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

[Wolf S.]

hi again,

everything started with the discovery that i had the wrong spark plugs on the engine. i had run 1600miles with the wrong plugs. as was recommended i changed to ac r45 instead of puting in the oe r43.

then i set the valve lash cold as per duke/john docs in .016" and exh .023"

as the car didn't idle correctly anymore so i tried to set the idle and idle mixture today but didn't succeed!

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

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. the motor slowly returned 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 orifice.

i haven't touched the idle mixture screws yet. or touched the ignition advance.

my vacuum canister on the distributor is a b22.
vacuum to the distributor is ported.
the distributor is fitted with the lectric breakerless ignition.
new 1.5ohm coil.

at the same time found that the little plastic white filter for the headlight/wiper door vacuum was broken. the little filter inside gone probably been sucked into the manifold!

so i called it a day.

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

first on the checklist will be to see if all the plug wires go to the their correct plugs! the #1 plug wire is fitted correctly.

[Duke W.]

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

A vacuum leak is certainly a possibility, however, you need to do the full idle speed/mixture adjustment procedure which involves setting the mixture screws to achieve peak vacuum/idle speed with a target idle speed of no less than 900. It's an iterative process that involves setting idle speed first, then adjusting the mixture screws. If idle speed/vacuum increases, readjust the idle speed screw back to 900 and repeat the process until there is no longer any increase in idle speed/vacuum with mixture screw adjustment.

The CSM should offer an initial mixture screw setting and then may go on to discuss setting the mixture using a "lean drop" method. This is an emission setting, which I would not recommend unless you have to have the car emission tested. The classical method outlined above will yield the best quality idle.

Prior to the doing the above, check the initial timing and remove the distributor cap/rotor and verify that the centrifugal mechanism is clean and moves smoothly between the no advance and full advance positions. Beware that the centrifugal may start at less than 900, but the initial timing must be set below centrifugal start speed. These data are in the CSM and you can verify where the centrifugal starts with a timing light. You will probably have to temporarily drop the idle speed to below where the centrifugal starts to set initial timing.

You should also check that all measurments/clearances for the carburetor including the choke/choke vacuum break are set to OE spec. The proper values are in the CSM. Also check the choke vacuum break and VAC for leakage.

The choke should fully close with the engine cold at room temperature, and immediately upon start up the choke vacuum break should open the choke valve slightly.

The C3 has a lot of vacuum systems, so original hoses and vacuum devices provide a rich opportunity for gremlins to take hold. It can take a lot of time and patience even with a well thought out systematic approach.

You will get better idle quality if you convert the ported to full time vacuum advance and this requires installation of a B28 VAC on the LT-1.

Duke

[Michael F.]

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

what is the rational behind the valve setting, seem to be way off factory specs?? also where to get the b28 can??

[Duke W.]

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

There is a nearby thread, similar subject, with a link to a paper posted on the Web. It has all the whys and wherefores.

B28 VACs are apparently out of production and getting scarce, but there have been a few tips recently here on where to find them, so do a search.

Duke

[Wolf S.]

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

thread
https://www.forums.ncrs.org/showthre...63406&uid=1362


doc

http://65corvette.nonethewiser.net/t...l/SBvlvadj.pdf

[Wolf S.]

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

short update
did the distributor check this afternoon

1. the distributor cap was loose not properly attached -- my fault. probable cause of my previous problems!

2. checked the plug wires and all are ok with each going to its correct spark plug. i also took some resistance meeasurements
#1 10250ohms
#8 7730ohms
#4 9050
#3 10010
#6 7680
#5 6920
#7 6520
#2 13580
is it normal to have so different values? i suppose wire length has something to do with it.

3. tested the vacuum advance with a vacuum pump. the advance canister works.

4. i couldn't take of the rotor (didn't have the right tools), so i couldn't check the centrifugal advance. it was attached with nuts.

[Jack H.]

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

Yes, you WILL see a variation in plug wire resistance. It's specified in ohms/foot since the exact length of a given wire dictates it's end-to-end resistance.

Plus, as plug wires 'wear' over time/use, their resistance will increase. That's because the internal conductor is stranded nylon impregnated with carbon. The carbon will 'cook' leaving micro-intervals where there's little/no carbon compared to adjacent areas.

If I remember correctly the original spec was on the order of 4 K-ohms per foot. Most techs discard/replace plug wires that measure in the 50K range. But, your readings all sound pretty fresh/new...

[Duke W.]

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

I don't recall rotors being fastened with anything other than a plain slot screw or a quarter-inch hex screw. Maybe the internal threads were stripped.

You need to get whatever tool you need to remove those nuts and check the centrifugal advance mechanism.

An improperly seated cap can certainly cause problems with spark distribution. The cap has a "bump" that should seat in a slot on the side of the dist. housing.

Like Jack said, your plug wire resistance looks okay.

Duke

[John H.]

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

I don't recall rotors being fastened with anything other than a plain slot screw or a quarter-inch hex screw. Maybe the internal threads were stripped.

Duke -

It probably has nuts because it has a Breakerless SE kit in it that uses a stamped shutter wheel mounted under the weight base.

[Wolf S.]

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

short update
didn't have much time to work on the vette this week.

anyhow, i cleaned the distributor cap/rotor and correctly reattached all the spark plug wires.

also checked the centrifugal advance, cleaned off the excess grease. the weights move freely. did a recheck of the vacuum advance and it works.

set both carb idle mixture screws turned in till seated lightly and turned them out 1 + 1/4 to 1 +1/2.

got the engine running but it would not idle even when hot. it just dies with the foot off the accelerator. backfires and then dies.
i can hold a steady 800-900rpm with the accelerator so i suppose i have to turn in the curb idle screw in further to raise the idle rpm. what's the base setting for the idle curb screw? one turn in?

i hooked up a vacuum gauge to manifold vacuum that also goes to operate the headlights/wiper door. vacuum gauge reads 5 at about 800rpm and 6 at around 900rpm. seems rather low to me.

once i get the idle going i'll recheck the float levels.

[Duke W.]

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

I don't recommend grease on the centrifugal advance - just get them clean and wipe down or spray with WD-40 to prevent corrosion.

The LT-1 cam at the recommended lash settings should pull about 12" @ 900.

Are you measuring manifold vacuum from a full vacuum source?

Possible vacuum leak?

Possible that the cam is not a real LT-1 cam?

There should be an initial idle mixture screw setting in the CSM. Set the idle speed screw to achieve the target idle RPM of 900 and adjust as required as mixture adjustment increases idle speed.

Duke

[Joe C.]

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

hi again,

everything started with the discovery that i had the wrong spark plugs on the engine. i had run 1600miles with the wrong plugs. as was recommended i changed to ac r45 instead of puting in the oe r43.

then i set the valve lash cold as per duke/john docs in .016" and exh .023"

as the car didn't idle correctly anymore so i tried to set the idle and idle mixture today but didn't succeed!

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

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. the motor slowly returned 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 orifice.

i haven't touched the idle mixture screws yet. or touched the ignition advance.

my vacuum canister on the distributor is a b22.
vacuum to the distributor is ported.
the distributor is fitted with the lectric breakerless ignition.
new 1.5ohm coil.

at the same time found that the little plastic white filter for the headlight/wiper door vacuum was broken. the little filter inside gone probably been sucked into the manifold!

so i called it a day.

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

first on the checklist will be to see if all the plug wires go to the their correct plugs! the #1 plug wire is fitted correctly.

So maybe you screwed up when you set the lash?
Do it again.........more carefully this time. And remember: it is better to err on lash being too loose rather than too tight.
I'd be willing to bet you have more than a few tight valves.
Did you check the lash BEFORE you set it? It would be best if you knew what they were before you fooled with them............just to ensure that you are, indeed, dealing with an LT1 camshaft.

[James B.]

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

Wolf, I have a '64 fuelie and recently found that I have an LT-1 camshaft. Just had the fuel unit rebuilt and thought I should set the lash (hasn't been done in the 10 years I've owned the car). Since I'm a novice at adjusting solids I opted to use the CSM procedure for cold setting ( half at TDC and the other half 180 d. out at 24/30) as opposed to the Duke/John method. The car runs great, i've got 12" vacuum at 900 & the car starts good hot. I probably didn't optimize performance but it was simple & i'm learning. The Duke/John method requires you mark the damper every 90 degrees; are you sure you marked it correctly? That's hard to do on an LT-1.
If all else fails you might try reverting to the factory recommended setting.
I restored a '71 LT-1 a couple of years ago and has been said, vacuum leaks are a bear.

[Wolf S.]

[Timothy B.]

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

Double check that the plug wires are installed correctly in the cap and on the spark plugs. You mentioned driving the car prior to changing plugs then after it ran bad.

If you had the distributor out could it be installed a tooth off.. You may also want to double check the TDC mark on the damper to make sure it's not moved, this could make a big difference setting timing.

Clean that grease from the centrifugal advance, it's only going to cause problems.