Chevrolet Nova Forum banner

distributor timing help - initial/total advance

3675 Views 8 Replies 6 Participants Last post by  Tom Mobley
running gm hei stock type distributor with crane adjustable vac can, and accel super cap. also recently lubed everything and put a fresh rotor, and also used the mech adv springs and installed the vac can as per crane's instructions. i recently tuned the carb and timing to idle great at 950 rpm in park with 19" of vacuum, and about 700 rpm in gear. i have a 406 with a 650 dp holley, dual plane, aftermarket heads, 10.5:1CR, 241/247 ud harold cam,and some headers in front of a 2500 stall. however - two things concern me:

1. i've got my 14* of advance without the vacuum can hooked up, this gives me more vacuum than at 12*, plus it idles a little better. is this alright? or should i just keep it at 12 where some people have told me to go? it seems to have a little difficulty idling at 12* unless i raise the rpm.

2. i measured my total timing, and i am reading more than 40* of idle while using 14* initial advance. i know i should really be running at 32-34*, and i don't want to hurt anything. how do i adjust this vac can so that the total timing will reduce itself? i spent time turning the wrench both directions to adjust the vacuum can, but it doesn't change the total idle timing.

can anyone give me some advice regarding whether to stick with 14* initial, and how to change my total idle timing please?

thank you!
1 - 9 of 9 Posts
Re: distributor timing help - initial/total advance

well for the top end timing you may need to limit the amount of mechanical advance your getting so that the total doesnt go so far as 14 intial is concerned I think your good to go with that. 12-14 is fine IMO.

Hopefully some one here will be able to tell you how to limit the mech advance(I believe you have to put a pin in the dist. that stops the travel of the weights) if not post this at Steves I'm sure ignitionman will be able to help ya out...I also think DriveWFO has done this too if memory serves me right
Re: distributor timing help - initial/total advance

Your initial timing is not that important. if you can crank the engine over when hot, you are ok. You should be more concerned with total mechanical timing (initial plus centrifigal), and then the overall total with the vac can hooked up. Most chevy engines respond well with 32-36 total mechanical all in around 3,000 rpm. The total with vacuum advance should not exceed 51, and probably be more around 48. Always check your plugs for signs of detonation if you are running a high compression engine or using low octane fuel. I always set my total mechanical timing and let the initial fall in wherever it lands.
Re: distributor timing help - initial/total advance

You don't worry about the vacuum advance when you calculate the total. At WOT(very low manifold vacuum) the vacuum advance does nothing. 40 deg at idle with the vacuum advance is fine as long as you don't get any surging at real light throttle.
Re: distributor timing help - initial/total advance

If he has 40 at idle and another 20 centrifigal, he will max out at 60 during part throttle load. Thats way too much. Really need to curve the distributor so you know what you're working with. Also need to know how much the vac can is adding. I he has 14 initial plus 26 vacuum, whats left for the centrifigal? Most distributor curves are set up to add 18-24 degrees. When you do the math, based on these numbers, the engine will be in detonation territory at part throttle.
Re: distributor timing help - initial/total advance

If it doesn't surge it will be fine. you won't get anywhere near 19in of vac. when there is enough load on the motor to cause detonation
Re: distributor timing help - initial/total advance

To set your timing with a dial back timing light:

1. Disconnect the vacuum advance line.

2. Start the engine and let it warm up.

3. Set the idle to about 600 RPM.

4. Set the timing at 10 degrees BTDC.

5. Rev the engine to 3000 RPM and check the total timing. Your looking for 36 degrees total.

6. If you had something other than 36 do this. 34 read at the balancer = add 2 more degrees for 12 degrees at 600 RPM and so forth.

7. Connect the vacuum advance back up to a ported NOT manifold source.

8. Run RPM back up to about 2500 RPM and check the timing. You want about 10 - 12 degrees of vacuum advance for a total of about 46 - 48. Adjust as needed to get this.

9. Test drive the car. If it pings drop the amount of vacuum advance until the ping goes away.

Hope this helps! Good Luck!!!
See less See more
Re: distributor timing help - initial/total advance

Do as Motorhead62 says, but remember to plug the disconnected vacuum line to the carb before any tune-up.
Re: distributor timing help - initial/total advance

Good to see you, Mike. Please feel free to drop in as often as you have time for.
1 - 9 of 9 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.