I would disconnect and plug the vacuum advance. Set the timing @ 36* BTDC at 3,000 RPM (or the point at which it no longer advances). This is your "total timing" Let the engine come back down to idle, and note where the timing is at idle. This is now your base point for tuning.

If you want to advance or retard the timing from this point, you don't have to stick your head under the hood with a screaming engine - just go off of the idle figure, with vac advance disconnected. Whatever you add or subtract from the idle timing will be added or subtracted from the total timing.

hook your vac advance back up when you're done.

 

Drag strip times and plug reading should guide you to your ultimate best total timing.
It can be anywhere from 32º to 44º on a SBC
You then work back to limit your Centrifugal advance so you can have a 8-16º initial advance.
If it is street driven, you can add ported?timed advance so you get maximum fuel economy, an adjustable advance can so you don't over/under advance is helpful.

Vacuum advance does not work at WOT, and shouldn't be an idle crutch.

 

my questions:
1.) what is a good idle timing starting point based on my specs below?

2.) can you walk me through how to set timing (process)?

3.) i have an hei dist so is it necesary to also set the timing at 3k rpm as well or will the dist automatically set the advance based on the idle timing?

4.) by retarding the timing do i basically lose performace at some point......so i'm guessing that i need to find the optimal point by trial and error?

1. More important at this point is total timing (somewhere around 2800 - 3200 rpm).

2. Keep reading...

3. Disconnect and plug the vacuum advance when checking/setting the timing (as Al suggests). Rev the engine to about 3000 rpm and watch the timing light to see where the total is. Do the same and set the total timing to 34 degs. (you won't see too many sbc strokers that like more than 34 degs total.) Now check the idle and adjust it to where you want it then check the idle timing. (vacuum advance still disconnected) Ideal would be about 14 degs showing you have 20 degrees of mechanical in the distributor. Now hook the vacuum adv back up to a manifold source (vacuum at idle) and check the idle advance again. subtract the first idle reading from this one to know what your vacuum can adds to this... 10 degs of vacuum advance would be ideal.

Write down your readings. Once you can tell us what the readings are, we can help you dial them in better...

4. More is not always better and less does not always reduce performance. Every combo is a bit different and requires it's own specific settings. Timing is critical in best performance, total timing is all about WOT, idle timing is critical to keep the engine from building too much heat and for off idle response and vacuum advance is a bonus adjustable advance that is mainly for part throttle cruising but also can be used to help idle quality...

Provide your current settings;
TOTAL =
IDLE =
VACUUM =

 

Do the same and set the total timing to 34 degs. (you won't see too many sbc strokers that like more than 34 degs total.)

Because of the stroke/bore ratio the max cyclinder pressure will be closer to 15 ADC than the 22 deg.

Tend to set the the total a little over advanced, plot the curve at 1500 rpms 2000, 2500 3000 etc...use springs/weight that give a nice round curve starting around the 45 deg then curves nicely off just underneath....that will get you close in the ball park, enough that dyno will make insignificant changes.
Start retarding the curve till you feel power drops off,(Do this with the rpms near where the curver is all in and above) it will drop off withing a couple degrees....advance it back up about 2 deg above this piont...any further makes the car sound stronger, but the power drops off very slowly as you get higher.

Once the all in is esptablished, its a matter of establishing your idle advance.
Still with VA disconected...play around...so you find say 16 deg is stable.
Return the dizzy to where you established the power...
If the cent is already at or very close to this, fine....use a VA that starts and comes in above idle vac but all in cruising up a slop vac
If the intial is too low, use a VA that is all in or no more than 1" above the idle vac with this curve and has the number of degrees in it to take the intial cent +vac upto what you want once the engine is fired...modern HEI dizies usually require more degrees to be put in the curve to run manifold vac.
A low intial cent (minus the VA) has far less load on the starter moter, easier firing, less load on batteries etc.

 

I've never done this hence the questions but on the timing marker it doesn't go above 16* so how do you set total timing at 34*? My timing gun is only a strobe light type.

I think I understand the process but what is ideal idle rpm?

 

See if the local parts counter will lend or rent you a "dial-back" timing light... With it you set the dial to the degs you want, rev the engine and adjust the timing so the pointer and mark on the balancer line up at zero.

 

Thanks. How can I do it with the timing light I have? Is it possible to do everything stated in the posts?

 

You can get timing tape and install it on your balancer so you can read total timing with your light.

 

I do like John says...use the wifes dress making tape...measure the circumferance. mm is the easest units to work in.
divide by 36 that gives 10 degs.
This part is easest to do from under the car.
measure with the tape each 10 deg then 2x mark with a punch out to 45 to 50 deg.
Always measure with the dressmaking tape and always measure from TDC.
using dividers and or from 1 10 deg to the next adds errors as u go higher.

I then mark the 5 deg inbetween.
Wipe down with a bit of thinners on a rag...then wipe white paint over the area....let tack off well, then gently wipe down with a clean rag with thinners.
The surface wipes off leaving the double and single punch marks easy read with the timing light....
To make things even better, punch each 10 deg with 1 2 3 etc...its sometimes easy to get lost where u are when the rpms are high..

OR head up the parts store...know what size balancer you have and but a ready made stick on tape...I have never used one so cant comment on how accrute or how well they last up.

 

"When the bottom (most clockwise) end of the tape aligns with the "0" mark on the timing tab, you're at 36* BTDC."

Ok I think I undertand this part but what do I do from there? Sorry first time.

 

The reason I suggested buying timing tape as opposed to making your own is it's already got the degrees marked out on it. Use the pointer and read off the tape to determine your advance...

If you use a piece of tape representing 36 degs then just divide it into 36 equal parts and your good to go up to 36 degs BTDC reading the same way...

 

See if this helps you..

 

This is awesome. Thank you.

 

Last night I decided to mess with the timing and ran into some issues. Followed the below instructions and set total timing to 35*. Let idle back down and want to say that idle timing was approx 17* and idle was a little low. Raised idle slightly and turned off engine..........then it wouldn't start. Does anyone know why this would happen?

Trying to undo what I just did I moved the dist back to its starting position before I set total timing (obviously with engine off since wont strart)...........and I got it to start.

I then set the idle timing to 17* which is where it was before I started all of this.

Can anyone help.

Thanks in advance.

 

You say it would not start..I assume it turned over but rather laboured right?

Read the 1st and last lines of the last parragraph of my 1st post....
Then read what is inbetween those lines.

 

Cranked over but I knew it would not start. No sign that it would catch at all.

 

If you set your total to 35 deg and it was 17 at idle, you have 35-17=18 deg of centrifugal advance in the distributor. I'm assuming you have the vacuum line disconnected from the distr and plugged. This could possibly be too much advance at idle. Retard the timing a little to get the engine started and set the idle timing to about 12 to 14 degrees. Hook the vacuum line back up to the distr and set your idle speed.Then take it for a test ride.

 

This could possibly be too much advance at idle. Retard the timing a little to get the engine started and set the idle timing to about 12 to 14 degrees. Hook the vacuum line back up to the distr and set your idle speed.Then take it for a test ride.

I think your right. Backed off timing to a point where it would start and set to 14* with vacuum line disconnected and it starts fine now. Need to take on test drive. Idle timing at 14* total at 29*.........will test drive but short of the recommended 35*.

 

Something doesn't add up...if you had 18 deg centrifugal before, you still should have 18 plus your new initial of 14 for a total of 32!

Correct.

I wouldn't worry about hitting a total of 35, if your car runs good, then leave it.

At 29 unless has a huge compession ratio or real low octane fuel it will be to far retarded, lot of potentual power lost and chew thru a heap of gas.
Usually from here people mes with the jets and then carb timing everything is so far out it becomes a mission and a pig.

See my 1st post in this thread...from establishing what the total and what the intial no VA is one then puts to required deg into the cent curve thats required
Then the VA is configured to meat the vac charactorists of the car and required degs.

 

ok i finally had time to mess with it today. after the last post i got it started and ended with 14* initial and 29 total. per steptoe something doesn't add up. when i drive the car at standing stop it hesitates/bogs then gets going.....top end is soft but that might be because of small 600 cfm carb?

anyways messed with it today and followed steptoes directions. rev'd engine and set total at 34* and let settle at idle. at idle timing was 20*. set both with vacuum hose plugged. if i adjust idle timing from 20* down to 14* the total falls by almost same amount.

when i hook vacuum line back up i noticed it makes very little to no change to idle timing but for total it jumps way up to over 60* to 70*+.

i ran it with the vacuum line plugged and the hesitation seem to go away. am i getting too much vacuum advance off idle at full throttle?

i'm about to buy a msd street fire to so i can rule out distributor. can someone pls help.

thanks

 

when i hook vacuum line back up i noticed it makes very little to no change to idle timing but for total it jumps way up to over 60* to 70*+.

i ran it with the vacuum line plugged and the hesitation seem to go away. am i getting too much vacuum advance off idle at full throttle?

i'm about to buy a msd street fire to so i can rule out distributor. can someone pls help.

thanks

You saw no increase because you have the distributor vacuum line connected to a PORTED vacuum source, which has no vacuum at idle; if you connect it to a FULL MANIFOLD vacuum source, you'll have 15* or so of additional advance at idle. Forget any readings for total timing with the vacuum line connected - total timing ONLY includes initial + centrifugal; that's why it's measured with the vacuum advance line disconnected and plugged.

No, you're not getting too much vacuum advance off-idle at full throttle; as soon as you step on it, all the vacuum advance disappears. With it connected to that ported source, you don't have any vacuum advance at idle anyway.

:beers:

 

thanks obviously im still trying to understand the whole timing thing. can sone explain why i have this hesitation/bog thing....it cant be too much carb. when i drive the car with vacuum advance plugged the hesitation went away for the most part. why is this?

if i set total at 34* and then check idle its at 20*......is this normal figures...i know every engine is different. when i backed idle down to 14* my total timing went down about the same amonth so i can never get to 14* / 34*.

 

I would think you have too little centrifugal advance built into the distributor, that's why when you set your total at 34 you end up with 20 at idle, which my small block wouldn't tolerate trying to start. You'd be better off with 14 at idle and 20 in the centr advance. And, like John said, you're better off with full manifold vacuum at idle. Your engine will run cooler and more responsive. You should read John Z's article 'Timing 101' on this site. I did and it helped me a great deal.

 

when i backed idle down to 14* my total timing went down about the same amonth so i can never get to 14* / 34*.

I would think you have too little centrifugal advance built into the distributor,

Correct..basically u are trying to run a late model post 73/74 pollution type dizzy and VA on a engine designed for manifold vaccuum.

These old engines had anywhere from 18 to 26 degs in the cent and a VA with 12 to 16 degs ....
From memory u are using a GM HEI correct...these are post 72/74.....the good news is it is not hard to put more in the cent....
round off about 0.3 to 1mm on the tail of the counter weights, tap one of the holes on the arm the rotor mounts to, take a allen nut, round the head off into a cam shape, to make a variable cent dizzy....done.

 

From memory u are using a GM HEI correct...these are post 72/74.....the good news is it is not hard to put more in the cent....
round off about 0.3 to 1mm on the tail of the counter weights, tap one of the holes on the arm the rotor mounts to, take a allen nut, round the head off into a cam shape, to make a variable cent dizzy....done.

How about if I just buy a MSD Street Fire Dist.....would that be suficient versus doing the above mods?

Thanks

 

How about if I just buy a MSD Street Fire Dist.....would that be suficient versus doing the above mods?

Supose so....I have moded pionts GM HEIs old accel duel points yrs back....I have not needed to run MSD dizzys cause my ign /HT circuits systems are kept in good order...Down side going on reports in other threads, I believe they are chinese inferior parts and people seem to have a lot of issues.

Cant really rem a thread re a GM hei having manufactoring or wiring issues, other than the thing has worn out over decades

Take the rotor off, look at the weight tails.....look at that hole, not hard to run a tape down it right?.....not hard to pick up an allen grub screw and put in the grinder right?
or round off 1/2 mm off the counter weight right?

All done in less time it takes to go up the road , buy a MSD, have a chat to the guy, pick up a bottle milk and doz eggs for the missus.

 

thanks steptoe i appreciate it.

bought the street fire and put it in. set total to 35* with vacuum plugged and let idle down. idle timing was at 20* so brought it down to ~14* and went for drive. looks like the hesitation went away but motor is idling a little rough. also when i rev it up at idle and let it off the rpms fall normally and decline down to about 1.5k then it will sit a little high and a few seconds and then setle in at 1k rpms. when looking at tiiming at idle the timing moves around on the timing tape maybe +1*/-1*. idle is a little rough....do i need to adjust idle mixture?????

Also i read john z's timing 101 but dont understand how to set vacuum advance....can you help?

last question....in timing 101 john z says to hook the dist vacuum line to the constant manifold port on the carb. on my edelbrock that would be the right side. i already have a hose on the right side but don't know where it goes........do you know what it might be?

i have my dist vacuum hose hooked up to the left side on the carb. edelbrock owners manual say this is for engines with emissions and right side is for engines without emissions.....did i answer my own question here?

thanks in advance.