70 LT1 ti distributor mainshaft comparison

[Dennis D.]

Visual opinions on these 2 LT1 distributor shafts. Is that the correct ID#
20191023_185339.jpg20191023_185440.jpg

[Duke W.]

Re: 70 LT1 ti distributor mainshaft comparison

I see what looks like "83" on the "football" in the left photo, but it's pretty grainy and no identifying characters in the right photo.

Note that with the centrifugal fully retracted the weights don't return to the football, but contact each other. This is the way some distributors limit advance. Also, the springs in the right photo look aftermarket... not sure about those in the left photo.

Look in your CSM or AMA specs. What are the '70 LT-1 centrifugal specs?

Duke

[Dennis D.]

Re: 70 LT1 ti distributor mainshaft comparison

Thanks Duke.
I increase rpm to bring in all mechanical of 26* plus initial. Vacuum blocked.(Comes in quick with the black springs). What seems wrong is if I keep increasing rpm the advance increases with it. Shouldn't it all be in already.
Heres a spec sheet on the distributor

[Duke W.]

Re: 70 LT1 ti distributor mainshaft comparison

I can't make any sense out of your statement, but those specs are in DISTRIBUTOR degrees and RPM. Converting to engine rpm and degrees they are:

0 @ 1000, 26 @ 5000 (very lazy)
0 @ 7", 15 @ 12"

The black springs should bring full centrifugal in at much less than 5000. Use a dial back timing light to find the point of maximum centrifugal , and report the number. Rev a few hundred above this new point of max centrifugal and set total WOT in the 36-40 range VAC disabled), as high as the engine will tolerate.

Since typical LT-1 idle behavior is 900 @12" The OE VAC does not pass the Two-Inch Rule. It's not important if the OE ported vacuum advance is retained, but I recommend converting to full time and installing an 8" B28.

Duke

[Dennis D.]

Re: 70 LT1 ti distributor mainshaft comparison

I can't make any sense out of your statement,
Duke

Sorry. Tried to keep is uncomplicated.
My question is, ( and it may be a dumb one), when initial plus mechanical advance total degrees are reached at a particular rpm ,as seen at the timing mark with a dial back timing light.
Let's call that 38* @3400rpm for argument sake. If rpm is then increased to 4000 ,will advance increase too as viewed on the db timing light. If yes, how since advance effects are maxed out?
Of course vacuum advance is blocked off.

[Duke W.]

Re: 70 LT1 ti distributor mainshaft comparison

Max centrifugal is just that - maximum. Advance increases with revs until it stops increasing and that is the maximum. If it "keeps advancing" it has not yet reached maximum. Above maximum it will remain constant, and I always recommend blipping the throttle when you THINK it's at maximum to ensure that there is no more additional advance.

Some curves are non-linear and add the last two or three degrees over several hundred RPM, so that's something to watch for.

Duke

[Dennis D.]

Re: 70 LT1 ti distributor mainshaft comparison

Now if the advance increases well past the established example in my previous post, would the distributor be defective or had been altered?

[Mark M.]

Re: 70 LT1 ti distributor mainshaft comparison

Dennis is the distributor the original 1111971 and has it ever been worked on? If it is original I'm curious as to what the number stamped under the pole piece is. Examples are ccw 730 to ccw736 would be likely for that engine. That pole piece has a slot of a size that correlates to the number stamped on it. ccw 730 would give 30 degrees. The pin that sticks down through that slot from the advance cam on top (the football), should have a rubber collar on it. If it's missing which happens, the pole piece would over advance more than it should. You should have enough room to see it with a small mirror if it's still in the car. Also is the vacuum can a 360?

[Dennis D.]

Re: 70 LT1 ti distributor mainshaft comparison

Thanks Mark.
Here's a pic I took today. I put everything back to stock to try to get some sort of baseline.TCS, timing at 8*. Swapped out the wrong vacuum can for the correct 201 15. The distributor is the correct 1111491.

[Timothy B.]

Re: 70 LT1 ti distributor mainshaft comparison

Dennis,

Some factory centrifugal curves advance the last few degrees at higher rpm's and I think it's by design so nothing is wrong with your distributor.

What I do is remopve the small springs from the weights and time the engine for total advance. Get a dial back timing light set at 36* and remove the springs then run the engine to approx 1500rpm's and set the total timing. When you are finished, install the springs and check the initial timing with your light for future reference.

You can install the brass bushing that comes in the aftermarket advance kits first as this will stay on the pin etter than the factory (soft) bushing.

[Duke W.]

Re: 70 LT1 ti distributor mainshaft comparison

Now if the advance increases well past the established example in my previous post, would the distributor be defective or had been altered?

All I can comment on is the actual centrifugal curve. So get out your dial back timing light, determine the start point with the black springs, then report advance in 500 RPM increments up to the maximum. Once you believe you have determined the maximum, blip the throttle to at least 5000 RPM to verify that it no longer advances.

Duke

[Dennis D.]

Re: 70 LT1 ti distributor mainshaft comparison

Thanks Tim. From Mark's comment , is it correct that the "540" indicates 40* mechanical advance.? That would seem to leave a lot of advance not used if set to 36* mechanical and initial.

I will try your method, if results are encouraging, may reinstall the b28 advance to see if that helps.

I should mention I've posted many results of different checks over the past few months. Lastly, compression and leak down with good results. Engine runs well driven at high rpms. Idle is rough with stubble of idle.
Set back to stock isnt helping. Getting heavy fuel smell with light smoke out the exhaust. Assume unburnt fuel from lack of advance.

U tube video of me moving the advance cam manually... https://youtu.be/3ECXeY1b9AA

[Timothy B.]

Re: 70 LT1 ti distributor mainshaft comparison

Dennis,

Mark's comments above mention a 730 or 736 weight base/cam, something with a 40* slot timed at 36* would have a initial timing of 4*ATDC if I'm thinking right, not what you want.

The weight base cam in your distributor should read something like 526 or 726, I am not sure what the first # means.

I think your car has a ported vacuum advance so set the timing at 36-38* and check where the initial falls and post back. Try the stock vacuum advance control and only change one thing at a time. How do the spark plugs look, can you control the idle A/F with the side emulsion screws?

When was the last time there was any carburetor work done, you could have some fuel leaking internally. The Holley's are good carburetors but if the surfaces are not flat they will drive you nuts.

[Dennis D.]

Re: 70 LT1 ti distributor mainshaft comparison

Tim
Carb uses manifold vacuum. It's in very good condition and was rebuilt by a well know west coast place I'm told. It does have the wrong base plate and was hooked up to ported vacuum. I have a nearly new replacement from holley I'll be switching out to eliminate that as well.
Spoke to a TI rebuilder and verified the "540" is in fact 40*, so there seems to be an issue there.
I've posted pics of the plugs in recent threads as well as compression and leak down. Heres a set with a very little miles I took out.
20191006_142731.jpg

[Mark M.]

Re: 70 LT1 ti distributor mainshaft comparison

Dennis my 1974 GM parts book lists 1967928 pole piece as the 70 LT-1 part. NOS examples are 736. I've seen a few GM service pole pieces vary from known original advance sized slots. Not much 2-4 degrees. You would have to get an original example to see if the 540 was correct. GM did have aggressive advance timing on some of their SP HI PO engines which the 70 LT-1 falls into. Is the small bushing in place on the pin? By 71 GM changed the LT-1 distributor with the 1964309 pole piece NOS examples showing 730 on them. As mentioned I would try total advance to 36* were it stops advancing and see what initial ends up. Vacuum blocked. If that engine has its hi compression pistons it will need the right fuel.