Base Timing/Centrifigal Advance w/Pertronix Ignitor II

fine69 · 10-09-07, 06:09 PM

Hello All,

With the points ignition system, I used JohnZ's method of fastening rubber bands around the centrufigal advance weights to prevent them from moving (the the vacuum advance plugged) when setting the base timimg.

I recently installed the Pertronix Ignitor II system. With this particular set up, the rubber band method won't work.

Does anyone know of an alternative? The Pertronix seems to be running just fine. I just want to be be sure...

Hey, if I can even better performance by optimizing the timing - I'll do it.

Thanks.

Ralph

RodsnRides · 10-09-07, 06:23 PM

Hi there -

I am running a Pertronix as well, but not sure of the difference between a I and a II.

Although it's a bit more effort, you could always remove the springs and weights altogether to set your base timing, then reassemble. This way would work. Also gives you a chance to lube them-I like to use the Harley-Davidson grease that was used on the older (70's) models for advance weights on the Shovelheads and Sportsters. It's a nice, heavier grease that won't fly off.

Good luck!

Rick

McMurdo Station, Antarctica

**Hib Halverson** · 10-09-07, 08:44 PM

First, let me say that, unless your idle speed is very high, if you must lock the centrifugal advance at zero before you set the base timing, there's a problem with the advance mechanism. This, also, means that you'd have some level of centrifugal advance with the engine at idle which is also a problem.

Now, if the engine won't idle properly without some level of centrifugal advance, then there's a different problem and it might be that the centifugal advance curve needs to be changed. A symptom of that is, when you set total advance (initial + centrifugal) at a given spec---for the sake of discussion let's use 38 degrees---and at that number, the initial advance is too retarded, some folks might modify the centrifugal advance so that it's enabled at idle to make the engine run right. The problem with that is you get idle speed instability and you may end up with the centrifugal advance curve too aggressive and the engine in detonation in the low-mid-range due to too much advance.

The right way to "fix" that usually is a combination of selecting the distributor cam assembly which has a centrifugal advance limit bushing slot of the right length and using the correct limit bushing.

Generally, high-performance engines need some additional advance limiting to shorten the length of the centrifugal advance curve enough that you can have 34-38 deg. total and 10-12 deg initial. In the "old days" when billions of old Delco distributors were laying around in wrecking yards or you could still get dist. cams from GM, it was easy to properly curve a distributor that way. Today finding the parts is tough and some have to resort to brazing up the limit pin slot to shorten it.

In years past, I've set up a lot of Delco distributors, both 62-74 and HEI type and I've NEVER had one that ran with centrifugal advance enabled at idle speeds because I had the right combination of limit pin slot length, limit bushing size and initial advance.

As for a Petronix conversion, I think all this holds true because I believe a Petronix conversion retains the OE mechanical advance.

petes74ttop · 10-09-07, 09:08 PM

Hi I had the Pertronix kit since Nov 2003, it changed the dwell to 34 and it works fine. Taking the weights off will work to give you the initial timing. This week I got a Accel Curve kit, I thought that I could just change the weights and the springs but they included a brass bushing,where does that bushing go?
Thanks, PG

JohnZ · 10-13-07, 04:27 PM

Quote:

Originally Posted by petes74ttop

This week I got a Accel Curve kit, I thought that I could just change the weights and the springs but they included a brass bushing,where does that bushing go?
Thanks, PG

That bushing presses on the limit pin on the mainshaft advance cam that protrudes down through the limit slot in the autocam; photo below shows the pin with no bushing on it. Varying the OD of the bushing limits its travel in the slot, which affects the length of the centrifugal advance curve.

**Hib Halverson** · 10-13-07, 08:01 PM

Quote:

Originally Posted by JohnZ

That bushing presses on the limit pin on the mainshaft advance cam that protrudes down through the limit slot in the autocam; photo below shows the pin with no bushing on it. Varying the OD of the bushing limits its travel in the slot, which affects the length of the centrifugal advance curve.

True, but in practice you can only vary the length of the curve by an amount equal to about .010" less than the width of that slot. If you're starting with an "emissions-era" dist. cam, even the thickest bushing you can get in there may not be enough to limit the advance. Sometimes you have to modify the cam assembly by shortening the slot.

petes74ttop · 10-13-07, 10:26 PM

Thanks John Z for the photo, that helped me to no where to look.
Thanks Hib Halverson, I'll double check that there is not to much total.

I'm a truck driver with a stock 74 T-Top and I thought this would be a good project considering my limited knowledge and ability.

I'll post the results, thanks again, PG.

big2bird · 10-15-07, 12:54 PM

Base timing is irrelevant. Set it to 36* with vacuum advance plugged at 3,000 RPM or so. Set this 36* as max, and let the base fall where it may.

JohnZ · 10-15-07, 03:24 PM

Quote:

Originally Posted by big2bird

Base timing is irrelevant. Set it to 36* with vacuum advance plugged at 3,000 RPM or so. Set this 36* as max, and let the base fall where it may.

Before he does that, he needs to know if the centrifugal advance system is working at all, and how much it's adding; easy enough to map with a dial-back timing light. If it's gummed-up or sticking, he could wind up with a way excessive base setting if he just sets total timing.

big2bird · 10-15-07, 03:54 PM

Quote:

Originally Posted by JohnZ

Before he does that, he needs to know if the centrifugal advance system is working at all, and how much it's adding; easy enough to map with a dial-back timing light. If it's gummed-up or sticking, he could wind up with a way excessive base setting if he just sets total timing.

"I recently installed a Pertronix unit." I assumed it was new and functioning unit.

My point is that total advance , with the correct curve, is more important than base timing. Chevy engines were designed to operate optimally at this setting.
Adding up to 16* vacuum advance under no load conditions for a total of 52* under no load is also desirable for most engines.

petes74ttop · 10-15-07, 09:47 PM

Hi everyone and thanks again for all the help. Here's what been done so far:
Installed new acr44t plugs, the round .035 would pass and the .040 would not so the gap is between .035 and less than .040.

The dwell is 34*, from idle to over 3500 rpm.

Screwed into the manifold, behind the carb, was a loose pipe threaded fitting with 2 elbow connections, one to the trans and one to the vacuum headlight system. Put some pipe tape on that and a 1/8 extra turn a that was fixed.

The choke pull off ( vacuum break ) is split in two, broken right in the middle, so that's on order.
Looked in Ruggles book, page 51 to learn about the choke pull off and on page 50 is the info and picture of the distributor cam weight assembly and the slot on the main shaft.

The new Cap, rotor and the weights and springs have not been installed yet, but the readings taken today were:
With vacuum advance pluged- inital 12* at idle.
- at 3000 rpm 35*, increasing rpm showed no more.
Can the centrifigal advance add 21*?
With the vacuum advance hooked up 34* at idle.
Can a vacuum advance bring in 22*?

Thanks again for the help on this project, Pete.

big2bird · 10-15-07, 10:00 PM

Quote:

Originally Posted by petes74ttop

Hi everyone and thanks again for all the help. Here's what been done so far:
Installed new acr44t plugs, the round .035 would pass and the .040 would not so the gap is between .035 and less than .040.

The dwell is 34*, from idle to over 3500 rpm.

Screwed into the manifold, behind the carb, was a loose pipe threaded fitting with 2 elbow connections, one to the trans and one to the vacuum headlight system. Put some pipe tape on that and a 1/8 extra turn a that was fixed.

The choke pull off ( vacuum break ) is split in two, broken right in the middle, so that's on order.
Looked in Ruggles book, page 51 to learn about the choke pull off and on page 50 is the info and picture of the distributor cam weight assembly and the slot on the main shaft.

The new Cap, rotor and the weights and springs have not been installed yet, but the readings taken today were:
With vacuum advance pluged- inital 12* at idle.
- at 3000 rpm 35*, increasing rpm showed no more.
Can the centrifigal advance add 21*?
With the vacuum advance hooked up 34* at idle.
Can a vacuum advance bring in 22*?

Thanks again for the help on this project, Pete.

Sounds like you are getting it in good shape. I would bump the advance to 36* at 3,000. If it pings, back it down. As for the vacuum, if you mean 12* to 22* at idle, yes. They generally can add 16* overall.

**Hib Halverson** · 10-15-07, 10:51 PM

You will find that with 91-93 oct. gas if you set up for 36 degress total and 10-12 initial you will need to limit vacuum advance to 10-12 degrees. Much more than that and you're really end-up with too much advance at light throttle. The engine may not ping but it may run rough and not be as responsive. I usually start to see this happen with initial + centrifugal + vacuum advance gets up to 48-52 deg. In many cases, an adjustable vacuum advance "can", such as what Crane Cams and others sell, will solve the problem of too much vac. advance.

big2bird · 10-15-07, 11:04 PM

Quote:

Originally Posted by Hib Halverson

In many cases, an adjustable vacuum advance "can", such as what Crane Cams and others sell, will solve the problem of too much vac. advance.

I find adjustable vacuum cans to be a nightmares at best. He could get a 12* unit if he has problems. They are easy to change.

petes74ttop · 10-15-07, 11:32 PM

Seems like your are correct 35* plus 22* with the can= 57* thats too much. Let me fix the vacuum break and well go from there. Are there vacuum cans with less advance? Thanks Pete.