70 LT-1 Ignition Timing

[Dave S.]

I'm in need of some assistance in clarifying what the timing for my 70 LT-1 should be. All but one original emissions sticker I have seen on 70 LT-1's, indicate the advance should be 8 degrees. The owners manual indicates it should be 14 degrees. The 70-72 TIM & JG refers to an emissions sticker that says 8 degrees is correct.

When I set the timing to 8 degrees from the 14 degrees I have previously used, I got some backfiring through the exhaust while decellerating. The car runs fine at 14 degrees advanced but that seems like an awful lot of advance. Is that motor enough of a hybrid to warrant that much advance.???? Is the Emissions sticker correct even though the Owners manual says differently. ?????

In my case I will use the 14 degrees but are some 70 LT-1 owners using an improper timing guideline, thus effecting their cars performance.???? Is the judging information correct. ????

Any thoughts are appreciated.

[Joe L.]

Re: 70 LT-1 Ignition Timing

Dave------

The emissions sticker is correct; the initial timing setting for the 1970 LT-1 should be 8 degrees BTDC. Somehow, the specification of 14 degrees got into the 1970 owner's manual and the 1970 Chevrolet Service Manual. However, if the engine runs well at 14 degrees, that's what I'd use.

[Duke W.]

Re: 70 LT-1 Ignition Timing

The initial timing is critical to emissions performance, but not for overall performance and fuel economy. What is the total centrifugal advance?

The sum of initial plus total centrifugal should be about 38 degrees. If it runs wells at 14 and you don't get any detonation, then 14 is probably better overall than 8.

Duke

[Chas Kingston]

Re: 70 LT-1 Ignition Timing

I ran mine at 14ºBTDC, but retarded it to 8º when I had it inspected.

Ol' Geezer

[Terry M.]

Re: 70 LT-1 Ignition Timing

Dave,
The reference in the TIM&JG is that the sticker with 8 degrees on it is a correct sticker -- NOT that it is the correct initial timing setting. This issue (1970 LT1 original initial timing) was hashed out in an old article in The Restorer back in the late '80s. That article cited a laundry list of sources, all of which had the same confusion between 8 and 14 initial setting.
Since that article appeared -- I'll let you in on the secret method, but you have to promise not to tell anyone else.
Since I know you have the original distributor and intake in place (this only works for the originally installed pieces), take a small mirror like we use for judging and look at the back of the distributor where it meets the intake. You will see a chisel mark that bridges the distributor and the intake. Set the marks to line up and you have the exact setting that was made on the hot test stand in Flint in 1970. No need to run the engine; you can set it cold. You can also now put back the distributor shielding you might have had to remove to see the mark. Now all you need is to find gasoline that is the equivalent of 1970 fuel -- fat chance.
FWIW: when I set mine this way it runs like the proverbial assaulted ape. A dial back timing light shows 14 degrees initial. I do run a bit of 110 octane low lead per tank to help the engine a mite, and I have never taken it for a smog test. I have run it on a five gas tester a couple of times, and it does fine -- even with the 110 low lead. One of these days when the IM 240 class gets to play at the testing station I'll bring it and run it on the rollers to see how it does in a real test.
And now all the rest of you can dash out and look at the same area of your engine. I am told that the UAW folks who set the initial timing did this for both large and small motors “forever.” Now y’all know another one of the items that Bowtie judges look for.

[Duke W.]

Re: 70 LT-1 Ignition Timing

Terry - as the contact surface and rubbing block wear the timing will change slightly, so if you align the chisel marks to check on the original plant set timing, make sure it has new points!

The 8 degree value was probably established for emissions compliance, but increasing the initial to the point where it plus the max centrifugal equals 38 will improve performance and fuel mileage as long as it doesn't detonate.

Duke

[Dave S.]

Re: 70 LT-1 Ignition Timing

Duke,

At 14 degrees initial I get about 38 or 39 degrees total. Seems like all is well there.

[Dave S.]

Re: 70 LT-1 Ignition Timing

Terry,
Thanks for the information and great response. Another story where we continue to learn interesting things about these cars every day. I looked closely at my distributor flange and the intake manifold. I can see a light file mark about 3/4' long on the intake but can't seem to see a file mark on the distributor. I do see two red dots, each about the size of a pin head, on top of the distributor flange which are close to aligning with the file mark on the intake. Could that be the technique used.?????? I'll have you look at it in Cape May and see if you can see something I don't see.

In any event I set the initial timing at 14 degrees and I'm getting about 38 degrees total. Car seems to run strong.

[Terry M.]

Re: 70 LT-1 Ignition Timing

Dave,
With my eyesight there is not a chance in the world I will see anything you don't, but I'll be pleased to look. The instructions (they are in the 1970 AIM) call for a chisel mark, but we know about those UAW guys.

[Terry M.]

Re: 70 LT-1 Ignition Timing

No points for me and the other 1970 & 1971 LT1 owners Duke, but your point (pun intended ) is valid nonetheless. Timing chain wear is, I am sure, another issue. All I can tell you is that my engine likes 14 degrees initial advance. When it's happy, so am I. I still have to try the Williams/Hinckley valve adjustment method. I believe that will make it even happier.