Took pics of my distributor in the car. Cam has 540 stamped. Thought there may not have been a bushing, but it appears there is. Looks strange to me and appears to take up most of the slot.
Last pic is of a 69 1111491 with the rubber bushing I'm used to seeing. Notice all the space still in the slot. Has a 536 weight. The problem I'm having is setting total advance, as the mechanical never stops advancing post 4k. Stock setting is 26 degrees max mechanical. Ideas?



69 distributor

 

It continues to advance past 4k because you appear to have the stock spring which hits maximum advance at 5k. See Gm Archive information below Electrical section.
The second picture looks like a home made bushing.
You need a spring kit with a bronze replacement bushing.
Try the silver and black springs to start with, 928 Mr Gasket

https://www.gmheritagecenter.com/doc...t-Corvette.pdf

 

think your saying the stiff spring are delaying the mechanical advance. Makes sense.
I have the spring kit. Also the 28b vac can. Tried both per Duke, but I think my carb had issues. Had it rebuilt and installed. Ready to try again.
I've been trying to get someone to I d the cam weight number. The 540 on mine is incorrect since 26 degrees is the spec. Yet to get an answer. The 69 in my post pic has a 536 which seems to be all I'm seeing. Like to sort all this out before I start the changes again.
you've been very helpful thank you

 

From what I understand GM had dozens of different weights, as well as cams The upper "cam" which holds the weights, and has the slot in it, also has number, it's upside down on under the plate for the weights. It's last 2 digits of the number designates the amount of mechanical advance, more than the weights, it's based on the length of the slot.

You are very fortunate. Not only for having a 70 LT-1 (congrats!) but for having a very short "slot". Most people have to weld theirs up to achieve this, you do not! I went thru a lot of effort to "tighten" up my slot from about 0.140" to 0.095" .You should really do a "performance tune-up" on the distributor. It will really wake the car up and make it roar! Good gas dependent of course.

Lars has great info on setting up the distributor curve.
I set my own up in my home garage as shown here: https://www.corvetteforum.com/forums...ing-chart.html

My 70 LT-1 that I drove hard for 27 years, loved initial timing. I know I ran 36 total at 3500, and IIRC inital was around 12-15? My vac can was limited to 10 degrees and it ran on manifold vacuum, not ported. It gained a lot of TQ below 3000 rpm with those settings, vs factory, and then it roared once past 3500. With true 11:1 CR I did have to pull it back 4-6 degrees to get it to run on 93 pump gas. That took like 30HP out of it. But it ran fine. The "track-tune" was never more than 5 gallons of race-gas away.

 

Leigh. Your saying the slot on my installed distributor appears to have been welded? The number on it is 540 which had me concerned. I does seem smaller the the slot on the 69 pic.The rest of your info is exactly what I was looking for. Have read Dukes. Lars, John H papers and had Duke tirelessly explain to me. You still have your LT1?
Thanks for all your input.

 

DUH. Just realized you have a T.I. distributor. the 540 cam number obvioulsy is not 40 degrees. it looks very small in fact. Can you measure the slot or gap?

Look underneath for the cam number: 530 or 722 etc. 30 degrees or 22 degrees. I have two now, one of each.

This is the 530 cam. It had 30 degrees advance and this much open space in the slot.

530

No I let the LT-1 go after 27 years. It was in a Z28 anyway. When I got the Vette after a 10 year "car" hiatus I decided to go bigger, LS6!

 

This says you should have a 1491 T.I. distributor and it should give 26 engine degrees at 5000 engine rpm.
If so your gap should look like my 530 cam.
Can you measure that gap with a dial caliper? My math says your gap should be 0.120" to give you 26 degrees. That and 8 initial is 34 total. Not bad just comes in at really high revs due to the stock springs.

1983 GM Parts Book

from Dave Fiedler at T.I. Specialties

 

Hey Leigh. Just realized you made that cardboard distributor measuring tool. Genius! Did you attach it to a assembled distributor as well as just the shaft and cam. How did you rig it up to distributor?
Yes I have a 1111491 distributor. The cam weight in is 540. I cant find a 526 to fit the spec of my distributor. The GM part number for the cam is GM #1967928 That has a 530 stamping. Don't get it?????

 

Yes I cannot figure what's going on with that number either. It is even on the flip side compared to my two cams. But you have TI and mine are points, that must be the reason.
The fact that the centrifugal advance comes in very late is because of the stiff factory springs. Even if you want to keep it 100% stock, you don't want to rev it to 5000rpm + to set the timing!!
I'd suggest use the two weakest springs from an advance curve kit, and then you can set the timing at a more reasonable 3000 rpm. Change the springs back when you are done. They should not have any other effect except for the rpm.

Now if you do that and find you do not get 26 degrees mechanical advance when the distributor moves, you may have to build your own distributor degree wheel like mine and figure out what is going on.
Glad you liked it!
It is actually attached near the tach drive, just has vertical supports to move it up over the rotating parts. I just kept adding cardboard and tape until it was stable. The copper wire pointer is wrapped around the two advance weight pins so it moves. All I turned was the gear, and the wire pointer moved smoothly. Modified my total mechanical advance and my vacuum advance. It worked very well. Especially for cardboard LOL. I could read it to 0.5 degrees consistently.

 

Ok made my timing tool. I rigged the wire around the 2 mainshaft pins. Help the weight cam and here's what I got. Btw this was using the spare 69 491 with 536 stamping.
figure I can rig this up to the 491 on the engine and move the cam weight instead of the mainshaft for obvious reasons

 

Wow that looks familiar!

If I read that right it looks like you have exactly 10 degrees advance (distributor) which doubles in the engine to 20 degrees (crankshaft)

I never even thought about using this setup while the distributor is installed in the car, but I guess you could, that's a good idea! As long as you can reach & rotate the right part without making everything "wiggle".

That's a good distributor curve. Mine started with almost 30 degrees.

 

Keep in mind that reading is from the spare 491. If you go back up to my 1st pic post, it's the one with the 536 stamping. Looking at the slot from the underside it doesnt appear to have been welded.
So if that "536" means 36 degrees is that rubber bushing what is shortening the degree read?

 

Nah. I don't think any distributor ever had 36 degrees of advance in it, so that number thing must not work all the time, too many exceptions.
The bushing on your TI looks original it was hard nylon? plastic and looks like you still have a retainer clip. Not so much the 69 one.

 

OK, I rigged it up on the car. Wasn't difficult. Here's the readings. Looks like 15 degrees, so 30 on the crank right? Also looking at my first pic of the underside close up of the car distributor, there seems to barely be any room there at all.

 

Cool! So the factory setting of 26 turned into 30 degrees advance after all these years.
So now you can just set your initial timing and you will know what the total really is.
6 degrees initial would give you 36 total etc.
Now the real question is how much timing will that 11:1 CR LT-1 really like without pinging on today's premium?
I needed 5 gal of race track 113 octane to run 36-38 total in mine.
I would guess no more than 32 total on pump gas. Just going by what mine liked.
For you 2 initial.
Are you sure you want to keep this 100% stock?
A performance curve in the distributor would help it run better at mid-range. Just sayin'

 

I had been running 112 leaded race fuel. Haven't been there lately so 93 ultra.
Runs good . Idles like crap. All the tinkering has a lot of unburned fuel ,(smoke),out the exhaust .
When I got the car timing was 6 degrees. Manifold vacuum went to the choke. Someone drilled a port vacuum and ran a line to distributor. Idle was crap, DC isnt run all that well with a slight light stumble on acceleration.
think I might change out distributors to see what effect that might have.

 

Yeah you should try a performance curve. Your idle troubles could be many, low advance, big cam, low vac, current gasoline, bad TI module, mis-adjusted carb, plugged carb orifice, etc. etc. (adjustable idle air bleeds too!)
Maybe try a 2nd distrib, that's a good idea, 'cause you don't want to mess with the original TI one, I get it.
I'd suggest near 18 degrees initial, with another 10 degrees on vacuum, with the vacuum on manifold. That would give you 28 initial at idle and it would really like that.
The vacuum advance has to be pretty much cut in half to do this, with a vacuum advance limiter, it just screws on. $6
But to run 18 initial you need to weld up the slot in the mechanical advance to cut that 30 degrees mechanical back to about 18 degrees. That's a pretty serious/permanent distrib mod. Then you could run 36 total. With 5 gal of really good octane (you shouldn't need a whole tank) it'll run terrific.
An electronic MSD distrib would make this really easy, or maybe you could just borrow a tuned one to try it.
I pulled the timing back 4 degrees to run pump gas (32 total) and bumped it back up when I had access to Cam 2.That alone made a 30HP difference. Ran it for 25 years that way. The last 8 years I didn't Autocross as much so I didn't ever put the Cam 2 in it.

That engine is Spotless and Beautiful! Keep it running like it should.