the gauge is at 10 and 14 inch of mercury. Each line has 2 inch of increment. You can see it idling at about 12 and dropping to 10.
Remember this is with all vacumlines disconnected and plugged.
Thank you
Racer
Joined: Feb 2007
Posts: 262
Likes: 2
From: FREDERICKSBURG VA
If your timing chain stretches, the cam timing will be retarded. This will have a negative effect on idle vacuum, low end power, throttle response & fuel milage. It sounds like you might also have a vacuum leak. If you have a distributor with vacuum advance, make sure you check the diaphram for leaks.
Thread Starter
Advanced
Joined: Jan 2007
Posts: 96
Likes: 0
From: Sandnes Norway
If your timing chain stretches, the cam timing will be retarded. This will have a negative effect on idle vacuum, low end power, throttle response & fuel milage. It sounds like you might also have a vacuum leak. If you have a distributor with vacuum advance, make sure you check the diaphram for leaks.
So it's maybe a timing chain problem I have. Is there a easy way to check if that is the issue?
Thread Starter
Advanced
Joined: Jan 2007
Posts: 96
Likes: 0
From: Sandnes Norway
If your timing chain stretches, the cam timing will be retarded. This will have a negative effect on idle vacuum, low end power, throttle response & fuel milage. It sounds like you might also have a vacuum leak. If you have a distributor with vacuum advance, make sure you check the diaphram for leaks.
If you look at the video recording you will see how it function with all vacuum lines disconnected and plugged.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2027 Corvette vs The World: Every C8 vs Its Closest Competitor
Joe Kucinski
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
I watched your video. It sounds like somebody has installed a more radical cam.
The more radical the cam, the higher the idle rpm needs to be.
To increase vacuum you have to use a higher initial spark timing. Like 14- 18 degrees with the vacuum advance disconnected. Then you have to adjust the idle mixture screws to get the smoothest idle.
The more radical the cam, the higher the idle rpm needs to be.
To increase vacuum you have to use a higher initial spark timing. Like 14- 18 degrees with the vacuum advance disconnected. Then you have to adjust the idle mixture screws to get the smoothest idle.
Thread Starter
Advanced
Joined: Jan 2007
Posts: 96
Likes: 0
From: Sandnes Norway
I watched your video. It sounds like somebody has installed a more radical cam.
The more radical the cam, the higher the idle rpm needs to be.
To increase vacuum you have to use a higher initial spark timing. Like 14- 18 degrees with the vacuum advance disconnected. Then you have to adjust the idle mixture screws to get the smoothest idle.
The more radical the cam, the higher the idle rpm needs to be.
To increase vacuum you have to use a higher initial spark timing. Like 14- 18 degrees with the vacuum advance disconnected. Then you have to adjust the idle mixture screws to get the smoothest idle.
The timing is a little above the area you describe. It was set at 40 degrees when I bought it, but I have retardet it to about 20.
The weights in the distributor is not working at all so I have ordered new ones. Hope it will help.
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
I have also been thinking about that someone has done that . Is there a way to find out about that without disassembling the engine?
The timing is a little above the area you describe. It was set at 40 degrees when I bought it, but I have retardet it to about 20.
The weights in the distributor is not working at all so I have ordered new ones. Hope it will help.
The timing is a little above the area you describe. It was set at 40 degrees when I bought it, but I have retardet it to about 20.
The weights in the distributor is not working at all so I have ordered new ones. Hope it will help.
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
You have already stated that the distributer was not functioning........
It is best to attack the problem with the easy things first. Like enshuring correct spark timing and tune of the fuel system.
Actually most cam grinders grind 4 degrees of advance into their American V-8 cams. The reason is to allow for chain stretch. So the 4 degree ground in only lasts for the begining part of the motors life.
I have my cams ground without the 4 degrees advance and I use quality billet steel cam timing sets that don't stretch like the cheap timing sets.
You have a big duration cam that is never going to have lots of vacuum. Once you get everything else running up to 100% and you think you need vacuum the fix options are. Install a vacuum reserve canister, buy a vacuum electric or belt driven pump, change to a milder duration cam.
I did forget one thing. If the motor is old and warn out then look at a rebuild with a new timing chain
Last edited by gkull; Jul 23, 2007 at 01:21 AM.
Thread Starter
Advanced
Joined: Jan 2007
Posts: 96
Likes: 0
From: Sandnes Norway
Thanks
The distributor will be taken care of as soon I get my parts.
Regarding the vacuum it is not a problem since everything works on the car with the amount of vacuum I have, it only bothers me because it's is not like it should be and I was looking for opnions on what it could cause. I've learned a lot out of your postings and I'm very thankfull for that.
So I think it is like you stated, a big cam in it
The distributor will be taken care of as soon I get my parts.
Regarding the vacuum it is not a problem since everything works on the car with the amount of vacuum I have, it only bothers me because it's is not like it should be and I was looking for opnions on what it could cause. I've learned a lot out of your postings and I'm very thankfull for that.
So I think it is like you stated, a big cam in it
