Hello to everybody, as suggested I open a new post instead use an old one. I'm working on my '69 L36 Corvette's distributor. It seems ok but it doesn't give more than 15 degrees in mechanical advance (it's really the maximum, I also tried with very soft springs). I read that it should be 26 degrees. If I don't pass the 36 deegres in total advance can I remove the stop advance bushing for have some more degree or it's better to have something around the pin?
Thanks a lot!
Filippo

 

Is your mechanism fully free to move? Seems like it might be binding. You should be able to see the bushing move through the full range of the slot. If not, you probably need to get that right first. Tear it all apart and clean/lube everything. While you have it apart, add shims to the bottom of the housing at reassembly to give you about .010" or so clearance between the housing and the drive gear. There is a kit for this.

 

Post a pic......see if it has the correct weights in it......

Jebby

 

Thanks for your answer. The mechanism is totally free, lubricate. The bushing reachs the end of the slot. The chain is new, the distributor seems in good conditions. If I take off the bushing and leave the pin 'nude', the mechanical advance goes from 15 to 22 deegres. It's enought for me. I just don't know if I can't leave the pin without the bushing

 

As Jebby said, it might help to see the weights and cam assembly. I don't know why you could not just leave the bushing off. I don't have one on my '69 distributor. My autocam plate has been brazed and file fit without a bushing. Works fine for me.

 

I post some pic. I don't thing it's a weight problem: I tried with very very soft spring and the situation doesn't change, I'm sure the plate opens until bushing. I have a rubber bushing now, I found a metal one in the spring kit (same size), so I was not sure if could run without bushing, that's all.

 

Depends on how much initial advance you want to run. I like having 20*+ initial. Now if the engine is dead stock....might be a little much....but for Hot Rod stuff with big cams etc...more initial is good stuff.

JIM

 

That picture of your black rubber bushing shows it so far off it probably does not work. It needs to be all the way up on the pin.
Distributor on left has brazed up slot and no bushing. Distributor on right has brass bushing. Notice the brass bushing is pressed on all the way and peened over. You can alternately crush the bushing a little bit before pressing it on as a way to retain it.​​​​​​​

 

You want pretty close to 36 degrees total mechanical advance if you have old school cylinder heads.
Depending on the size of the cam (duration) you probably want 12 degrees initial if it is close to stock, and 16-18 initial if your cam is a little more moderately aggressive camshaft. 427 hotrod recommended 20 degrees initial but his idea of a street cam is very aggressive, so it works.
Well that means you either need 36-12 = 24 degrees mechanical advance, or 36-18 = 18 degrees mechanical advance.

It matters not how you manipulate the mechanical advance, as long as the degrees are what you want. (18-24 degrees). Having the initial somewhere in the 12 to 18/20 range is probably where you want to be. If you are a few degrees lower than ideal here (initial), it's no big deal, it's only at idle. But do NOT mess up the Total number.

I have welded the slots, ground out the slots, brazed the slots, changed bushings, machined bushings, changed the football, changed the pole piece, etc. etc. As long as it gives you the tune you want. People have been curving these distributors for a long time, so who knows if it has been modified, or not, already anyway. But I do not like the aftermarket weights, they do not seem to work correctly, we only use the factory weights.

Now I do know of a road racer, that determined the ideal timing for the engine, at every rpm, on a dyno, and then we shaped the weights until the timing curve matched what the engine wanted, all the way from 2000 to 7000 rpm. But then again that actually was THE Penske Trams Am Javelin, that I got to work on in college, when it was a "PITT" racing team car, in the late 70s. That car had more than a few tricks up it's sleeve! It was very cool. If only I had a camera back then...sigh.

 

Thank you all for your detailed answers!
In the picture I moved the bushing fron the pin just for show you better the size, I found it in the correction position when opened
My engine is original so I don't need a big advance. I like to set the initial advance between 8 and 10 degrees, the engines runs good with this and it's quite ready yet. If I set more advance I have some resistance when I start the engine and also a little bit of diesiling when I turn off with warm engine, also if I use the high octane gasoline (we have also 100 octanes in Italy).
The mechanical advance gave me just 15 deegres, now without the bushing it take 22 degrees and 10+22=32 is enough for me, I don't need to reach the "magical" 36 of total advance (from where does this number come?). I just don't understand why the distributor doesn't give me the 26 degrees of maximun mechanical advance (the plate reach the pin so opne completely...)

 

This magical 36 number comes from thousands of people who have dyno'd thousands of engines and determined best power on most older designs is 36 degrees.
You most certainly DO need that number.....4 degrees retarded is huge as far as mid to high end power is concerned.....and the engine will just run more efficient there.....
32 is not enough.....some SBC and BBC setups like close to 40 degrees....but the hard baseline rule is 36.
And as I implied by sending a photo....you probably have the wrong weights in it....
10 degrees with the vacuum advance unplugged is not enough either....12-15 is the number for stock to mild cams.....so bump your initial to 14.
If it is hard to start at 14 then you need to check cables and starter....
These timing numbers have been around since the 60's.....

Jebby

 

Did not know you were in Italy.
That makes certain things a bit more of a challenge, but we are here to help.
Chevy published these timing curves back in the 1970s in a book for their racing customers, called the "GM Powerbook".
Things haven't changed much since then except for trying to deal with the added ethanol in the gas here stateside.
The main accomodation there is to still run the vacuum can as mentioned 50 years ago, but limit it to 10-12 degrees max to work better with the ethanol gas.
So you very likely will need a new one or an adjustable one. The old ones had huge vac advance and do not like the 'new" gas.

The dieseling after shut-off is aggravated by several things: (should be easily fixable)

• Idle speed too high
• Timing too retarded causing hot cylinder/valve temps
• Vac advance can hooked still up to "ported" source on carb, instead of "manifold" vac source on manifold as recommended, (also causing the above).
• Carbon deposits on piston/valves
• Cast iron heads and intake hold heat
• Exhaust/carb heat crossover in intake not plugged up yet

The cylinder temps in the engine are much cooler when the advance curve is "correct"
This helps with over-heating issues as well.

The factory timing curves in 69-70 are as much as 20 degrees retarded from the ideal curve as installed on some 1964 cars, or described in the GM Powerbook, at a typical 2500 rpm cruise anyway. This was done for early emissions HC compliance, and it definately hurt performance, under 4000 rpm anyway. But "power-tuning" the distributor has been a common occurance since 1967 because of that, and it really helps.

 

Thanks a lot! I understand a lot of new things now. I'll work on the car during the week end!

 

Hi! I closed the work last week end, the engine runs good. Just a tought: I have a cam a little bit retarded on my car, has this got some influence on timing advance? should I give more or less advance in this case?
Thanks