Timing with a vacuum gauge

[perko]

Hi all,
I've been trying to time my 318 after I put on a new carb and intake but I think that the balancer has slipped or something because I can't even see the timing mark with my gun.

I've seen some information about timing an engine with a vacuum gauge on the internet so I gave it a try. I adjusted the distributor to max vacuum (about 22) and then brought it back a little to about 20-21.

My question is have i done this correctly? Is this an accurate enough way to time the engine that i can move on to tuning the carb and stuff? Or do i need to find another way to make sure I get this right? Thanks!

 

[MopaR&D]

Hi all,
I've been trying to time my 318 after I put on a new carb and intake but I think that the balancer has slipped or something because I can't even see the timing mark with my gun.

I've seen some information about timing an engine with a vacuum gauge on the internet so I gave it a try. I adjusted the distributor to max vacuum (about 22) and then brought it back a little to about 20-21.

My question is have i done this correctly? Is this an accurate enough way to time the engine that i can move on to tuning the carb and stuff? Or do i need to find another way to make sure I get this right? Thanks!

That probably got you pretty close to the mark, it's basically how you would tune the initial timing anyway even with a light. However you really need to know what the advance is as the RPMs go up, most stock distributors have mechanical advance designed to work with only a few degrees initial advance so when you crank up the initial for better efficiency the total advance goes too high.

Next course of action should be locating TDC on cylinder #1, there are multiple ways to do this just do a search on the forum. Then you will know how much the balancer has slipped, which you should probably get a new one of sooner than later anyway.

 

[perko]

weird. The mark is dead on at TDC. Its a 67 motor with a 73 timing cover and ballancer though. Maybe that's why. The 67s had the mark on the other side of the engine.

 

[318willrun]

weird. The mark is dead on at TDC. Its a 67 motor with a 73 timing cover and ballancer though. Maybe that's why. The 67s had the mark on the other side of the engine.

that answers why you can't see the mark

 

[Mike69cuda]

I bought a bolt on timing marker off the forum for the passenger side. You can bolt that on and get your timing scale where you need it. I bet if you point your light on the passenger side you will see the balancer tdc mark light up about 10 o’clock

 

[66jim]

Advance dist. a little at a time see how it runs,starts to ping uphill under load retard timing until it just stops. Before that decide on what octane this (318) may require
for top performance. Then work on the vacuum advance, then throw the timing
light in the trash.

 

[jbc426]

Hi all,
I've been trying to time my 318 after I put on a new carb and intake but I think that the balancer has slipped or something because I can't even see the timing mark with my gun.

I've seen some information about timing an engine with a vacuum gauge on the internet so I gave it a try. I adjusted the distributor to max vacuum (about 22) and then brought it back a little to about 20-21.

My question is have i done this correctly? Is this an accurate enough way to time the engine that i can move on to tuning the carb and stuff? Or do i need to find another way to make sure I get this right? Thanks!

You have found what the engine wants for it's initial timing. Now you need to limit the total timing by reducing the amount of mechanical advance in the distributor.

There are at least two ways to do that. One is to use the limiter plate offered by Four Seconds Flat. The other is to weld up the slots in the mechanical advance portion of your distributor and then file the slot to obtain less mechanical advance. I'm guessing you target timing will be anywhere from anywhere between 34 to 36 degrees total including your new higher initial timing plus your vacuum advance rate.

Once you get the initial and mechanical advance done, you can adjust your vacuum advance to get greater efficiency at steady state cruising.

There's a great article about this process on the Four Seconds Flat website.

 

[nm9stheham]

weird. The mark is dead on at TDC. Its a 67 motor with a 73 timing cover and ballancer though. Maybe that's why. The 67s had the mark on the other side of the engine.

So are you saying that the timing marks on damper and timing cover lined up at true TDC for #1 when you were assembling the engine? If that is the case, then the damper ring has not slipped. I read your posts to say that both your timing cover and damper are 1970 and later; as long as the damper and timing cover are the same vintage, the marks should line up properly at TDC, regardless of the block's year.

Do you have the aluminum bodied water pump? (Which is the 1970 and later version) If so, then the timing marks should be visible on your '73 damper and timing cover. Do you have power steering? Sometimes that setup blocks the timing marks on the left side.

If in post #1 you mean that you cannot see the timing mark on the damper versus the timing mark with the timing light after using the vacuum method, then the mark on the damper probably is back under the water pump when the timing light fires. That means either the damper slipped, or your timing method has advanced the idle timing too much and the damper mark is back under the water pump, where you cannot see it.

I never use the vacuum method except maybe to get in the ballpark. I've seen guys end up with 30-40 degrees of initial advance that way. IMHO, there is too much interaction with carb settings to make this reliable.

 

[Mattax]

My question is have i done this correctly? Is this an accurate enough way to time the engine that i can move on to tuning the carb and stuff?

No & Not really.

Because this is what happens.

I never use the vacuum method except maybe to get in the ballpark. I've seen guys end up with 30-40 degrees of initial advance that way.

Don't feel bad. I think most have thought vacuum and/or rpm was a good way.
Part of the problem with this method is the interaction with carb settings, in particular throttle position and mixture. RPM is equally related.

Increasing initial timing will lead to increased idle rpm which naturally creates more vacuum under the throttle blades.
The problem with this goes back to what spark timing is all about.
Timing is the degrees of rotation needed for complete burn.
Denser mixtures burn more quickly, and lean mixtures take more time for the flame to travel.

from MTSC 259. Ignition system analysis. 1969

The goal is to create maximum pressures with the mixture, heat, compression available while the pistons are in position to apply maximum leverage on the crankshaft.
As Chrysler reminded their techs in 1959

from MTSC-136

Running lean mixtures is great when the engine loads are low for the rpm.
At idle speeds, just slipping the engine into gear is a big load.

For maximum efficiency at idle, a relatively rich mixture is needed.
With an automatic transmission, its possible to set the timing and mixture, shift it into gear - and then note rpm, vacuum when it steadies. Can't do that with a stick shift.

For a basically stock engine with no emissions equipment, go back and see what the factory recommended for initial before CAP or CAS. Then you can experiment with increasing timing a couple degrees, resetting idle speed, then mixtures. The carburetor throttle blade positions must stay within a reasonable range of the factory setting or off-idle fuel delivery will be miscalibrated.

Best procedure for a hot rodded engine is to start with initial timing suggested in the "How To" articles. Usually 12- 16 BTC or 14-18* for typical hot rod street cams. Using a Holley type carb, keep the throttle position so transfer slots under the blade look squarish. Adjust the timing and mixture to get enough rpm the engine will idle nice. Probably a little more than a stock non-smogger, but should go into gear (automatic) at less the 900 rpm.

If you are using a smog era distributor, almost certainly will have to weld up the inside of the slots to get the advance correct. The FBO plate doesn't usually work out too well 'cause it results in the mechanical advance too early unless you fiddle withthe springs.