R.H.. -   I have a 66 396 in a 68 SS. I replaced the point distributor with an Accel HEI. The cam was replaced by the previous owner, I was told it is a Lunati 108 centerline cam (not sure of the numbers, but it is a mild street cam with a lot of lope). I was trying to set the timing, but at the book 4 deg before, it dies. If I set it around 10-12 it seems to run ok. I was told with the HEI I may have to go to 8, but it doesn't run well there either. I'm no expert tuner, but this doesn't seem right. I did replace the timing tab, so my thought is maybe it is just reading incorrect. Any suggestions, or would taking it and having it diagnosed be the answer.
G.M. -   Without the stock cam, the manufacturer's timing specs are useless. You will need to either set the timing for full advance, which is usually 35° ATDC @ the rpm when the mechanical and vacuum(if used)advances are maxed out (3000 rpm ?). Or you can just set it for where it runs good and will still start, whereas the starter can still turn it over when the car is warm. Too far advanced, and it will make the starter drag.
B.S. -   Rob, Unless you have something else wrong, you don't have a "mild" cam if you have a loping idle. A loping idle is caused by a long duration cam with lots of overlap! These cams require LOTS of advance in initial static timing in order to have any bottom end at all. It is quite common to run at up to as much as 14 or 16 degrees BTDC static timing, as long as you limit TOTAL timing, with all mechanical advance in, to 36 or 38 degrees.
Here is some info I provided to another Chevelle enthusiast who was new to engine timing procedures:
If you have never set TOTAL timing before, it may not be immediately obvious exactly how that's done.
As mentioned, get a timing tape and wrap it around your balancer. (Make sure you get the correct one -- its based on the diameter of your balancer!) Once you have that tape on the balancer, it will have timing marks up to at least 40 or 50 degrees that you can use to set total advance.
All mechanical advance should be in by between 3000 to 3500 rpm... so hold your revs steady at that point and turn the distributor until the 36 degree mark (or whatever you want for total advance) on the balancer tape is next to the ZERO mark on the timing tab mounted to the front of the timing chain cover. At that point your total advance is set to 36 degrees.
That is kind of backwards from the way you check initial static timing at idle (with the primary zero timing mark on the balancer next to the 12 (or whatever) degree BEFORE mark on the timing tab.
Also, the more bottom end you need, or the bigger cam you've got, the more intial advance at idle you should set, up to as much as 16 degrees before TDC. If you do that, you have to remember to adjust the mechanical advance in the distributor back to 20 degrees (10 dist. degrees x 2 = 20 crank degrees ) to get your total advance back down to 36 degrees.
Yes! You MUST disconnect and plug the hose from your carb to the vacuum advance can on the distributor before you make any initial or mechanical advance settings. If you don't your settings will be junk.
No! Setting total advance (initial plus mechanical) does not "reset" the TDC reading on the damper. I think that what is confusing you here is how the total mechanical advance is read by using a timing tape and timing marks on the damper instead of the timing marks on the timing tab on the engine. The only reason that this is done is that it is much easier to put timing marks up through 38 degrees on the damper AND they will be visible when the engine is running at 3,000 to 3,500 rpm. You certainly could read total advance using the timing tab attached to the timing cover, BUT you would have to get a timing tab that had marks on it up through at least 36 or 38 degrees! (Most only go up to 12 degrees or so.) That would be a very long timing tab AND even if you did find one, once the stock TDC timing mark on the damper lined up with the 36 or 38 mark on that very long timing tab, it would probably be out of your view, way up underneath the water pump or some other piece of the engine! That is why the industry has standardized on putting those total timing marks on the damper itself or on a tape on the damper! Its easier, and you can see them when lined up from the same angle that you view the damper when setting static initial timing at idle.
So, they are not really two different settings. They may appear that way to a person new to making timing settings because of the two different ways that they are observed when setting them.
If your timing tab is not a mile away from your balancer or rubbing on it, it is probably mounted on the correct screws on the timing cover.
Bye the way, your particular ignition system has no real effect on all of this. Regardless of what kind it is all it does is get the spark to the plug. You are responsible for telling it when to spark by setting the timing according to your particular engine configuration (cam, auto transmission torque converter stall speed, rear gears, etc. all affect this decision).
Hope this helps you out.
F.O.T. -   Great advice BOB that is one of the reasons they came up with the setback timing lites, you set it at 36 or what ever and when it reads 0 on the balancer you are there.Makes it so much easier, don't leave home without one...FRED
J.B. -   Well I'll like to hear more on this timing light
J.H. -   John, Try going to sears they carry them or if you want the best (in my opinion)find your local snap-on dealer the snap-on light has a digital read out,tells you the rpm,has advance and retard by using buttons,(one hand operation)switable to rpm and advance with touch of a button.Check one out you will like it also bring your old timing light most snap-on dealers will give you some trade in value not much but better than nothing the light cost about $295.00 and worth every penny after having and using mine I can't live without it
R.H. -   First of all, thanks for the explanation. I understood most of it, so I guess I'm not totally stupid. I understand how to set total timing. I assume there are more ways to set timing than just moving the distributor, but I'm unsure what they are. If I set total timing to 36 deg at 3000 rpm, then let it idle and check the static timing. If it is say, 8 deg, how can I adjust it to 14 or so without moving the distributor and affecting the total timing? You lost me when you said to set the mechanical timing. I assume this internal to the distributor, mechanical parts? Is this difficult?
Just for clarification: Initial is set at idle using the timing tab, Total is set at 3000-3500 using tape, Keep vacuum line off for all settings.
thanks for the help, and patients,
B.S. -   Rob, You've just about got it. Only the static initial timing is set by rotating the distributor in the engine. Once you have that set to what you want, say 14 degrees BTDC, you then check the total timing by reving the engine to over 3000 rpm (or whatever RPM that the timing finally stops advancing) and check the timing with the timing light again using the total advance tape on the balancer. If it is 36 to 38 degrees BTDC you are all set. If it is too high or too low, you must adjust the mechanical advance in the distributor to get it where you want it... you DO NOT get it by rotating the distributor in the engine again since that would change your initial static timing setting. Some distributors, particularly aftermarket units, make it very easy to set mechanical advance. Most use some type of lever or stop that allows you to limit how far the centrifical advance weights rotate out from center when the distributor spins up. Many are marked with small lines indicating how many degrees of mechanical advance you are setting by placing the tab or stop in a particular position. That way you don't just have to guess at the correct setting and make an actual timing light check after each adjustment to see if it is correct. Remember that mechanical advance as set in the distributor must be multiplied by 2 to get its affect on the engine (crank advance). So if you set the distributor's mechanical advance to 11 degrees that would be 11 x 2 = 22 crank degrees. Adding that 22 to your 14 degrees initial static setting gives you a TOTAL advance of 36 degrees. Note that most stock GM distributors do not have an easy internal mechanical advance device built in. Some folks would accomplish this by brazing a small bushing around the post that rides in the slot to limit the advance weight travel. That works, but is not a very exact science... it is hard to get the right amount of advance set that way. If you are still running a stock points-style distributor, now would be a good time to upgrade to an aftermarket electronic unit that has an easily adjustable mechanical advance method. Hope this gets you going, good luck... Bob S.
R.H. -   Thanks Bob, I think it finally sunk in.
J.B. -   Bob, ever thought about teaching, now I got it too thanks...
F.O.T. -   Bob: if you know the avance built in to the dist. and you use an adjustable vacuum advance to a set the total timing is that not the same? I set the timing on my 350 at 38 degrees total and I don't care about initial timing can't even tell you what it is. With no problem,thats on reg.fuel and 9.5to1...FRED
Archivist: Tom Wilson