I need some advice on timing settings and methods..

I have set dwell on new points to 32

Now my questions:

I have an advance type timing light.. and not sure if I'm using it correctly

Should I be checking timing in this manor

Pull and plug vaccum advance
Set RPM to 600 RPM
Crank in 10 degrees (Initial) into the advance in the light and turn dist until timing mark on crank is at 0 degrees

Reconnect vaccum advance and increase degrees in light to realign to 0 degrees on the crank (Read degrees advance on light) 28 degrees

WOT checks with Vaccum disconnected and plugged increase RPM until max advance in obtained then crank in advance in light until crank is realined to 0 degrees.. (read max advance on light). 40 degrees

Is this the correct procedure???
If not, what do I need to do to get this right

Are the values I am getting correct??

Thanks
Tony

 

Procedure is correct, but the max advance reading (with engine revved up until it stops advancing) should be the sum of the initial advance (10 degrees) plus the centrifugal advance the distributor weights/springs are providing (in your case, adds 30 degrees). Your engine wants 34-36 degrees total timing (40 is excessive). Short-term, if you back off the initial timing to 6 degrees, that will give you 36 total. Longer-term, you should install a larger bushing on the pin in the slot under the autocam mechanism to limit the centrifugal advance to 20-24 degrees, and you can then run more initial timing. Your vacuum advance is adding about 18 degrees, which is about right - most cans add 15-16 degrees. :thumbs:

 

Thanks John,

I reset for max 36 degrees and the engine now runs strong on top, but
I almost think it is laboring some in low to midrange power

I rechecked my numbers.
I think I am seeing that I am already getting several degrees of centrifugal advance already cranked in at 600 RPM..
If I drop the RPM to 450 advance drops to 6 degrees
Springs appear loose without much tension.
Could these springs be stretched? Allowing 4 degrees of advance at 600 RPM?
Would that cause this laboring in low to midrange power?

Any thoughts?

I am going to spend a little time tomorrow closely recording these numbers again, I want to be sure I am seeing what I think I am..

First time I've used one of these advance timing lights

I think I'm actually learning what I'm supposed to be looking for... :D

Thanks for the help..
Tony



[Modified by pittsaj, 11:31 PM 7/19/2003]

 

Yours is a typical case of centrifugal advance starting too early due to weak springs, which influences "true" initial timing; if you put a rubber band around the weights, it will allow you to set correct initial timing. From that point, you need to work with various combinations of the available "advance kit" springs (usually three steps - light, medium, and strong) to find the setup that holds off the start of the centrifugal advance until you're above normal idle, yet brings it "all in" between 2500-3000 rpm. A shop with a Sun distributor machine can do this in about 20 minutes (and they usually have LOTS of springs in their collection to pick from), but you can do the same thing at home with your dial-back light - just takes longer. :thumbs:

 

One always has to be careful that the centrifugal has NOT started at the normal speed used for checking initial timing, and also, it's best, but not absolutely necessary if centrifugal does not start until above idle speed. For most OEM spring sets this is the case.

You can either drop the idle speed until the engine barely runs to check initial or tie up the centrifugal temporarily with a rubber band.

Using a dial back light it's very easy to characterize the centrifugal curve. Bring the speed down to the point where you are certain that the centrifugal is not deployed, then slowly raise the speed until is just starts. This is your start point: 0 @ xxxxRPM. Raise the engine speed until the timing no longer advances, this is your maximum centrifugal advance, and on some engines the speed may have to be raised to as much as 5000 RPM, but if your dist. has been modified with light springs the the centrifugal might start as low as 500 and be all in by 2500.

Once you have a start and end point, you can also check some values at one or two points in between.

The sum of initial plus centrifugal should add up to 34-38 degrees on a SB.

You should also characterize the vacuum can. You can do this with a Mighty Vac type vacuum pump, or just use oral vacuum and a vacuum gage. Disconnect the vacuum can from manifold vacuum and rig up your vacuum pump or a gage and hose to apply oral vacuum. Apply vacuum until the timing mark starts to advance. This is your start point, 0@xx". Now increase vacuum until the timing mark stops advancing. This is your maximum vacuum advacne xx@xx". Compare the vacuum for max advance to your normal idel vacuum. The vacuum can should reach full advance AT LEAST two inches BELOW what the engine normally idles at.

Now connect the vacuum can to manifold vacuum, and set the idle to your normal idle speed. This is TOTAL IDLE timing, which should be the sum of initial plus full vacuum. If you have a quick centrifugal curve, you might also have a few degrees of centrifugal. You want the total idle timing to be in the range of 20-25 degrees for medium performance engines and 25-30 for SHP engines.

Now you have characterized the timing map for your engine, and you can work from here to make any corrections such as swapping the vacuum can to one that is more suitable for your engine, or modifying the centrifugal curve. As a rule, better low end torque will be achieved the quicker the centrifugal curve, and the limiting factor is usually low rev detonation with too quick a curve.

To give you a idea of your finished data set, here are the final numbers I came up with after retuning the fuel and ignition maps on my Cosworth Vega after I reindexed the cams to different than OEM timing.

Normal hot idle speed/vacuum: 900 @ 18"
Initial timing: 14
Centrifugal: 0@1000, 9@ 1500, 18@2000
Vacuum: 0@4", 11@8"

Total WOT timing = 32
Total Idle timing = 25

Duke

 

One more thing to check.... The bushing in the slot under the weights may be worn or may have fallen off. Mine did, and I had the same problem you are describing (too much total advance). My distributor ('64) has a plastic bushing that is supposed to be pressed on over the pin. Some other distributors have a bearing or metal bushing of some sort in the same location. I finally glued a new one on with some RTV and have not had a problem since.