Timing W/ Manifold vacuum

[Mlcdc1]

I have a 1977 stock 350 with an Edelbrock 1906 carb and would like to switch from ported vac to manifold vac after reading through the posts. the timing is currently set at 10 BTDC. Would I have to reset the timing if I make the switch and if so what would be a good starting point? Thanks for any input.

 

[Matt69olds]

Read this:

Timing & Vacuum Advance 101 -good article by GM engineer

www.460ford.com

The vacuum advance needs to compliment the mechanical advance. Get the initial, timing curve, and total timing dialed in with the vacuum advance disconnected. Once that’s all set up, connect the vacuum advance to manifold vacuum and drive it. Whatever you do, DO NOT MESS WITH THE MECHANICAL ADVANCE. If it pings or rattles with the advance hooked up, limit the amount or rate of advance.

 

[75gmck25]

- When setting base timing, disconnect and plug the vacuum advance line. Then turn the idle speed down to about 650 and set base timing to about 10-12 BTDC.
- Then run the rpm up and see when the mechanical advance comes in and how much you get. It should add about 18-20 degrees, and it will help acceleration if it’s all in by about 2500 rpm. However, to change the curve of rpm/advance you will have to buy a kit with new weights and springs.
- Let it come back to the 650 rpm idle and then reconnect the vacuum line. Check timing again. Most stock vacuum cans will add about 18-20 degrees, but there are a few that provide less advance.

One general rule is to not exceed 34-36 degrees for base+mechanical, and do not exceed about 54 degrees for base+mechanical+vacuum. However, you also need to drive it to check that you don’t get any pinging during light throttle acceleration.

 

[Mlcdc1]

Thanks for the advice. I’m a rookie when it comes to timing so this may be a dumb question but how do you tell when the mechanical advance comes in?

 

[Matt69olds]

You either need a timing tape on the balancer, or a dial back timing light.

Warm the engine, disconnect the vacuum advance. Note the engine rpm, see what the timing is at that speed, and make a note. Set the idle to 1000, read the timing again, make a note, set the idle to 1500, read the timing, keep going until the timing stops advancing. It might be as high as 3500-4000 rpm. Connect the dots on your chart, that’s your timing curve.

You can change the rate at which the curve comes in by changing the advance spring. Lighter brings it in faster, heavier springs flows it down.

GM made a ton of different advance weights and cams with different shapes, that tailors the shape of the curve.

Your vacuum advance canister will have a number stamped on it stating the amount of advance that particular canister will provide.

Hope that helps. There are lots of YouTube videos on ignition timing.

Once you have

 

[75gmck25]

You can see where mechanical advance starts by just pointing the light at the timing marks and slowly increasing the engine speed. The line/mark on the balancer will obviously “move” as rpm goes up. However, the markings on the timing tab only go up about 16, so as already mentioned, you will need to either use a timing tape to see higher values of degrees on the balancer, or use a dial-back timing light.

With a dial-back light you pull the trigger on the light, point it at the marks, and then turn the dial until the 0 mark on the timing tab lines up with the line on the balancer. Whatever the dial shows, that is the current timing BTDC. For example, if you have 12 degrees base timing and then speed the engine up to about 3000 rpm to get 18 degrees more from mechanical, the dial on the light should be at 30 when the marks line up.

Mechanical advances can be modified, so yours might not be stock. Check base timing at a low rpm (about 650) to ensure there is no mechanical advance yet.

 

[Mlcdc1]

Thanks for all the help. I worked on setting the timing this weekend and here is where I ended up. Took it for a test drive and no noticeable ping, seemed to run good.

idle 650 = timing 12
mechanical advance = 32-12=20
idle 650 timing with vac on manifold vac = 28
3000 RPM with manifold vac = 44

 

[Matt69olds]

If you don’t hear any rattling, add a couple degrees and see how the engine likes it.

If you have 20 degrees of CENTRIFUGAL advance, your total mechanical timing is 32. You add the amount of mechanical advance to the initial. Most low compression engines like 34-38ish of mechanical advance. Couldn’t hurt to try

 

[Mlcdc1]

You’re right - couldn’t hurt to try it. Thanks!

 

[Mlcdc1]

Finally got a chance to work on the timing again - here’s what I’ve got
timing curve: 750rpm=14 degrees
1000rpm=15
1250rpm=19
1500rpm=24
1750rpm=26
2000rpm=26
3000rpm=26
16 degrees @ idle 750 no vacuum advanced
34 degrees @ idle 750 vacuum advanced hooked up to manifold vacuum
50 degrees @ 3000rpm vacuum advanced hooked up to manifold vacuum

engine is stock 350 with Edelbrock AVS 1906 & HEI distributor
test drive = no pings, seems to get down the road pretty good

I would appreciate thoughts and any suggestions-THANKS

 

[Ricko1966]

Same thing I tell everyone,disconnect the vacuum advance plug the line. Find 2 land marks about 1/8 th mile apart,accelerate from point 1 to point 2,note your speed when you cross the second landmark,bump the timing 2 degrees check your speed,repeat until speed drops or you get ping,then go back 2 degrees. That's what your engine wants for total,then we figure how fast to bring in centrifugal, then we figure how much vacuum advance to add.

 

[JBswth]

I have a 1977 stock 350 with an Edelbrock 1906 carb and would like to switch from ported vac to manifold vac after reading through the posts. the timing is currently set at 10 BTDC. Would I have to reset the timing if I make the switch and if so what would be a good starting point? Thanks for any input.

Never tried it on Chevy engines, but I know from experience that 70s AMC engines didn't like that - they would at first try to jump smartly off the line, but then hesitate and spit back at you if you hooked up to vacuum advance to manifold vacuum.

J. B.