I have a 71 350/330 engne with 4 speed. I have read Lars papers several times but I am obviously doing something wrong.
1. Disconnect vaccum from distributor and plug end.
2. Let engine warm up.
3. Set dial-up timing light to 36 degress. Make sure it goes to TDC at max rpms around 2500-3000.
4. Check timing at idle should be around 12 BTDC--but instead mine is at 24 degrees.

What am I doing wrong. Base timing should be around 12 BTDC. If I set it at that then when all in it is at 25-26 degrees.

Can someone please help me.

 

Have you changed the advance springs yet?

 

The 71 has a transistorized ignition. The distributor was just rebuilt and set for the optimal power curve by TI Specialists--Dave Fiedler.

 

Sounds more like he set it for a stock ignition curve. Did he provide you with the advance curve data?

 

No, he did not send me any timing advance curve data. I assume he set it for what was stock. If he did wouldn't the normal advance all be in by3000 rpms? I emailed him and asked. I will post the info.

 

The book I have shows your stock base timing for a '71 with TI, 4 speed, as 8 degrees BTDC. I believe the Lar's papers you have are for a performance timing curve which is typically 36 degrees by 3,000 RPMs for a SBC.

 

If it idles fine at 24* let it stay there, if there are problems with the idle at 24* but it idles fine at a lower value then you may have to work on it some more.
The "36*in by 3000" is for the best possible performance, stock timing is to meet emmissions and stop idiots blowing up their motors under warranty.
Check you also have no more than 52* total with all your vacuum advance in.

 

I'm going ask a very silly question...but when I read that you "Disconnect vaccuum from distributor and plug end"...are you plugging both the carb vacuum port as well as the vacuum from the distributor? I read it as you plugged just the distributor end....

If you didn't plug the carb port, that would cause the engine to run in a high RPM at idle.

 

I only plugged the end going to the carb, NOT the end going to the distributor.

How or when do you check for total 52 degrees. Is that after reconnecting vacuum? What happens if there isn't 52?

 

Run the engine up to the point where all the mechanical advance is in (36*) then suck on the hose to the distributor can until it advances no more. If the rebuilder has done his job, it should be a total of 52*.
If not, then check there is the rubber/plastic tube on the end of the Vac can arm. if the tube is there then change out the can for one with 16* advance in it. Lars did a list of compatible Vac cans a while ago, do a search.
Does it idle OK with 24*? you may have to reset the idle speed on the carb to bring it down to spec.

 

Centrifugal advance curve is: starts at 1100rpm, 17 degrees @ 2400 rpm, and full 26 degrees @ 4600 rpms. Other 350/330 owners have told me they are setting idle at 10 degrees with stock setup (book says 12). I just had the distributor rebuilt and don't want to get into changing distributor springs.

 

If you dont want to change springs then you are pretty much stuck
with a stock advance curve and should probably set your timing
around 10° btc at idle and just live with it.
If you get ready to set up a performance curve then you can
concern yourself with 36° @ 3000 rpm.

 

Changing springs is no big deal and the recurve is the biggest performance boost you can do for the money.

 

How hard is it to replace the springs and/or the weights?

 

you can change the springs easily. just use a softer spring to bring the timing in sooner.
leave the weights that Dave put in. Dave has an excellent reputation for the TI stuff and probably has the correct weights in there and the aftermarket weights are typically junk

 

Several people have emailed me and questioned the all in at 3000rpm logic.

First--one performance curve does not fit all cars--so how can saying 36 degrees in by x rpms be good for all cars.

Second--if all cars needed the advance all in by 3000 rpms why did GM engineers spend so much time developing performance curves for each car?

Thoughts?????? to good questions

 

first-- correct, all cars are different but dyno tests over the years have proven that the older generation small block motors produce the most power when the timing is in the 34-36º range. Timing below that reduces power (and can cause overheating issues) and increasing the timing over 38º can also reduce power at WOT. 34º-36º timing is what have been proven to be the most optimum in most cases.
One exception is if the motor has Fast Burn heads on it which are more efficient. These require a lower timing, typically in the 28º-32º range. There are several factors that affect total timing and combustion chamber efficiency is one of the major ones. The more efficient the chamber, the less spark lead you need. The fast burn heads have a superior combustion chamber and thus require less timing.

second-- it's not that the motors NEED the timing all in by 3000rpm but rather having it all in by that point significantly helps performance. Generally, from the factory, the timing curve was much different and in some cases the timing didn't come all in until well after 4,000, or 5,000 rpm. Keep in mind that Chevy is in the business to sell cars and they put warranties on these cars. Having the timing come in later is "safer" on the motor and GM doesn't wants cars out there coming back with destroyed motor because of advanced timing curves set up too soon, etc. They would rather leave some performance on the table than deal with tons of motors coming back for warranty repairs they have to pay for.
This doesn't mean that by setting your timing to come in by 3,000rpm that you will destroy your motor, but GM has to pay it very conservative for their own benefit. Bringing in the timing VERY quickly can be overdone. If you set up your timing curve for example to come all in by let say, 2200rpm that's way too soon and can cause detonation problems that can destroy a piston or motor quickly but to bring in the timing by 3000rpm is pretty safe and will significantly increase your performance over the stock factory curve.
Also, again, each car is different and GM had different curves for different cars based on the motor, the tranny, etc. but it still comes back to GM "playing it safe" and willing to leave some amount of performance on the table while doing so.
I wouldn't call the factory advance curves in most stock distributor set-ups as "performance curves" from GM. By bringing in the advance faster and having it all in by 3000 is taking the GM stock curve and changing it TO a performance curve.

 

did he ever send you the info?
I' would bet that Dave set up the distributor with the stock factory curve and therefore the timing is probably not coming all in by 3000 rpm. Check your set-up to see if there is still timing coming in past that point.

If you read the info on Dave's website and in his tech articles he mentions at the bottom of one of them that feels that the STOCK timing curve is the best so that's how he most likely rebuilt yours to be. If that's the case than you may still have timing coming in much later than 3000 rpm.
If you simply revved the motor to 2500-3000, set the timing light for 36º and moved the distributor to hit the "0" mark on the balancer to set your timing and than walked away there is a very good chance that there is still more timing coming in past that. did you check to make sure there was no more timing still coming in past 3000?

EDIT: opps, sorry, I had missed post #11. I see now that your timing doesn't come all in until 4600rpm. Therefore, by setting at 36º at 3000 you are not setting it with the timing all in by that point.

 

You really only have three important things.

1. What is the max total mechanical advance timing.

2. What rpm is the max mech timing all in.

3. how much does the vacuum advance add at max vacuum.

You said that you don't get 26 degrees of mechanical till 4600 rpm. IMO - hot rodded motors like all in at more like 3000 - 3200 rpm. Cars with stock exhaust, EGR, stock intakes, 40 year old combustion chambers can use all in at 2800 or so rpm.

So you set a motor up by the max mechanical advance without vac. connected. like your 36 degree. Initial can on only be changed by recourving the distributer.

I'm a believer in adjustable vacuum advance cans like the Crane cams unit. Vacuum advance should only add @8 degrees of timing

 

If you do that you are creating a vacuum leak at the carb when you come off the idle circuit (assuming you're using ported vacuum). You need to either unplug the hose from the distributor and plug that end of the hose - or - unplug the hose from the carb and put a vacuum cap over the vacuum port of the carb. Regardless, that will not affect your timing unless you are having to set the curb idle screw so high (to achieve idle) that the mechanical advance is coming in at idle.
I would get a basic advance curve kit from Summit and install the gold springs to get a good baseline.

What kind of carburetor are you running and what is your curb idle rpm?

Hang in there man - you'll get it.