1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

[Duke W.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

I've never personally done it, but I know it's a frequent practice with racers, and it's also common on stationary industrial engines (usually based on automotive engines) that run at fairly constant speed and load.

The problem is starting the engine, as it may want to run backwards.

Welding the breaker plate to the distributor housing has been done since at least the sixties if not before. The points actually place a pretty high bending load on the plate, and over the years, rotation of the plate by the VAC can cause wear. The result is a wobbly breaker plate, which can cause point bounce and significant timing variation, if not complete ignition breakup.

I forgot what distributor configuration you are running on your vintage race engine. It probably makes sense to weld the breaker plate to the dist. housing, but I think you should retain the centrifugal advance getting it all in by about 3000.

Why do you think your distributor is overadvancing?

Duke

[Jerry G.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

It's an aftermarket breakerless conversion. i have welded the plate vacuum rod. i don't have any indication of overadvancing, but as you know there have been teething problems with going back to vintage standards for the race motor.i've hung up the exhaust valves on #5 and #7 twice now.Current thinking is lean A/F but i'm trying to eliminate as many variables as possible.
Apparently the starting trick is to turn the engine over before turning on the ignition. for now i think i'll leave this modification alone and concentrate on the air meter testing.

[Duke W.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

The electronic switch doesn't put any bending load on the breaker plate - nothing compared to point's mechanical spring, and the high breaker arm tension points load the breaker plate much more than standard tension.

Welding the VAC rod should stabilize the breaker plate as long as it fits snuggly on the dist. housing and the VAC rod hole is not worn allowing the breaker plate to wiggle more than about a half-degree, which is one degree at the crank.

Duke

[Stuart F.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

Guess I'm probably wasting a good "Dyna-Flyte" plate using it with the Pertronix II Electronic unit in my 63 L-76, but I like the certainty of knowing I'm controlling the Vacuum Advance action spot on. I've gone through the experience of tipping plates on both an early style (55) distributor and a (57) model. Each one drove me nuts trying to find the problem. The 55 was my first Chevy small block in my 50 Chevy coupe (1956). It would accelerate like a bandit to a certain point then crap out big time. After much trial and error with fuel pumps, spark plugs and wires, carburetors, etc., I finally disconnected the vacuum advance and found the problem. Fixed that with a new Corvette dual point distributor for about $25. back then. I stayed away from vacuum advance units until I bought the 63. Didn't like it then until recently when I changed to the B28 and found religion.

Stu Fox

[Timothy B.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

Jerry,

Here is a pic of the Breakerless SE installed on my 67 distributor. The mainshaft spins like there are no points, I am very happy with this conversion and other than factory side pipes it's the only non stock (born with) item on the car.

https://www.forums.ncrs.org/showthread.php?t=69728

[Duke W.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

Guess I'm probably wasting a good "Dyna-Flyte" plate using it with the Pertronix II Electronic unit in my 63 L-76, but I like the certainty of knowing I'm controlling the Vacuum Advance action spot on.

What's that?

A few years ago I ran across a guy who had a NOS ball bearing breaker plate in the box. I think it was Niehoff. I'd never seen one before, but I wish I found out about them decades ago.

I'm not aware that new breaker plates are available from any source.

Duke

[Stuart F.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

Duke;

I have one more "out of the box". It's mfg. is as I said; Dyna-Flyte #956. It has a ball bearing diameter of 2", and consists of two plates riveted together with the bearing pressed/encapsulated into the cup of the bottom plate which engages the distributor hub in a press fit, typical of the method used on a standard plate. I had to modify the one I have in my distributor for fitting the Pertronix sensor as the locating nubs for the points would not allow the base of the sensor to sit flat on the plate. The one I have "out of the box" I am leaving as is in case I want to go back to points in a future life (I guess).

The action I've seen on a distributor machine is very smooth and positive, with no reason to believe it will be any less so in the future.

Stu Fox

[Wolf S.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

what do you think is best for a lt-1 engine -- full advance in at 2500rpm or 3000rpm?


happy new year

[Wolf S.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

hi,
today i connected the vacuum hose from the distributer b22 vac to
manifold vacuum at the base of the holley carb.

i warmed up the engine and found that i now have a steady idle of 1500rpm.
before with the vac hose to ported vacuum it was at about 900pm.

i suppose that the vac is now fonctionning at idle which it did not
before when the hose was connected to port vacuum at the top side
of the carb.

should i just reset (with the idle speed screw) the idle to 900-1000
and continue driving?
or definitely replace the vac b22 by the b28 as was suggested?

thanks for the input.
merry xmas

[Duke W.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

Total idle timing for Corvette engines, which is the sum of full vacuum advance, initial timing, and maybe a degree or two of centrifugal if the curve starts below idle speed, should be in the range of low twenties to low thirties for lowest idle fuel consumption. High overlap cams need to be in the top part of this range.

But emission requirements lowered total idle timing with ported spark advance to increase EGT to facilitate HC and CO oxidation in the exhaust system.

The 15" B22 DOES NOT pass the Two-Inch Rule on the LT-1 engine, so if an LT-1 is converted to full time advance it must also be equipped with a VAC that passes the Two-Inch Rule, which means the 8" B28.

Before you proceed any further, google my name along with San Diego and Corvette to get the pdf of my presentation, which tells you what you need to know to set up the spark advance map for various Corvette engines, both pre-emission, and emission era.

1. Install a proper VAC

2. Install the lightest centrifugal springs you have

3. Set the idle mixture screws at the initial setting called out in the CSM.

4. With the VAC disconnected and the signal line plugged use a dial back timing light to set the advance to 38 degrees at a few hundred revs higher than the point that the centrifugal stops advancing.

5. Let the engine back to idle and go as low as you can on the brink of stalling. Check timing. Now increase engine speed and document where the centrifugal starts advancing. It could be as low as 600-700.

6. Connect vacuum advance, set engine idle to 900. Go through the idle speed mixture adjustment procedure for non-emission controlled engines.

7. Test drive. If it doesn't detonate, you're done. If it does detonate, the centrifugal must be slowed with heavier springs or total WOT advance/initial advance can be lowered up to four degrees or a combination of both.

Your high compression LT-1 should accept a fairly aggressive spark advance map with 98 RON European gasoline.

I don't recall if you ever told us what the centrifugal curve is from the AMA specs or CSM, but if the total is less than 24, other work needs to be done.

So what are OE the centrifugal specs?

Duke

[Terry M.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

Duke,
From the Delco green book (I don't have the CSM in the house and it is too cold to go to the garage for it -- no wise cracks) all for 1970 LT1:

111491 dist # centrifugal
575 rpm = 0-2 deg
850 rpm = 4-6 deg
2500 rem = 12-14 deg
3000 rpm = 11.3-14 deg footnote (c)

(c) After maximum centrifugal advance is reached and distributor speed is increased above that needed to obtain maximum centrifugal advance, it is characteristic of this unit for centrifugal advance to decrease approximately .5 degree for each increase of 500 distributor rpm

1116201 vac unit
inches Hg to start advance = 7-9
inches Hg for maximum advance = 15-16
Maximum distributor advance in degrees = 8

Merry Christmas

[Duke W.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

Those are distributor degrees, so you have to double them to get crank degrees. The spec has a wide tolerance range, so you need to test your engine to see what the maximum centrifugal of your distributor actually is. Install the lightest springs (probably gold) and rev the engine until it stops advancing with the VAC disconnected and hose plugged. Set it to 40 with a dial back timing light. This will compensate for the TI that retards as revs increase. That's what the footnote is about.

With the lightest springs I expect the centrifugal will be all in by 2200-2700, which is the correct range

Then you will have to get the engine below 575 to actually read initial, which is tough as the engine might stall. You can tie up the centrifugal with a rubber band and read the initial at a higher speed that will yield a stable idle.

The 201 VAC will not work properly with full time advance. It needs a B28, period.

Did you download, read, and absorb my San Diego presentation?

Also, look at the last week's postings. I did a blueprint overhaul of a L-79 distributor and set up the spark advance map to the recommendations in said presentation, and documented the job in a thread a week ago.

Find it and read it.

Duke

[Tim S.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

Duke,

Facinating presentation! Great information in this thread that applies to most SHP engines.

All the best!

Tim

[Duke W.]

Re: 1970 LT-1 Valve Lash, Idle and Idle Mixture Setting

It actually applies in general to all engines. The presentation explains why base cam engines need a somewhat different spark advance map setup than SHP engines, and suggested starting points for all OE engine families are there. From those starting points owners are free further fine tune the spark advance and fuel maps for their specific engine and driving conditions using the suggested test and evaluations.

Duke

[Wolf S.]