70 LT-1 tuning checks

[Dennis D.]

Had my LT-1 in for rear diff fluid change, and some tuning. Havent driven much since, but yesterday was a beautiful day, so out it went.

Fired right up, ran quite well. With one exception. Backing off in gear would create popping. My first reaction would be lean condition. Checked mixture settings and found 1- 1/2 turns out on passenger side and 1/2 turn out on driver side. Made adjustments twice. First time, 1 turn out on both. Better but still popping. Then 1-1/2 out on both. Better again, but getting little smoke out exhaust. Put back to initial for the day.

Decided to get back at it today checking some basics. Primary float level was low. Seconary level ok.

Timing checked in at 35- 36 degrees with advance. 15-16 degrees no advance, hose plugged. (Engine wanted to die)
17 degrees, no advance, hose not plugged. Idles was a tad smoother than with advance.(strange). (28 v Can)Vacuum steady at 12.5 " with advance. Under 10" no advance. Idles 900-940 rpm.(advance). 850 (no advance), adv hose not plugged.
.
Always interested in opinions.

[James G.]

Re: 70 LT-1 tuning checks

Did you remove your fuel cap from the tank when performing the operations?

Regarding your 35-36deg adv is that TOTAL distributor advance (vac + mechanical) or TOTAL ignition advance (Vac + Mechanical + initial)?
You say under 10" with no advance - is that with the adv hose plugged or open?
as I would expect the vacuum readings to be 2"+ lower if the hose were unplugged.

[Dennis D.]

Re: 70 LT-1 tuning checks

Did you remove your fuel cap from the tank when performing the operations?

Regarding your 35-36deg adv is that TOTAL distributor advance (vac + mechanical) or TOTAL ignition advance (Vac + Mechanical + initial)?
You say under 10" with no advance - is that with the adv hose plugged or open?
as I would expect the vacuum readings to be 2"+ lower if the hose were unplugged.

Total= initial +vacuum+mechanical
10" with vacuum disconnected/ unplugged.

[Terry M.]

Re: 70 LT-1 tuning checks

I like to find the chisel mark made when the initial timing was set at Flint Engine. Look at the back of the distributor and you should find a mark bridging the distributor and intake manifold. Align mark and timing is set. This only works with the original distributor and intake manifold. There may also be issues if the timing gear and chain set was replaced depending on how they were installed.

[Dennis D.]

Re: 70 LT-1 tuning checks

I like to find the chisel mark made when the initial timing was set at Flint Engine. Look at the back of the distributor and you should find a mark bridging the distributor and intake manifold. Align mark and timing is set. This only works with the original distributor and intake manifold. There may also be issues if the timing gear and chain set was replaced depending on how they were installed.

My 70-L-46 had the stake mark. Have to check the LT-1 for it.

[Duke W.]

Re: 70 LT-1 tuning checks

Timing checked in at 35- 36 degrees with advance. 15-16 degrees no advance, hose plugged. (Engine wanted to die)
17 degrees, no advance, hose not plugged. Idles was a tad smoother than with advance.(strange). (28 v Can)Vacuum steady at 12.5 " with advance. Under 10" no advance. Idles 900-940 rpm.(advance). 850 (no advance), adv hose not plugged.
.
Always interested in opinions.

I can't make much sense out of the above, but we've got to start talking on the same frequency.

There are THREE "total advance" numbers, and it's really the first two that are important.

TOTAL WOT ADVANCE: This is initial plus maximum centrifugal. The VAC hose MUST be disconnected and plugged for this test and the engine must be revved a few hundred RPM above the speed of max centrifugal THAT YOU MUST DETERMINE BY TESTING. You can use the centrifugal specs in the service manual and AMA specs as a reference, but they might have been modified over the years.

OE total centrifugal advance comes in as low as 2350 and as high as 5100 on some engines. That's why you have to test your specific engine to determine.

The '70 LT-1 OE spark advance map was determined by EMISSION CONTROL REQUIREMENTS, not performance of fuel economy. Optimum would be the 365/375 HP 327 spark advance map, with full time vacuum advance, of course, not the OE LT-1 ported.

TOTAL IDLE ADVANCE: is initial plus full vacuum plus maybe 2-3 degrees centrifugal if the centrifugal starts below idle speed, which is the case for the 365/375 HP engines. Centrifugal starts at 700, but they won't idle stably much below about 900. Best total idle advance is low 20s to low 30s, low overlap cam engines at the lower end of the range and high overlap cam engines at the upper end.

Total cruse advance is initial plus full vacuum plus full centrifugal. A 365 HP engine with a 3.70 axle in top gear is turning right about 2350, the point of max centrifugal, at 50 MPH in top gear and manifold vacuum is more than enough to keep the VAC pulled to the limit for an additional 16 degrees. So if you set total WOT advance at 38, total cruise advance at typical highway cruise speed should be about 54.

A 300 HP engine will be less because of lower revs and a centrifugal advance that not all in until 4600 to 5100.

If you set total WOT advance in the best power range of 36-40, as high as you can go without the engine detonating you don't have to worry about "initial advance". Forget about it! Get the total WOT advance right and get the total idle advance right with the correct VAC that passes the Two-Inch Rule and you're done!

And PLEASE... when you talk about "total advance", specify whether you mean.

Total WOT advance
Total idle advance
or
Total cruise advance, which is just a FYI value. Get the first two right and total cruise advance will be okay.

Duke

[Terry M.]

Re: 70 LT-1 tuning checks

My 70-L-46 had the stake mark. Have to check the LT-1 for it.

It is not always easy to see. A mirror and bright light are helpful. Once you locate where is it will be easier to locate later.

[Dennis D.]

Re: 70 LT-1 tuning checks

Thought I was keeping it simple. Sorry about that. Talking strictly to idle advance as it might relate to the popping issue.
Intial 15 degrees,(advance plugged), and 35 degrees, (advance connected), seemed alot of advance increase with the b28 advance can. Didnt verify the distributor mechanical advance spec yet, but what is the max for the b28?

Also, my 70 LT-1 used manifold vacuum.

[Duke W.]

Re: 70 LT-1 tuning checks

The B28 VAC should provide a nominal 16 degrees crank advance at 8" and over manifold vacuum. Do you have a set of light springs in the centrifugal? It's possible that, like the 365/375 HP 327s the centrifugal advance starts at a lower speed (700) than what you find to be the minimum acceptable idle speed, like 900, thus adding 2-4 degrees of centrifugal at idle speed.

Typical 365 HP best set up is total WOT advance, 38, and since max centrifugal is 24 this implies 14 initial. Measure total idle advance at 900 and it's likely in the range of 32-34, which implies 2-4 centrifugal is already in at idle speed, and that's okay. High overlap cam engines like lots of total idle advance because the high valve overlap causes a lot of exhaust gas dilution at idle, which slows combustion propagation, so the fire needs to be lit early for minimum idle fuel consumption and lowest EGT.

Afterfire on trailing throttle is likely a lean idle mixture. Go through the non-emission control idle speed/mixture adjustment procedure using the initial position for the mixture screws stated in the service manual. Target a 900 idle speed and find the number of turns out that yield the highest vacuum and idle speed.

It's an iterative procedure. If idle speed increases with a mixture screw change, set it back to 900 with the idle speed screw, and readjust the idle mixture screws looking for more improvement. I use 1/4 turn in (leaner) or out (richer) and maybe 1/8 turn when I think I'm close to optimum until I find the sweet spot, and each idle mixture screw should be out an equal number of turns from the seat when you're satisfied that it's optimized.


Duke

[Dennis D.]

Re: 70 LT-1 tuning checks

Ok then. If I'm reading right, my 35-36 degree at 920ish rpm is in you range if I get the idle down a tad. I'll keep on tinkering. Thanks all.
Terry, got me curious, hope to find the stake marks. My L-46 was practically dead center on the intake.

[Timothy B.]

Re: 70 LT-1 tuning checks

Thought I was keeping it simple. Sorry about that. Talking strictly to idle advance as it might relate to the popping issue.
Intial 15 degrees,(advance plugged), and 35 degrees, (advance connected), seemed alot of advance increase with the b28 advance can. Didnt verify the distributor mechanical advance spec yet, but what is the max for the b28?

Also, my 70 LT-1 used manifold vacuum.

Dennis,

Another contributing factor to the after fire on deceleration could be to much initial advance, this will make the engine very efficient and raise idle speed. When you slow the idle back down you will close the transfer slot opening and further reduce fuel flow to the engine then when you coast with your foot off the throttle the mixture could go lean as Duke suggest.

The best thing I would recommend is to start with the factory settings, timing, carburetor etc. and see how the engine runs.

[Dennis D.]

Re: 70 LT-1 tuning checks

Tim
I've been fanatical about maintaining the transfer slot reveal. I haven't touched the idle since I put the carb back on. Never thought the shop might have messed with it. Thanks

[Timothy B.]

Re: 70 LT-1 tuning checks

Sounds like you have a good handle on it, Dennis. How many turns out are the side screws when it's idling at the factory setting.

[Dennis D.]

Re: 70 LT-1 tuning checks

When I first checked the right mixture screw was 1 1/2 out and the left was a 1/2 turn out. This was what the shop set. Ran well, just starting getting the decel popping. I did re-adjust twice. First ,1 turn out less popping. The 1 1/2 out. Better again still popping. Notice slight smoke out the exhaust after that adjustment.