Hello,
I finally got around to working on my 1972 SB with AT. I was going to check the timing and found that the vacuum advance hose was not connected to any vacuum source at all. I think that it went to the TCS solenoid, which was plugged at the output. The vacuum port on the carb. was also blocked off, which should mean that I did not have any vac. advance on the car.

I started checking the dwell, rpm and timing. I am asking if this is a normal situation.

1. With the vac. advance hose disconnected and blocked, my starting point was:

dwell 35 degrees
rpm 800 rpm
timing >16 degrees * my guess. it was past the last tip on the indexer.

2. I adjusted the settings to the following.
Dwell 30 degrees
rpm 600 rpm
timing 12 degrees.


3. I then connected the vac. hose from the carb. to the vac. advance unit. The timing and the rpm changed.
Dwell 30 degrees
rpm 900 rpm
timing > 16 degrees * way beyond the last tip on the indexer. but still visible with timing light.


Summary
.............Pre.........Post..........C onnected
Dwell......35 deg.......30 deg......30 deg
rpm........800rpm......600rpm......900 rpm
timing.....> 16..........12............> 16
............* my guess................* my guess


So, is this normal for the rpm to increase after the vac. hose is reconnected, and for the timing to increase.

thanks,
kdf1986

Here is a photo of the Bubba connection.

 

You first need to find out if the vacuum is on a manifold vacuum or a ported vacuum( I think that is what it is called) the manifold will have vacuum at idle because it is below the throttle plate so it is correct. The ported vacuum is above the throttle plate and has no vacuum with the car at idle. This is why you must remove the vacuum advance from the dist. before checking initial timing on a manifold vacuum and actually all cars. Mark

 

You should really read Lars paper on timing. To answer your question, if you connect the vacuum advance up to a constant vacuum source your rpms will increase because you are advancing the distributor via the vacuum can. If you connect the distributor to a timed vacuum source there will be no change in rpm because you won't get vacuum until the throttle is cracked open. As far as your initial timing goes you should disregard it and set your distributor up to get 34~38 degrees mechanical advance all in by around 3000 rpm. Ideally you should shoot for about 36 degrees but each engine is different and some like a little more while others like a little less. Once you have the mechanical advance set for max advance let the initial fall where it may. If you feel that the initial advance is too low you can put a bushing on the distributor vacuum advance stop to shorten the amount of advance thereby giving you more initial advance. To make that statement clearer let's suppose you have your max mechanical advance set for 36 degrees causing your initial advance to be at 10 degrees. That means that you have 26 degrees of mechanical advance. If you put a bushing over the advance stop and set your max advance again to 36 degrees your initial would probably end up around 16 degrees because you have limited the mechanical advance to 20 degrees by using the bushing. Once you have the mechanical advance set up the way you want it reconnect the vacuum advance to a timed port and give the car a test drive. If you find that the car bucks or pings it means that the vacuum advance is adding too much additional advance to your performance advance curve. You can look at Lar's paper and get a cannister that limits vacuum advance to 10 degrees which when added to the 36 degrees mechanical advance would give you about 46 degrees advance during light throttle cruise (remember that at WOT there is no vacuum therefore no vacuum advance). You may have to play with these settings + or - a degree or two to see what your car likes but overall that's the correct way to set up a distributor. Good luck.

 

Small correction - althought it is correct that there is no vacuum at idle at the PORTED vacuum nipple, it is still MANIFOLD vacuum (BELOW the throttle plates) - it's just "switched" off at idle.

 

Thanks for the help. I have so many articles by Lars printed out I dont know where to start. I guess I'll get out my highlighter this weekend and start making notes.

The car ran great last night on my test spin. I guess it should after connecting the vac. advance to a vac. source. Good acceleration, no pinging....pretty smooth.

One thing I need to get is a timing light with the advance function on it. I cant tell what my timing is past the last point on the indexer. I saw some digital timing lights with an advance function that will let you dial in the timing. For 36 degrees that is what I need.

KDF

 

$69.95 at Sears. The best tool purchase you'll ever make.

 

Thats where I got mine..but be careful with the plastic pick-up!

 

Lars, on a 1980 corvette with HEI distribtor can you still pull the springs and set it to 36degrees @ 2200-2500 rpm, put the springs back on and continue on... I read one of your papers that does not mention it, then I think I ran across another one that says HEI cannot be done the same way.
Also, how does a slightly lumpier cam affect timing?


Thanks
Mike

 

Thanks for the corrections. Mark

 

On the HEI you cannot remove the springs to see total timing at a low rpm. Removal of the springs on an HEI will cause an artificially over-advanced condition that will not be duplicated with the springs in place. Instead, on HEI, you have to rev the engine until the timing advance pegs out. You can install a random set of softer springs to get the timing to peg out at a lower rpm in order to keep the operation safer, but you must have a set of springs installed.

A lumpier cam will normally require more initial timing in order to maintain an acceptable idle quality - I usually shoot for at least 18 degrees initial, and as much as 24 with a radical cam. Total timing does not change, and should remain at 36. Also, a lumpier cam will require use of a "softer" vacuum advance control unit due to the degraded manifold vacuum.

 

Lars I have a question.
It seems some people rev the engine to get max mechanical advance and while there also set their timing. I can see this as a no difference thing but wouldn't the same be done at idle knowing this mechanical advance value?
Seems safer and easier to set static at idle with the known mechanical value. Then just check it.
Timing light, distibutor, thottle. Somehow they found an extra hand too.

 

Techno -
Yes, you can do as you state: If you rev the engine to see and note the total advance, you can then correct the timing at idle by the number of degrees needed to obtain the difference between the noted and the desired.

However, you actually end up reving the engine about the same number of times for either method: If you dial in 36 degrees on your timing light (desired total advance) and rev the engine to note the total, you'll end up with a brief rev, a quick turn of the distributor, and another rev or two to nail the timing at 36 total. If you just wish to measure the actual total timing, you'll end up reving the engine, turning the timing light **** to correct, reving the engine again to see if you hit it, adjusting the **** again, and giving it a final rev to verify. You'll then have to do some quick subtracton and set the timing at idle by the noted difference. Not much gained... With a little practice, you can set and nail down the total timing at elevated rpm with only 2 revs of the engine, with each rev-up lasting only a few seconds.

 

could someone link me to The papers that lars has been so kind to provide us on timing and carb tuning? I tried the search and keep coming up with threads like this, and no links to the papers thanks for your help

 

Yeah, I learned the hard way. Melted the darn plastic on the induction pickup. It still works, but it's contorted.

 

Just drop me an e-mail request for the paper and I'll send you the latest updated version. Most of the papers posted on various sites are not the current version, and they're not kept up-to-date:
V8FastCars@msn.com