Power Timing

[73 Centurion]

Hello everyone,

There's a magnificent thread on powertiming on the V8 Buick board. I suggest you get a cold drink and read through all of it. It explains how to change the spark advance in your distributor to make your engine happier. It gave me the courage to try it for myself and I've been amazed at the results.

In a nutshell there are 3 types of advance in our distributors. The sum of these 3 types determines how much timing the engine receives and at what RPM. Initial advance is how the distributor is installed in the engine. I think the factory setting for my car is 2 degrees advanced. The second advance is mechanical advance this is the weights and springs under the rotor that advance the timing as RPM's increase. I beleive the factory setting does not allow full advance until somewhere past 4,000 RPM. The third type of advance is vacuum advance. When the vacuum is high it advances the spark for better gas mileage. I think the factory advance allows 20 to 30 degrees.

The goal is to change the distributor so that you have 30 degrees advance when you combine initial and mechanical and another 10 degrees on top for the vacuum advance. Cruising on the highway with all three advances maximized you should have around 40 degrees. If you floor the pedal the vacuum drops the vacuum advance goes away and the engine is at 30 degrees advanced which is just right for power.

I now have my initial set at 13 degrees, 17 degrees of mechanical and 9 degrees of vacuum advance. The car starts easier, runs 15 degrees cooler, has more power and gets better gas mileage. It worked out so much better than I would've guessed.

[73 Centurion]

I had replaced my distributor at some time in the distant past so none of the factory settings seemed to be right. The adventure began with the purchase of an advance kit with springs and bushings and an adjustable vacuum advance that has not been installed yet.

Following the directions from the V8 Buick thread I tried to adjust the timing curve. But the readings I was getting were not consistant and the timing seemed to bounce around at idle although it settled down when the RPM's increased. The first step is to limit the vacuum advance. If you don't do this the engine will get over advanced and ping which is very bad. Some suggest limiting the vacuum to 8 degrees some say to 10. It all depends on what your engine likes. By chance the block off plate I made came in at 9. I then put in what I guessed were the lightest springs (the Mr. Gasket set doesn't actually tell you which springs are which). With the vacuum advance unplugged and blocked off I set the total timing (initial plus mechanical) to 30 degrees. This left my initial timing at 18. I didn't think the car could live with that but on the contrary it really perked up and the engine temps went down 15 degrees. I left it like that for a few days to see how the car liked it. I did notice the starter was sluggish if I tried to restart the engine when it was hot, a sign of too much initial advance. I also didn't like the timing bouncing around at idle and sometimes the mechanical advance would not go away after revving the engine. I decided to pull the distributor and investigate.

Turns out the end play on the distributor was way too much. Here is the distributor disassembled and the worn out bushing.

[73 Centurion]

I was guessing that part of the fluctuation at idle was the distributor bouncing up and down advancing the timing on the helical cut gears. After some searching on the internet I found that 0.10 inch of end play and my distributor was probably 3 times that. I installed a pertronix module when I replaced the distributor and I remember being frustrated at setting the proper gap because there was so much play in the distributor. I think this distributor was loose when I bought it and it got worse over time. I purchased a shim kit and decided to give it a try.

I found I could have .05 (too tight) or .15 (too loose) with the shims in the kit. After a little fiddling I found that reusing the worn bushing with a 0.2 from the kit gave me just under 0.1 where I wanted to be.

I also noticed the distributor was rusted inside and the grease had dried out and the pivots posts for the weights were rusted.

[73 Centurion]

I cleaned off the grease, sanded away the rust. I added new grease and made sure everything worked smoothly.

I noticed that the srpings in the Mr. Gasket kit have a top and a bottom, whereas the original srpings had their loops in the middle and could go in either way. Here's a picture with on spring facing down, you can see how it would interfere with the wieghts. I think this might've been why my mechanical advance didn't always back out.

The second picture shows the srpings installed correctly.

[73 Centurion]

To get to 30 degrees I needed 18 degrees initial because there were only 12 degrees mechanical in the distributor. I found there was a rubber bushing that limited the mechanical advance. Most of the threads I've read people are limiting the mechanical advance to allow more initial. My situation was reversed, I wanted to add mechanical advance because 18 was too much initial. I pried the bushing off and put the distributer back together and stabbed it back in the engine.

We all know the releif when an engine we're working on actually starts up again.

When I reset the timing the initial came in at 13 degrees advanced with 17 degrees mechanical. The lighter springs bring the full mechanical in around 2,500 RPM. The engine starts great hot or cold, runs cooler and there's definitely more power under the hood.

Here's a picture of the mechanical advance slot and the post that had the bushing on it.

[73 Centurion]

the final chapter of this tale has a bit of a comical ending. I was working on the distributer on vacation and had a 4 hour drive to get home. The engine was working beautifully but my exhaust system was getting louder as the vacation progressed and I hoped to deal with it when I got home. Fate had a more interesting idea. About an hour into the trip the passenger side exhaust broke right before the muffler leaving the muffler in front of the axle and the tail pipe on the other side. The pipe was too long to untangle and too solid to cut through. I borrowed a coat hanger from the nice policeman who stopped to see what was going on. This cop really was a great guy. Not only did he give me the coat hanger but he drove down the highway and found some scrap lumber on the side of the road and brought it back so I could drive up on it and get a bit of clearance. They really are heroes in big and small ways.

A pair of vice grips clamped the hanger to a cross member some creative work with the remaining hangers got the peices of f the ground and allowed me to get home. I spent 3 hours of driving a couple of miles under the speed limit avoiding every imperfection in the roadway. Although nerve wracking we did make it home.

The drivers side was complete but the passenger side was a set of headers and about 3 feet of straight pipe. The nice police officer wrote me a warning for a noise violation. He then explained that any other officer would be reluctant to give me a ticket for something I'd received a warning on. Yes, he was that thoughtful.

Normally it takes a full tank of gas to make the journey between the timing changes, driving exceptionally gently and a "freer flowing" exhaust system saw me home with more than half a tank of gas. I haven't checked the MPG (it's a topic I try not too pay attention too). But, I'm going to check it out.

I've since had a new exhaust system installed but that's a story for a different thread.

John

[Gum,beest]

Thanks John
Great thread
I did the cleaning lubricating and adjusting the play in the shaft when i was installing my Pertronics II ignition module with the stock springs and weights
Set my initial advance at 6 degrees (4 degrees is in the manual) so it runs good but according to this tread there is always room for improvement
What kit did you buy and where
In the thread on the Buick V8 forum was mention of a Mr Gasket set from Jegs but the only set i found there was for a Hei ignition
An other problem i have is my timing light it is not adjustable so i am i cant adjust the vacuum advance in the correct way

[73 Centurion]

Hi Erik,

I bought my kit from Summit although I'm sure Jegs carries one as well.

http://www.summitracing.com/parts/MRG-928G/

You have to buy the vacuum advance seperately. I'm still using my "stock" vacuum advance. I put stock in quotes because this is a replacement distributor and it seems to be quite different.

There are 2 things that need to be adjusted on the vacuum advance.
1) The first is the total number of degrees of advance. This is limited with a plate that limits the movement of a pin. This is the least well defined part of the process. You have to remove the vacuum advance (it can be removed with the distributor in the engine) and make a block off plate out of scrap metal. You then have to secure the plate to the vacuum advance in the right place. It's not particularly hard, it just takes a little creativity and fabrication. You absolutely must take this step or you could over advance the engine and run into pinging.
2) The strength of the spring that resists the vacuum. If the spring is strong the vacuum advance goes away quickly but if it's too strong you won't get full advance, if it's weak you'll get full advance but it will be slower to go away. If it doesn't back out quickly enough you could have too much advance momentarily when you stomp on it.

Some vacuum advances adjust both with one adjustment others have them in two separate adjustments. I've adjusted #1 above and am using the stock spring. I will eventually switch to the adjustable vacuum advance and fine tune it.

You don't need a dial back timing light (although it's much easier with one). You can make a peice of tape 1.76" long that you add to your balancer that indicates 30 degrees. With this you can set your total timing using light springs and the stock timing marks.

[73 Centurion]

With the timing sorted out I had to turn the idle setting way down and I also had to reduce the amount added when the A/C was turned on. I was really surprised how much the engine woke up with these changes. It ran smoothly and reliably before but would stumble and sneeze at anything more than half throttle. After the changes I can floor it without any stumble. If I leave it to the floor it begins to backfire through the exhaust so I have more tuning to do but it's a huge improvement.

I think the springs I have in maybe too soft because the RPM drop between park and drive feels a bit too much. I've got to put a tach on and see what the numbers really are.

The best thing about this whole process is I feel more confident in making changes and feeling how the engine reacts. I hope to make some modifications to the engine and I now feel like I'll be able to dial in the combination after the changes. The next bit of voodoo to learn is tuning the carb, but I've read in many places you need to get the ignition right first and based on my results I can see the wisdom of this.

I encourage you to read through the threads and consider giving this a try. It is the type of modification that is very easy to undo if you don't like the results. Just swap in the stock springs remove the vacuum block off plate and reset the timing and you're back to bone stock in no time.

The factory set the timing curve very conservatively intentionally reducing performance because they need all of the cars to run well in Denver or Florida, summer or winter. My car doesn't see winter and is unlikely to see that kind of altitude. I'm glad to have some of that power back.

John

[Gum,beest]

Thanks John
That is a very cheap kit so well worth to try it
I put this on my to do list for this winter
First on my to do list is installing my new top that i have bought
I am now waiting on dry weather to do that (got a wet summer over here at the moment)

[73 Centurion]

I just wanted to post an update on this.

My parents have a summer house on an idyllic lake in Maine. It's about 3.5 to 4 hours away depending on traffic. I filled my tank just before I left after a relaxing long weekend. Today I refilled the tank and was able to calculate the MPG.

17.1 MPG !!!

I knew the mileage had improved with the timing changes and the new exhaust system but I was pleasantly surprised to see that number. I'm going to keep tweaking the timing sneaking up on the point where it pings. I wonder how much more I can squeeze out of it.

John

[Gum,beest]

Wow John
That is amazing
What where the driving conditions ? cruizing on the highway or average driving
Did you have the adjustable vacuum advance installed yet?
I have no numbers of the Centurion from this year i had very little opportunity to drive here and have cleaned and refurbished the carb and put in a Pertronics ignition system in the mean time ,but the last time i check t the fuel consumption it did 10.4 MPG :confused4: while cruizing on the highway doing 50 MpH

[73 Centurion]

The trip is all highway. I had the roof down and the weather was perfect the whole way. That doesn't affect gas mileage but it definitely improves the smile per hour!

The vacuum advance is hooked up. I made a limiting plate that holds the vacuum advance to 9 degrees.

John