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

[Wolf S.]

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

thanks for all the input.

i know that long distance troubleshooting is very very difficult but a little info here and there is great help.

my car ran ok, until a friend mentioned that the car had no punch compared to his 350cu' vette. idle was a little low then--around 600-650rpm. that's when i decided to verify the valve lash, ignition advance, etc.

i'm a little stressed that i can't get it running as it should. i haven't given up yet and will work on the car this coming weekend again.

the 70 lt-1 ddin't have the seleinoid/damper. originally it was fitted with a 780cfm vacuum secondary holley list 4555. something in the car's life it received the double pumper that is on it now.

[Terry M.]

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

the 70 lt-1 ddin't have the seleinoid/damper. originally it was fitted with a 780cfm vacuum secondary holley list 4555. something in the car's life it received the double pumper that is on it now.

Wolf you are correct -- the 1970 LT1 originally had neither the throttle damper nor the anti-diesel solenoid, and List 4555 is the correct carburetor. It did originally come with a TCS (Transmission Controlled Spark) solenoid. This solenoid allowed full vacuum to the distributor vacuum advance only when the transmission was in third and fourth gears. At that, this signal was ported vacuum -- that is it was not available at idle, however, since the vacuum went to the distributor in only gears 3 & 4 the ported vacuum is not much of an issue. With your double pumper I expect photos of an original set-up would not be helpful to you.

[Robert S.]

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

Terry, are you sure that the vaccum to the distributor for a 1970 LT-1 is ported?

On my 71 LT-1, there is only one vacuum port on the carb and it is manifold vacuum (as measured at idle). The vacuum line from there branches to the choke pull off, the diverter valve on the AIR pump, and the distributor vacuum can (via the CEC switch).

[Terry M.]

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

Pretty sure, but I am willing to take a look. I think the vacuum connection to the carb is different in 1970 than 1971 -- but I will have to research it. I can't test it on the car.

[Robert S.]

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

Terry,
I have never measured the vacuum on a 70 car, but the port appears to be at the same location (lower front on the throttle plate) as on 71. There are differences in the vacuum line arrangement between the two years (TCS vs. CEC) but the 70 appears to be functionally the same with the choke pull off and diverter valve tied in as for 71.

However, one main difference for 70 is that there is an additional vacuum line from from a port on the right side of the carb body that tees into the line to the distributor after the vacuum switch. Maybe this one is ported vaccum? Or course I'm just guessing, but it does indicate some changes were made from 70 to 71.

[Terry M.]

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

However, one main difference for 70 is that there is an additional vacuum line from from a port on the right side of the carb body that tees into the line to the distributor after the vacuum switch. Maybe this one is ported vaccum? Or course I'm just guessing, but it does indicate some changes were made from 70 to 71.

I think the vacuum line you are referring to goes from the TCS solenoid to the top of the air horn. It is a vent line. In 1971 and 1972 this vent line was eliminated.

I have yet to look at the emissions book, AIM, and my car for the 1970 vacuum hook up answers. I have some pressing family issues in the next few days. I'll get to it, but it will take some time.

[Wolf S.]

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

here' a quick update:

i had left the car sunday with a curb idle of 650-700rpm.
today i took some time off and decided to seek out my vacuum leaks, if any!
as i had taken the carb off for a rebuild, so i took a look at its mounting. i had put in 3 gaskets to get close as possible to the original 1/4" thick gasket that came with the lt-1.
this time i torqued down the 1/2" mounting nuts to 80in/lb. (before 60in/lb).

started the engine, and what a surprise, as the engine warmed up the curb idle was slowly going up and way past 900rpm. i had to turn out the idle speed screw to lower the idle and stabilized it at ~800rpm.

i also fiddled a bit with the idle mixture screws. they have become a litte more sensitive.

vacuum is now about 7hg

so found one of my possible vacuum leaks.

now the questions:

what are the torque specs for the carb mounting 1/2" nuts?
should i use a gasket sealant and if yes, any suggestions?

[Larry E.]

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

FWIW:

Here is a picture of the proper routing for a 1970 LT-1.

Larry

[Stuart F.]

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

Wolf;

Please don't use stacked gaskets and torque the hold down nuts. You are asking for a vacuum leak for sure. You might get away with two gaskets, but that is max. Stacked gaskets tend to squash down at the hold down studs/nuts, and you take a chance on warping your carb base plate which will lead to all kinds of problems. I suggest you get the correct gasket/spacer and other parts for your application. Don't take any chances.

Stu fox

[Terry M.]

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

There is also a good chance you will crack the base plate with stacked gaskets. Don't ask how I know.

[John H.]

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

something in the car's life it received the double pumper that is on it now.

Wolf -

Does that double-pumper have 4-corner idle (idle mixture screws on both the primary and secondary metering blocks)? They cause all manner of tuning problems.

[Wolf S.]

[Terry M.]

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

Wolf,

1970 LT1 initial advance is listed at the two different numbers you found. Back in the 1985 to 1990 era Scott Linde and I did an article for The Restorer

[Wolf S.]

[Jim T.]

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

[QUOTE=Terry McManmon (3966);521384]Terry I am original owner of a 1970 Corvette that left the factory with Q-Jet carb. My 70 has never had a throttle damper. 350/300 turbo 400