I've been playing with the ignition curve on my 74. Here's some background. I changed out my Pertronix to a Crane XR1 and I'm very happy with it. While doing this I decided to bump up the ignition curve a bit. With my current springs I have 10 degrees initial and 38 degrees full mechanical in by 2800 rpm. I tried a bushing from one of the kits I had and ended up with 16 degrees initial and 36 degrees full mechanical. This ran nice but on initial cold startup you could hear the starter strain. So I took out the bushing and I'm back to my original. So here's my question. Can you get different thicknesses of bushings for the distributor to fine tune the curve and what would the source for these be? Also, has anyone used brass tubing like you get from a hobby shop to make your own? I'd like to end up with 12 to 14 initial and 36 total. Thanks.

 

I don't know about the bushings but I had my dist. set up by the machine shop that machines my block. He has a dist. machine and will make the curve whatever you need. Mine had to much advance so he brazed up the slot tha tthe bushing rides in and then ground out the proper amount af material. Mine is set at 12 to 14 and a total of 36. If you can find different size bushings that would work also.

 

Gordon, I live in the Wilmington area and need a good shop to look at my heads. Anyone in the area able to flow test? Thanks in advance.

 

Hi Gordon. What shop did you take it to?

 

It is a little shop in Collingswood NJ called Withers automotive. Nothing impressive to look at but the shop is immaculate. They do top notch work. The shop is actually behind a house. They have been there for a lot of years. Sandra and Paul are the owners and Paul has forgotten more about SBC then I will ever know. One word of caution he is not an advocate of vacuum advance. He is mainly a racer. I had him setup the dist and then I put on the vacuume can.

I know he does head work but I don't know if there is a flow bench in his shop. He would know where to go for one though.

 

Thanks Gordon. I remember Withers from when I was a kid. I didn't know he was still in the business. I'll have to look him up if I don't come up with something else.

 

I've used plastic ball point pen casing to make a bushing. It worked perfect with perfect inner diameter. I'm talking about the all black bic type that have a taper so you could choose outer diameter as needed. A little bubba but I couldnt find right replacements I needed.My .02

 

I haven't done it, so I hope i'm not speaking out of turn. MSD sells advance curve kits with Bushings that work in their distributors. Maybe one of those will do the trick.

 

vettefixr -
The curve you had actually sounds very good: 16 initial with 36 total is exactly what it ought to be. You can even shorten it a touch more to get about 18 initial. This should not normally cause startup problems, and when it does, it's hot start - not cold start - that tends to suffer.

But if you want to lengthen the curve to get less initial while maintaining the 36 total, you can do it as follows:
Install the brass bushing and see how much advance you get. Once measured, you can then pull the distributor and measure the distance that the advance slot allows the bushing to travel. By dividing the length of the slot into the number of degrees of advance, you can determine how much the slot length needs to change for each 1 degree change in advance curve length. You can then simply take a Swiss file and lengthen the slot by the amount you determine.

For example:

You have 16 initial and 36 total. (When measuring initial, make sure you install a stiff set of springs to fully retract the advance to its seated position - use of light springs may partially deploy the advance at dle, resulting in an incorrect measurement.) So you have a 20 degree advance curve.

You then pop the distributor out and measure how much travel there is of the advance stop bushing within the advance slot. Let's say you measure that there is .100" travel.

.1" divided by 20 equals 0.005". That means that every 5 thousands change in the length of the slot will change your curve length by 1 degree. If you want to lengthen your curve by 5 degrees, you need to lengthen the slot by 5 times 0.005, equals .025". Use your calipers and a swiss file to produce the desired change.

...but I wouldn't change it from your 16/36 setup unless you really need to - that's a pretty nice timing spec right there!

 

Thanks all especially Lars. I'll try the bushing again and see if I have the hard start problem. After 2 days of sitting it started right up which it didn't do before with the shorter curve. When I tried to start it with the short curve it cranked very very slowly. I guess I can even file down the bushing a little to give me a slighly lower initial. Trial and error. At least I don't need a laptop to do it.

 

Just get a mini starter that can crank a 500 ci 17:1 compression motor and gear it down with a 168 flex and cranking it over will be a thing of the past.

I've always tried for max advance in the 3000-3200 range and get a dial type like crane cam vac advance unit. Then try 4-8 degrees additional

 

Thanks Gkull.. I have a Crane adjustable vacuum advance that I tried. The problem is that the adjustment controls the application of the advance and not the total. In the kit that Crane sells there is the vacuum can and also an advance limiter. The limiter is screwed into the distributor at the end of the advance actuator and, depending on how you have it set, forms a stop. I found that going with a 10 degree can was easier. I think you may be right though. I started it yesterday and it spun right over but the temps were in the high 40s to low 50s. I had problems starting it when the temps were in the low 20s to high teens. It could be that the 20 year old starter is getting cranky, no pun intended. The problem could also be a slow drain on the battery. I installed an Optima Redtop about 6 months ago and it's great but when the car sits for a week or two it cranks slow. I'll probably have to do a current test to see how much drain there is on the battery when the car is off. It's possible that something is not grounded right and causing a problem.

 

All vettes has a certian amount of drain. My battery was also low after two weeks. Years ago I got one of those cig lighter plugin 1-2 amp trickle chargers.

I have since went with the next step. JCWhitney sells this key type removable battery cable switch. I just unplug the juice. I don't have a radio to worry about my preset stations anymore. It's also within reach. So I can shut down power in case of fire.

 

I have a battery disconnect but I hate using it. Every time I disconnect the damn thing I have to pull out the books to set the anti-theft code in the radio, then set the presets, then set the rf frequency of the CD changer, then set the amplitude of the CD changer and then make sure it all works. It takes about 5 seconds to reconnect the battery and another 10 minutes to get the sound system working. And as an added kicker I replaced the guts of the clock with a quartz movement about 5 years ago. It keeps great time with one exception. When you reconnect the battery you have a 50/50 chance that the clock is going to start turning in the right direction. Sometimes it takes 3 or 4 reconnects to get it to spin clockwise.