Vacuum Advance
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Joined: Apr 2003
Posts: 51
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From: Penticton BC
Vacuum Advance
Hey guys
my motor runs great...well better...with the vacuum advance unhooked. But if i plug it in it runs rough at about 2000 rpm or when im just cruisin at 30mph it surges and screws around.
Garrett :confused:
my motor runs great...well better...with the vacuum advance unhooked. But if i plug it in it runs rough at about 2000 rpm or when im just cruisin at 30mph it surges and screws around.
Garrett :confused:
Race Director
Joined: Sep 2002
Posts: 10,362
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From: San Diego - Deep Within The State of CONFUSION!
Re: Vacuum Advance (Smokin80)
Hmmm do you have a speed shop in your area that can curve the distributor? If so use it!
One more thing: You may have a bad vacuum advance unit. If it is leaking vacuum then it's not only NOT doing it's job but also is a vacuum leak.
Food for thought.
One more thing: You may have a bad vacuum advance unit. If it is leaking vacuum then it's not only NOT doing it's job but also is a vacuum leak.
Food for thought.
Melting Slicks
Joined: Jun 2001
Posts: 2,340
Likes: 2
From: Moro IL
Re: Vacuum Advance (Smokin80)
Well the first thing you need to do is check where your at as far as your timing goes. Most engines don't like much more then 36 degrees mechanical advance, if your running more then this mechanical and then add on the vacuum advance you could be overadvancing, which would cause the symptoms you described. Check things out and let us know what you find.
:cheers:
Pat Kunz
:cheers:
Pat Kunz
Melting Slicks
Joined: Mar 1999
Posts: 2,845
Likes: 2
From: Apex NC
Re: Vacuum Advance (Smokin80)
sounds like your total advance is too high at cruise. The sum of initial and centrifugal should be 34-36 degrees for a small block chevy, at around 3000 rpm. Disconnect the vacuum advance and bring the car to 3000rpm and measure the advance. Then connect the vacuum advance and see how much additional advance you're getting. If the vacuum can is giving more than 12 degrees advance, that's going to cause you to buck and surge at high vacuum conditions, like cruising at 2500 rpm.
Go to corvettefaq.com and read up on what to do.
Go to corvettefaq.com and read up on what to do.
Melting Slicks
Joined: Mar 1999
Posts: 2,845
Likes: 2
From: Apex NC
Re: Vacuum Advance (mvftw)
Doesn't matter. The first time with the vacuum advance disconnected will give you your total mechanical advance. Second time with vac advance connected gives mech + vacuum. Subtract second number from the first and see what the vacuum can is giving.
The vacuum advance hose on the wrong port will only affect idle operation, not part-throttle cruise (as I understand it).
The vacuum advance hose on the wrong port will only affect idle operation, not part-throttle cruise (as I understand it).
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Melting Slicks
Joined: Mar 2001
Posts: 3,365
Likes: 5
From: Lake Villa IL
Cruise-In IV Veteran
Cruise-In V Veteran
St. Jude Contributor
Re: Vacuum Advance (Smokin80)
I just added a Crane adjustable vacuum advance cannister, to address some light throttle freeway cruising issues.
You might want to try the same. I picked mine up at Summit for $29. The cannister allows you to adjust two things: the total amount of advance and the rate of advance. It takes some playing around. I don't have mine completey shaken out yet. But I can see definite advantages.
Good luck with it...
:cheers:
You might want to try the same. I picked mine up at Summit for $29. The cannister allows you to adjust two things: the total amount of advance and the rate of advance. It takes some playing around. I don't have mine completey shaken out yet. But I can see definite advantages.
Good luck with it...
:cheers:
Le Mans Master
Joined: Jul 1999
Posts: 9,206
Likes: 17
From: Sewell NJ
Re: Vacuum Advance (Smokin80)
I had the same problems with my 74 until I used the parts that Lars recommends. The problem is that the stock vacuum advance cannister adds too much timing to the distributor. You can get away with this with a stock curve because the stock curve is very lazy and depends on the vacuum advance to supply most of the increase in timing. When you recurve the distributor using lighter springs and more initial advance the stock cannister gives you too much too soon and you get the symptons you describe. You need to get a can that gives you about 10 degrees of advance. I tried using the Accel adjustable unit with the timing limiter but never quite got it right. I used Lars recommended vacuum advance and never had a problem since. Read his paper and use what he tells you to use and you'll save yourself a lot of aggravation. Good luck.
Thread Starter
Advanced
Joined: Apr 2003
Posts: 51
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From: Penticton BC
Re: Vacuum Advance (vettfixr)
AHHHHHHHHHHHHHHHH
well.....all the things you guys have told me i have done for the most part. I added the crane adjustable advance and kept limiting it but it didnt seem to help. So i go to check my timing marks and i have a problem...they dont line up nemore. I have a feeling maybe my timing chain has jumped a tooth. could this be possible because i know a week ago or so my timing marks lined up. i was having problems then too but i had a leaking carb gasket which was probably causing most of the problems. Any ways to see if the cam is now not in time. the car still runs obviously but it just doesnt seem happy. With the mods i have i would expect around 250hp at the wheels but im not gettin that for sure. it feels about the same as last year so im digging for answers here.
Edelbrock 600, performer intake, 3 angle grind, port and polish, msd 6a, comp xe256h, hooker long tubes, underdrive pullies, 2.5 inch duals w/ h-pipe, full engine balancing, 9.1:1 CR.
Id think this should put 300 down at the flywheel but i could be wrong.
Garrett
thanks for your help guys this is a great forum
well.....all the things you guys have told me i have done for the most part. I added the crane adjustable advance and kept limiting it but it didnt seem to help. So i go to check my timing marks and i have a problem...they dont line up nemore. I have a feeling maybe my timing chain has jumped a tooth. could this be possible because i know a week ago or so my timing marks lined up. i was having problems then too but i had a leaking carb gasket which was probably causing most of the problems. Any ways to see if the cam is now not in time. the car still runs obviously but it just doesnt seem happy. With the mods i have i would expect around 250hp at the wheels but im not gettin that for sure. it feels about the same as last year so im digging for answers here.
Edelbrock 600, performer intake, 3 angle grind, port and polish, msd 6a, comp xe256h, hooker long tubes, underdrive pullies, 2.5 inch duals w/ h-pipe, full engine balancing, 9.1:1 CR.
Id think this should put 300 down at the flywheel but i could be wrong.
Garrett
thanks for your help guys this is a great forum
Melting Slicks
Joined: Feb 2003
Posts: 2,331
Likes: 6
From: Long Island NY
Re: Vacuum Advance (Smokin80)
I have read all of LARS' papers, and there are full of great info...but I believe that LARS does Not have a kit for HEI's...I'm thinking of going to Accel or Crane for the adjustable vacuum & weights/springs.
Pro
Joined: May 2001
Posts: 596
Likes: 0
From: Yardley PA
Re: Vacuum Advance (1979toy)
More likely than your timing chain jumping is that your balancer slipped. Check the condition of the rubber on the balancer.
The chain would amke it run HORRIBLE, even with just one tooth jumped. Don't ask how I know :rolleyes:
I'd bet on the balancer.
BTW: How do you know your mark isn't lining up? How did you check it?
Prost:cheers:
Stefan
Team Owner
Joined: Jun 2001
Posts: 23,381
Likes: 1,067
From: Virginia USA
Re: Vacuum Advance (Stefan69)
More likely than your timing chain jumping is that your balancer slipped. Check the condition of the rubber on the balancer.
:iagree:
The chain would amke it run HORRIBLE, even with just one tooth jumped. Don't ask how I know :rolleyes:
I'd bet on the balancer.
BTW: How do you know your mark isn't lining up? How did you check it?
Prost:cheers:
Stefan
:iagree:
The chain would amke it run HORRIBLE, even with just one tooth jumped. Don't ask how I know :rolleyes:
I'd bet on the balancer.
BTW: How do you know your mark isn't lining up? How did you check it?
Prost:cheers:
Stefan
Race Director
Joined: Mar 2001
Posts: 11,322
Likes: 1
From: Living in the Hartland
Re: Vacuum Advance (theandies)
Back in college I had a nylon tooth gear give out and the car would not start. I had it replaced by a shop and about 2 weeks later I spun a main bearing. Took it back to the shop and they said proably some nylon clogged the oil pump. Of course it wasn't their fault although they pretty much new what caused and they wold not do anything about unless I forked over my $. :mad :cuss
Thread Starter
Advanced
Joined: Apr 2003
Posts: 51
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From: Penticton BC
Re: Vacuum Advance (Stefan69)
I put a stop in the number one piston spark plug hole. hand turned over the motor until the compresion stroke and when it hit the stop...prolly within a few degrees of TDC the timing marks were nowhere to be seen. I have heard a lot of people say about spinnin the balancer so im thinkin that is probably what it is. I think ill change that, use some propane and look for any vacuum leaks and have my dist curved and ill be good to go!!! :)
