Timing issue chasing my tail!
Thread Starter
Heel & Toe
Joined: Dec 2012
Posts: 23
Likes: 0
From: Fellsmere FLORIDA
Timing issue chasing my tail!
Hello, quick intro: 1979 vette owned about 5 years. I have rebuilt most of it except the bottom of the engine, which I know very little about. I am sure its been altered but to what full extent I am unsure. I have replaced intake (Performer) Eldebrok 1405 carb 600CFM, summit racing HEI, headers, entire car is rewired with painless wire harness and so on. I do not know what cam is in it or if block has been altered.
So here is the problem: Runs ok, but never quite right (if you know what I mean) runs a little hot 220 at 70 MPH (new radiator, dual electric fans).
So I started at the very beginning one step at a time: Initial timing with vacuum plugged and RPM around 700 was 0 to 4 degrees. Again I was just testing not altering anything.
Then I plugged vacuum back in, RPM went up to 1500 and timing at idle was 34 degrees. So just to see effect I turned idle down to about 700 where I started and that brought the timing down to 24 degrees. If I set initial timing at around 12, then at 3000 RPM it is well over 50 degree!
Feel like I am missing something quite obvious but cant seem to figure it out. Any ideas or suggestions would be greatly appreciated! I have not altered the timing curve, springs, weights or anything else in the distributor...
Steve
So here is the problem: Runs ok, but never quite right (if you know what I mean) runs a little hot 220 at 70 MPH (new radiator, dual electric fans).
So I started at the very beginning one step at a time: Initial timing with vacuum plugged and RPM around 700 was 0 to 4 degrees. Again I was just testing not altering anything.
Then I plugged vacuum back in, RPM went up to 1500 and timing at idle was 34 degrees. So just to see effect I turned idle down to about 700 where I started and that brought the timing down to 24 degrees. If I set initial timing at around 12, then at 3000 RPM it is well over 50 degree!
Feel like I am missing something quite obvious but cant seem to figure it out. Any ideas or suggestions would be greatly appreciated! I have not altered the timing curve, springs, weights or anything else in the distributor...
Steve
Team Owner
Joined: Jan 2007
Posts: 53,648
Likes: 878
From: Southern Cal Ca
St. Jude Donor '22
yes you are.
base time= fixed and starting point of advance induced by rpm and vacuum
centrifugal time= changes by engine rpm
vacuum time=changes by amount of vacuum.
and what the three all add up to is total time.
you see that.
52 at cruise with no load is not unheard of.
what are you after?
you have a start in that you measured what is going on.
there are countless threads about dist timing.
base time= fixed and starting point of advance induced by rpm and vacuum
centrifugal time= changes by engine rpm
vacuum time=changes by amount of vacuum.
and what the three all add up to is total time.
you see that.
52 at cruise with no load is not unheard of.
what are you after?
you have a start in that you measured what is going on.
there are countless threads about dist timing.
Thread Starter
Heel & Toe
Joined: Dec 2012
Posts: 23
Likes: 0
From: Fellsmere FLORIDA
That's a good question, going to dig into the distributor today and make sure its all good! Also trying to find the specs on this distributor to find out what its total timing is at what RPM. Been Reading Timing to Win by Don Gould, good read!
Race Director
Joined: May 2006
Posts: 18,735
Likes: 2,582
Summit Racing has 7 different HEI distributors listed for a 1979, large cap, small cap, different color caps, etc. Which distributor number do you have? In addition the vacuum advance is listed as adjustable, did you read the instructions that came with the distributor? There also is a kit to customize the mechanical curve https://www.summitracing.com/parts/s...make/chevrolet
Last edited by MelWff; Apr 23, 2018 at 12:07 PM.
Thread Starter
Heel & Toe
Joined: Dec 2012
Posts: 23
Likes: 0
From: Fellsmere FLORIDA
Summit Racing has 7 different HEI distributors listed for a 1979, large cap, small cap, different color caps, etc. Which distributor number do you have? In addition the vacuum advance is listed as adjustable, did you read the instructions that came with the distributor? There also is a kit to customize the mechanical curve https://www.summitracing.com/parts/s...make/chevrolet
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Thread Starter
Heel & Toe
Joined: Dec 2012
Posts: 23
Likes: 0
From: Fellsmere FLORIDA
Sorry got a little sidetracked! Got my wife a 1992 Vette and I am in the process of replacing the oil pan and timing gear gaskets, as well as all the others, new optispark and new water pump.... Turning out to be a bigger project than expected... Will get back to the 79 soon..