Hello Forum,
I'm in the process of setting up my timing for the best performance I can muster out of the old girl.
I am working on a stock 1968 427/390hp iron heads with an M-20 4speed.
She just turned 89k. I have my idle set at 850 rpms.
I am using a dial back light.
I have the initial base timing configured for 12 degrees with the vac. advance disconnected.
I have 36 degrees at 2500 through 2800 rpms. It appears all in at 2800. I will verify again.
When I hook up the stock advance can I get 24 degrees at base.
I took it out for a ride and I am getting the low speed hitching or surging and when I shut her down I had to wait about 10-15 min. to restart.
She was acting like I had a hot start problem.
I know I have to much initial timing so my question is what does the stock 360 marked vac. advance can add in terms of advance?
It must be more than 12 degrees because 24 degrees initial should not be causing a difficult restart or surging.
Vehicle runs fine above 2000 rpms with normal driving through the gears.
Thank you in advance.
Marshal

 

I don't remember the values and how.they are numbered . I know Napa can get the size you need but is check with Lars for guidance. It sounds like something is screwy. Do your weights fit on the pins or are they sloppy? Also do they move freely?

 

Inspect your mech'l advance mechanism to ensure it is not binding and the springs snap back the weights.

 

Thank you both for your responses. As I remember when I attempted the recurving of the distributor a while back using the latest 2012 Rev. of Lars Papers, I used the original factory weights and replaced the heavy orig. springs with the two gold springs from a Mr. Gasket 928 kit. I will recheck the condition of the original weights and centrifugal plate to make sure it is moving freely. It was as of last check some time ago. I am still curious if anyone knows what the factory can adds as currently it mimics to much initial advance. If I am at 12 degrees base and the timing light show 24 degrees total with the Vacuum advance connected it leads me to believe the unit is adding 12 additional degrees. However the car is responding at start up and low rpms, surging, like I've added much more than 24 degrees initial?
Any one else have any additional thoughts.
Marshal

 

I think you might need Lars' vacuum advance limiter. I used one on my 350 in conjunction with the can he recommended. I have a points dstributor, you may not. I dont know if that makes a difference. (I run 12 initial.)
Its $15 shipped directly from Lars' cookie factory!

VS

 

VERYSOON,
Yes, I am running a stock Delco points distributor in my car. I kinda go on the same premise of John Hinkley. "I like stock looking cars with some performance enhancements" I saw this in his timing paper. I would like to run this car as well as can be possibly conceived with the inherent ability present in the current configuration. I have disconnected the vacuum advance and plugged it. I am going to see if that temporarily cures the hot start and surging. Then after I isolate the vac can I can be sure that I have to much initial advance with the current can.
Marshal

 

The operating condition you described is quite common in modified vehicles. One minute, it feels like you are dragging a boat anchor. The next minute it feels like a semi is pushing you down the road. This surging is known as "trailer hitching". And its cause is, a little too much vacuum advance. The Vac canister is searching for it happy place but is battling varying RPMs and vacuum signal. Every engine is different, but the happy place seems to be around 9-12 inches of Vac applied.

Like others, I too run a Vac Limit Plate, set at eleven degrees. Here is one from Summit:
Crane Cams Vacuum Limit Plate 99619-1
$6.
I have a different style with hooks on each end and it mounts under the canister mounting screw. I believe its an MSD unit.
You can also purchase adjustable Vac canister for tweaking the timing. An allen wrench is inserted into the Vac port on the canister to adj.

One thing you may want to try. Disconnect the dizzy vac line from the carb & plug that carb port. With the engine idling, attach a Vacuum Hand Pump Tool to the dizzy hose and watch the gauge as you apply vac. If the engine continues increasing RPMs as you continue increasing vacuum and beyond, then you know you need to limit the vac.

Your hot hard starting issue I believe is from all that advance and you are on the edge of detonation. You can not always hear pinging due to the exhaust system.
That is why the cylinders need to cool down 10-15 min before re-starting.

 

I had a similar trailer hitching on my 350, and ultimately found out my advance mechanism had basically seized up......never found out why. It apparently was siezed up at a very advance position?? Make sure your mechanical advance mechanism is working freely, and then use a Miti-Vac and make sure the vacuum advance is also free to work.

 

If you're using an old, outdated 2012 version of my paper, you're working to bad info. E-mail me for current, valid info.
Lars

 

Lars,
I will email you in the morning for the latest revision.
Thank you for taking the time to respond to my thread.
Marshal

 

I read somewhere but haven't been able to find it again that some of those aftermarket spring kits were only designed to use with electronic distributors not points type, does the timing jump around at and above idle without the vac advance moving?

 

Your story rings a bell Haggisbash.

I think it had to do with Mr. Gasket 928 kit or maybe Mr. Gasket 929 kit? Something about incorrect fitment with the plastic bushings maybe?

 

Timing is not jumping around the balancer and timing tab show steady. The colored springs in the kit are for points distributors. A combo of colored springs gives you the full advance desired for tuning. I installed 2 gold springs from the kit as per an older version of tuning papers by Lars. My distributor is an orig. points style dizzy.
I am getting too much initial which needs to be sorted out.
Marshal

 

If you post the distributor number i can give it's oem settings including the vac can number and it's factory advance specs.

 

I believe I read Lars recommends for points style distributors one black and one silver spring.
Did you try disconnecting the vacuum advance when cranking on a hot start?
Did you install the bronze bushing that comes with the points style advance springs?

 

I will post the requested info when I get home from work.
I did not install the bush as there was already a bush still on there.
I used 2 gold springs from the 928 kit so might have to try another combo.
I thought I read 2 golds in his paper?
I have not had the chance to run it again with the advance disconnected but this is next to see how she starts hot.
Thank you everyone for your help.
We will get this right even if it takes an entire forum village.
Marshal

 

first thing to do is try 10 then 8 degrees to see how it starts. then when you have that sweet spot you need to figure out how much mech advance you need to get to 38 degrees total. stock iron heads on a big block? 38. usually you time for total advance and it is fine at whatever initial it ends up with. but usually ain't always.

 

Sounds like you are on the right track Marshall.
Every car does like a slightly different tune.
And yes there is a specific sequence to the tuning.
Get the centrifugal curve where the engine likes it first. And remember that is basically for full throttle only.
The "right" way to do this is on a dyno or with a stopwatch. Many have done that, I did. And most of us found that the curve you have works just fine.
Now depending on your compression and gas octane, you can tune it to the as you use. Basically increase timing in increments until power stops increasing then stop.
But basically your curve sounds OK. It is straight from the Chevy PowerBook.

Now the Vacuum is the last part to tune, and with a performance centrifugal curve installed, typically the original vac can has too much advance.
The engine would like a little less, and possibly to pull off easier/quicker. (at a higher vacuum).
On my LT1 I found I had to first limit the degrees to about 10 vac, with a limiter. (Like the Crane one mentioned, or Lars)
Then at certain rpm/vac levels it was still a little too much, so I used an adjustable vac can, where I could adjust the vac that it released at.
I adjusted the internal screw until it pulled off a little quicker than stock, with less vac drop.
The vac settings are all by experimentation, and seem to vary the most from car to car. Due to load, gearing, temp, cam, cranking psi, etc.

Here's an Accel one I found quickly, but it is the HEI version: https://www.summitracing.com/parts/acc-31035
Here is a tuning article from Super Chevy: http://www.superchevy.com/how-to/eng...er-drivability
I remember literally driving around for a day with a big Vac gauge duct-taped to my dash. I found that my "problems" always happened at "X" vacuum. Then I would put a mity-vac on the vac advance can, see what it was doing at that vac, and change it.

Eventually even got my 11:1 LT-1 to run on pump gas! It can be done.

 

I have the same Accel vacuum can on my 79. Below is an image from the bench tuning, when I was confirming that full engine vacuum would result in only 12 degrees of vacuum advance (it did!). My application was a stock L48 SBC with a new ZZ4 takeoff distributor. The adjustable hard-limit method that Lars suggests may work just as well, or better, and will also work with different vacuum cans working in different vacuum ranges, for example on cars with a big cam. You can see in my image below the vacuum range the Accel can expects.

One additional thought. I replaced my stock, worn-out distributor because it was not advancing smoothly. I used the ZZ4 with the stock (to that dist) weights, and used one soft and one medium spring from a Mr. Gasket kit. I also shimmed a brand-new, out of the box distributor to minimize endplay. This gave me all-in at 2800 RPM or so, which I set to 36 degrees (stock SBC). Equally important, I got zero mechanical advance at idle. My total initial timing was 15 degrees BTDC: 36 full, minus the 21 mechanical. Add another 12 degrees from the vacuum advance for an actual 27 degrees advance at idle.

Were you able to confirm that your distributor is returning to "zero" at idle, with the vacuum disconnected? You had mentioned soft springs earlier, so it is worth confirming.

 

Gentlemen,
Here's where I am at. It looks like from the distributor tag 1111493 came out of a 1969 350/350 originally. The timing specs for this unit pegs out assuming I have the original weights at 26 degrees at 5000 rpms. The centrifugal advance begins at 1150 rpms. I have a base timing of 12 degrees. Now that is assuming the springs are holding the weights back and no mechanical advance is in play. My curb idle with a 4speed is 850 rpms. I will need to confirm this and if so go to a stronger spring set up.
When I hook the MS360 can up to full manifold vacuum my dial back reveals that I add 12 more degrees on top of the base timing. The can is listed as the correct original application for a 1968 427/390&400hp engines. The can is marked 12. I presume this is crank degrees as opposed to distributor degrees as it's adding 12 degrees crank to the timing light reading.
My symptoms as listed above are a result of this idle combination. I have ordered an Accel point dist. adjustable vacuum can#31304. Will this work as a stand alone or do I still need a liming plate as Leigh1322 has mentioned?
Thank you again,
Marshal