timing question

speedracer · February 23, 2007, 08:23:26 AM

Seems like I asked this question before. When timing a my engine 350 SBC. that has the vacuum advance plugged, MSD distr w/msd box 6al, this engine has the edelabrock power package w/cam, alum heads, dual plane intake, 750 holley, bored .30 over, headers etc. At what degress do I set it with the timing light. I think it's set at 30 degress advance, but it sounds like the valves are rattling. Any suggestions?
Thanks, John

model a vette · February 23, 2007, 09:07:45 AM

Does this dist have mechanical advance? Is it capable of using vacuum advance?
A really "hot" engine would have no more than 12 degrees advance when idling with the vacuum advance disconnected and the vacuum hose plugged.
If it is set at 30 degrees when at idle that noise will be the death rattle of the engine.

speedracer · February 23, 2007, 09:16:05 AM

Quote from: "model a vette"Does this dist have mechanical advance? Is it capable of using vacuum advance?
A really "hot" engine would have no more than 12 degrees advance when idling with the vacuum advance disconnected and the vacuum hose plugged.
If it is set at 30 degrees when at idle that noise will be the death rattle of the engine.

Man I'm so lost on this, it does have a vacuum advance port which is plugged. When I check the timing I use the very first plug wire drivers side closest to the radiator. And the light hits on the damper at 28degres, I thought it should be set around 6 or 8 degres advanced, seems like 28 or 30 is way to far! Thanks, john

Dave · February 23, 2007, 09:20:28 AM

Id set it at 28 to start but you have to have the engine running about 2500 or so rpm. Its more like 8 degrees at an idle.. The best way to set em is high rpm and at full advance.
Dave

EMSjunkie · February 23, 2007, 09:23:26 AM

Quote from: "model a vette"Does this dist have mechanical advance? Is it capable of using vacuum advance?
A really "hot" engine would have no more than 12 degrees advance when idling with the vacuum advance disconnected and the vacuum hose plugged.
If it is set at 30 degrees when at idle that noise will be the death rattle of the engine.

I hafta agree, 30* initial advance is gonna cause alot of problems

I would advise you to hook the vacuum advance back up.
make sure you hook it to a constant vacuum source, not a "ported" one.
I have a SBC that has initial timing of 10* BTDC.
(no vacuum hooked up)
the total timing is 34*BTDC
(vacuum hooked up)

runs pretty good.

just my .02

Vance

dragrcr50 · February 23, 2007, 10:56:09 AM

and to add to your problems if you have a msd 6 box or the like you cant use a timing light with a knob for degrees on the back of it because it has a delay in it and you will read retarded usually even at 3k or up... good luck

speedracer · February 23, 2007, 11:12:42 AM

Quote from: "dragrcr50"and to add to your problems if you have a msd 6 box or the like you cant use a timing light with a knob for degrees on the back of it because it has a delay in it and you will read retarded usually even at 3k or up... good luck

MSD box forgot about that, wonder if you time one different with hooked up? And I still don't understand why my mechancic insist on keeping the vacuum pugged up on the distr. And those marks on the damper used for timing there like one notch per degree or what? On my damper it says 0 the 10 then 20 etc, at idel should I set it at around 6 or 8 degrees (notches). As you can see I'm so dumb when it comes to timing! Thanks, john

model a vette · February 23, 2007, 01:20:07 PM

If there is a vacuum can on the distributor you should definitely use it. Find a port on the Holley that is below the throttle plate and hook up the vacuum hose there. Usually there is one in the lower casting (the one the hold down bolts go thru).
The timing tabs are usually stamped so that every notch on the tab is 2 degrees. The factory setting for a 302 Z-28 was 4 degrees so start around there. I don't think any SBC had more than 12 degrees initial.
To set the initial timing the idle has to low, something in the area of 750 -950 rpms. The vacuum hose is taken off the dist and the hose plugged. The light is hooked up to the #1 plug (driver's side most foward plug).

Yoy will probably have to readjust the idle while you twist the dist to keep the engine running. Sometimes that requires three or more hands so enlist the help of a buddy.

As Vance wrote most SBCs have a TOTAL of 34 degrees at high rpms, not at idle.

speedracer · February 23, 2007, 02:53:58 PM

Quote from: "model a vette"If there is a vacuum can on the distributor you should definitely use it. Find a port on the Holley that is below the throttle plate and hook up the vacuum hose there. Usually there is one in the lower casting (the one the hold down bolts go thru).
The timing tabs are usually stamped so that every notch on the tab is 2 degrees. The factory setting for a 302 Z-28 was 4 degrees so start around there. I don't think any SBC had more than 12 degrees initial.
To set the initial timing the idle has to low, something in the area of 750 -950 rpms. The vacuum hose is taken off the dist and the hose plugged. The light is hooked up to the #1 plug (driver's side most foward plug).

Yoy will probably have to readjust the idle while you twist the dist to keep the engine running. Sometimes that requires three or more hands so enlist the help of a buddy.

As Vance wrote most SBCs have a TOTAL of 34 degrees at high rpms, not at idle.

Hey guys thanks alot, so I adjusted the timing to 8 degres
and had to adjust the idel and took her out sfor a test drive and guess what no rattling sounds like the pushrods were going to go thru the cover and she runs alot stronger! Now I still didn't mess with the vacuum advance, its still plugged off at the distributor. The brake cannister has been enlarged and hooked up to the rear of the intake manifold and the vacuum ports on the holley are still plugged off. Does this sound about right. I think the engine may run alittle cooler now too, I bought new VDO gauges for it just hadn't got around to replaceing those piece of crap Sun Pro from advanced auto, ha ha---------------john

model a vette · February 23, 2007, 03:40:20 PM

There is no reason to plug the dist. The vacuum comes from the carb. You should try running the vacuum advance. It will probably make the engine run cooler. It will also improve the gas milage.

enjenjo · February 23, 2007, 04:39:38 PM

Quote from: "model a vette"There is no reason to plug the dist. The vacuum comes from the carb. You should try running the vacuum advance. It will probably make the engine run cooler. It will also improve the gas milage.

And it won't hurt horsepower output.

EMSjunkie · February 23, 2007, 04:49:25 PM

Unless you are running WOT most of the time, there is no reason to NOT run a vacuum advance.

you will be suprised at how much better it will idle, cruise, and when needed....,.haul azz!!
plus it will run cooler.

I know your engine builder told you you didn't need vacuum advance.
but as far as I know, and its possible that I'm FOS, a street driven engine needs vacuum advance to be streetable.

Vance

Dave · February 23, 2007, 06:01:37 PM

Quote from: "EMSjunkie"Unless you are running WOT most of the time, there is no reason to NOT run a vacuum advance.

you will be suprised at how much better it will idle, cruise, and when needed....,.haul azz!!
plus it will run cooler.

I know your engine builder told you you didn't need vacuum advance.
but as far as I know, and its possible that I'm FOS, a street driven engine needs vacuum advance to be streetable.

Vance

Ya know Vance I just threw in the full advance at hi rpm and i really didnt think. And i figured 28 deg was a good starting point. Ok so i assumed that he knew something bout timing. I am glad he got it back to 8 or whatever he said at an idle. Yes do use vacuum advance especially on the street the engine will run smoother and cooler. Ive had this discussion so much that i really kind of shy away from it most of the time. I like to set my timing at hi throttle then run er a abit and see how it acts. Whats gm specs? 28 to 32 or 28 to 34 i ferget but i can find the info if i need it..
Your doing good for an old emt.........
Dave

dragrcr50 · February 23, 2007, 09:27:38 PM

Quote from: "jusjunk"
Quote from: "EMSjunkie"Unless you are running WOT most of the time, there is no reason to NOT run a vacuum advance.

you will be suprised at how much better it will idle, cruise, and when needed....,.haul azz!!
plus it will run cooler.

I know your engine builder told you you didn't need vacuum advance.
but as far as I know, and its possible that I'm FOS, a street driven engine needs vacuum advance to be streetable.

Vance
isnt the rule of thumb 10 initial, 12 mechanical and 10 vacum for a total recomended gm at 32 at 3200 rpm?? I think that is it ..

Ya know Vance I just threw in the full advance at hi rpm and i really didnt think. And i figured 28 deg was a good starting point. Ok so i assumed that he knew something bout timing. I am glad he got it back to 8 or whatever he said at an idle. Yes do use vacuum advance especially on the street the engine will run smoother and cooler. Ive had this discussion so much that i really kind of shy away from it most of the time. I like to set my timing at hi throttle then run er a abit and see how it acts. Whats gm specs? 28 to 32 or 28 to 34 i ferget but i can find the info if i need it..
Your doing good for an old emt.........
Dave

39deluxe · February 24, 2007, 01:51:33 AM

The only thing I would add to this is to install an adjustable vacuum advance unit. Depending on your gas quality, compression ratio, combustion chamber shape and a few other variables you may pick up some pinging once you connect that vacuum advance (part throttle when vacuum is high). Your choice then is to turn back your timing until it stops, which will hurt your performance, or delay the point at which the vacuum advance starts to turn the plate in the distributor. By tightening the vacuum advance you are delaying the point at which vacuum advance occurs and dialing out the ping. This way you can have your 6-8 degrees initial, your 34 degrees total and also tune the vacuum advance rate to eliminate the pinging. This is trial and error but with your aluminum heads and heart shaped chambers you should be able to pull this off. I like to run 10-12 degrees initial and this should be within range with a little distributor tuning.

Also if you get the initial set where you want it and get the vacuum advance tuned so you have no pinging but you find that the distributor is not advancing to total advance by 3000-3200 rpms get a spring kit and experiment with lighter springs until it reaches total advance where you want it. It is also ok to run a lighter spring on one side with a heavier one on the other if that's what it takes to get it right.

Tom