1971 LT-1 Valve settings?

[Duke W.]

Re: 1971 LT-1 Valve settings?

Per the 2008 revision of the article referenced in a previous post I recommend .016/.023". That's it. Set them there and make a note to check them in 15K miles unless you have a nut that doesn't have much preload, in which case I would recommend replacing it.

At .034/.040" there must have been a hell of a lot of valve train clatter. At .016/.023" most guys would think it has hydraulic lifters, which is the way it should be. Properly set valve clearance on mechanical lifter cams should not be noisy.

Duke

[Wolf S.]

[Duke W.]

Re: 1971 LT-1 Valve settings?

After setting the lash you must go through the idle speed/mixture adjustment procedure. Set the idle at 900, adjust the mixture screws to achieve maximum engine speed/vacuum, then set the idle back to 900 again and adjust the mixture again. Repeat this process until it's dialed in.

Chevrolet's recommended idle speeds for mechanical lifter cams are way too low to achieve reasonable idle quality. I recommend 900 for all and add 100-200 for Fuel Injection with the 30-30 cam.

Set the idle speed using a known accurate test tach unless your indash tach has been recently calibrated and is known to be accurate to within a few percent in the idle speed range. Most are not!

Since LT-1s have ported vacuum advance I recommend changing it to full time, which requires that you swap the OE VAC for a B28.

Since the tighter lash specs increase effective overlap, especially in your case where the previous lash was way over even the "loose" OE recommendation, there will be a dramatic effect on idle behavior until it's readjusted.

The engine may feel a little softer down low, but will pull harder at the top end, and it should sound much less strained.

Duke

[Robert S.]

Re: 1971 LT-1 Valve settings?

My 71 LT-1 uses full manifold vacuum for the vacuum advance, but the vacuum is switched on and off by the combined solenoid/vacuum switch, which is part of the CEC system. So, as designed, it is possible to not have any vacuum advance even when the engine is well above idle speed. Since the same basic carb is used for 70-72, I would guess that 70 and 72 do not use ported vacuum either.

The CEC system on my car is complete and fully functional, but I wanted to have full time vacuum advance. I simply unplugged the switch on the 3-4 shift lever on the transmission. So the system "thinks" the car is always is 3rd or 4th gear, and therefore the solenoid that activates the vacuum switch is always on. I adjusted the stop on the solenoid so that it does not contact the carb linkage and increase the idle speed.

I changed to the B28 vacuum can as required. I also replaced one of the advance springs with a weaker one to provide full mechanical advance just above 2900 rpm.

[Wolf S.]

Re: 1971 LT-1 Valve settings?

duke,
thanks very much.

without your precious advice i would have been lost. i was a little depressed after i came home with the results.

i'll work on the idle part tomorrow with an electronic tach meter.
as you mentioned mine is way off. 750rpm car tach is about 600rpm on the electronic meter.

the carb on the vette is not the original holley #4555 one. (the #4555 i had restored and is packed away). the car came equipped with a holley 650cfm dp #4777-2.

the tcs system is there but non fuontional.

i have two idle screws to work with. that will be fun and challenging.

[Duke W.]

Re: 1971 LT-1 Valve settings?

My 71 LT-1 uses full manifold vacuum for the vacuum advance, but the vacuum is switched on and off by the combined solenoid/vacuum switch, which is part of the CEC system. So, as designed, it is possible to not have any vacuum advance even when the engine is well above idle speed. Since the same basic carb is used for 70-72, I would guess that 70 and 72 do not use ported vacuum either.

The CEC system on my car is complete and fully functional, but I wanted to have full time vacuum advance. I simply unplugged the switch on the 3-4 shift lever on the transmission. So the system "thinks" the car is always is 3rd or 4th gear, and therefore the solenoid that activates the vacuum switch is always on. I adjusted the stop on the solenoid so that it does not contact the carb linkage and increase the idle speed.

I changed to the B28 vacuum can as required. I also replaced one of the advance springs with a weaker one to provide full mechanical advance just above 2900 rpm.

So what changes in operating characteristics do you observe with the remapped spark advance?

Duke

[Robert S.]

Re: 1971 LT-1 Valve settings?

[Duke W.]

Re: 1971 LT-1 Valve settings?

Once a suitable VAC is installed that offers full advance at no less than about 2" less than typical idle vacuum, the next step is setting the initial such that initial plus full centrifugal is equal to about 36-38 degrees.

In dyno testing that you will read about in the next issue of The Corvette Restorer, 36 degrees total WOT spark advance reduced power about one percent relative to 38 degrees, but sometimes total WOT advance might have to be less than 38 to avoid detonation.

The final step is to make the centrifugal curve as aggressive as possible without detonation, and it you can get full centrifugal in by 2000 that's probably about optimum. The more advance the engine will tolerate at low revs, the better the low end torque.

So you're pretty close to optimum, and if you get detonation with a real aggressive curve you can always use a higher octane fuel, but chances are you can have an aggressive curve and still use 87 PON fuel. Though the advertised CR of the '71-'72 LT-1 is 9:1, if you took all the measurements to compute it, it would probably be closer to 8.5 than 9.

There is usually a lot to be gained by optimizing the spark advance map on any engine. Most pre-emission controlled engines had conservative centrifugal curves because they had to be set up for worst conditions - hot weather and poor fuel - and some OE configurations' VACs are not well matched to the engine's idle vacuum.

Emission controlled engines can gain even more because their spark advance maps were generally more retarded from optimum than pre-emission engines.

Since we usually drive our cars in more narrow operating conditions (not super hot weather) and modern fuel octane is more consistent, we can usually set up our spark advance maps at the ragged edge of detonation, which yields broader torque bandwidth and better fuel economy.

Duke

[Robert S.]

Re: 1971 LT-1 Valve settings?

Duke,
I am using an original TI distributor that had been rebuilt to stock specs. I only changed the vacuum can and one advance spring as I noted earlier. I tried a more aggressive centrifugal advance curve with weaker springs (max advance at 2400 rpm), but this yielded several degrees of centrifugal advance at idle. 2900 rpm was as far as I could bring down the speed for max advance and then have the advance just start to kick in above idle speed.

For now I'm hpapy with the timing map that I have.

The engine had been rebuilt by a previous owner and I do not have a complete record of that work. Therefore exact compression ratio is unknown, but from what I have seen I suspect nothing dramatic was done to change it from a factory build.

[Duke W.]

Re: 1971 LT-1 Valve settings?

A few degrees of centrifugal at idle is what the '64-'65 L-76/84 engines have being as how the curve starts at 700 and all 24 degrees is in at 2350, and it takes about 900 to get acceptable idle quality from them.

I modified my '63 L-76 curve to the above specs circa 1966, and it works very well, so there's no reason not to use the more aggressive curve you mentioned as long as it doesn't detonate.

These high overlap cams create huge exhaust dilution at idle, which slows flame propagation speed substantially, so they like at least low thirties degrees total idle timing and can tolerate mid-thirties.

Duke

[Robert S.]

Re: 1971 LT-1 Valve settings?

Duke, that's good information. If I recall I was getting about 6 degrees centrifugal at idle for the case with max at 2400 rpm, and I was concerned that it might be excessive. But from your L-76 experience, it sounds like I might be ok.

I going to recurve the distributor for that case again and see how it behaves.

[Wolf S.]

Re: 1971 LT-1 Valve settings?

hi again,
a little update

well today i planned to get the idle and idle mixture together today but really didn't get anywhere!

to start the car was a small horror trip with flamed carb backfires. anyhow got it running by not pushing too hard on the accelerator and warmed the vette up.

with some difficulty got the idle 800-900rpm by turning the idle set screw, pushed on the accelerator to about 1800rpm, took my foot of the pedal but the motor stayed at 1800rpm and wouldn't return to idle. the accelerator cable was not stuck. i had to turn off the engine by the ignition key.

i tried once more, again the same situation but this time didn't turn off the motor but turned back the idle set screw a little at a time. the motor slowly returned without stopping past 900rpm to around 500rpm and then died abruptly!

i checked the float levels and they're ok with the fuel just touching the check orifices.

i haven't touched the idle mixture screws yet.

my vaccuum canister on the distributor is a b22.

at the same time found that the little plastic white filter for the headlight/wiper door vaccuum was broken.

so i called it a day.

the idle mixture is probably way to lean! or is it something else?

[Duke W.]

Re: 1971 LT-1 Valve settings?

If you've converted to full time vacuum advance you should install a B28 VAC. If it's still the OE ported vacuum advance, the B22 is okay.

Backfiring through the carb can be caused by either ignition of carburetor problems.

I recommend you start another thread on the subject. Review the history/symptoms and what you have done so far, and we can have a fresh go at it.

This thread is getting confusing since we are dealing with two members with separate issues.

Duke

[Wolf S.]

Re: 1971 LT-1 Valve settings?

ok

"1970 LT-1 valve lash, idle and mixture setting"