Timing issue
04-21-2018, 07:18 AM
#1
Heel & Toe
Thread Starter
Timing issue
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...yet....
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...yet....
Steve
04-21-2018, 07:43 AM
#2
Safety Car
Set the timing with the vacuum disconnected. The max mechanical advance with the vacuum should be 36 degrees. Check this by reving the engine up while watching the advance when it stops advancing even with more rpm that should be 36 degrees. Then idle down to 700 rpm and see where the initial advance is for future reference. After that hook up the vacuum advance. A correctly functioning advance curve will be over 50 degrees at light throttle at 2500 rpm. It helps with gas mileage and cruising power.
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Steven B Hammer (04-21-2018)
04-21-2018, 11:18 AM
#3
Melting Slicks
Set the timing with the vacuum disconnected. The max mechanical advance with the vacuum should be 36 degrees. Check this by reving the engine up while watching the advance when it stops advancing even with more rpm that should be 36 degrees. Then idle down to 700 rpm and see where the initial advance is for future reference. After that hook up the vacuum advance. A correctly functioning advance curve will be over 50 degrees at light throttle at 2500 rpm. It helps with gas mileage and cruising power.
But I would add that your timing at idle should be 10-12 advanced, assuming you have a stock or mild cam. If you have that and more than 34-36 degrees of mechanical advance all-in, you will have to modify the distributor to reduce the total mechanical advance. After you have that worked out, reconnect the vacuum advance and it should add another 14-18 degrees of advance. If you truly have 0-4 degrees at idle, that is likely contributing to your hot running problem.