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

[Joe C.]

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

initial the lash setting, as i mentioned previously, was a little more that .040" for the exhaust and as little more than .034" for the intake -- measured cold.

i suppose i have the lt-1 cam in the engine but can't be sure as bubba was everywhere.

maybe it would be best that i measure the exhaust/intake valve lift. what are the correct numbers for the lift? in the book 'corvette by numbers' the lt-1 cam lift is given as intake lift .458" and exhaust lift .484". i've seen other numbers.

You didn't mention your old lash before, unless you edited it out. Did the valve train clatter before you set the lash? It should have, and if it didn't then you have a hydraulic cam installed.

You need to measure lobe lift, which is .3057/.3234. Unfortunately, even if you manage to get those measurements accurately, which won't happen unless your dial indicator's axis is exactly parallel with the pushrod axis, the result is inconclusive in itself.

Just for fun, set them as if they were hydraulics and see how it idles.

[Wolf S.]

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

sorry didn't explain myself clearly. the old lash cold was exh .040" and int .034". these are 'about' figures as the feeler gauge moved in and out without any resistance. so the lash was really more important than the figures i gave.

coming back to hydraulic lifters, how do i know i have hydraulic lifters?
as i was setting the valve lash for each valve, on turning the engine the rocker arms moved opening and closing the valves. so i deduced that i have solid lifters.

[Duke W.]

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

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.

Your lash is waaaaaaaaaaaaaay loose, and no doubt your valvetrain is very noisy. The above method is only valid for hydraulic lifter cams as stated in the service manuals. And it's stated clearly in the 2008 revision of the paper.

Wolf - I don't doubt that you have mechanical lifters. If hydraulics have any clearance they would be frightfully noisy.

A dial indicator should reveal lobe lift to ID the cam. Pull off the rocker arms and set the indicator directly on the pushrod tip as close to in line with pushrod as you can eyeball. As long as the indicator is eyeball in line with the pushrod, readings should be accurate enough to ID the cam.

If the indicator is off five degrees the error is one minus the cosine of five degrees (0.99619), which is less than half a percent.

C3s have a lot of vacuum systems, which give rise to potential vacuum leaks.

Duke

[Wolf S.]

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

no update
no time to work on the vette

[Wolf S.]

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

update
latest

i got the engine running for a short while.

on cleaning the carb i've discovered that i been running the vette without fuel filters! i always thought that there were small bronze filters in the inlets of holleys--i didn't have any! i have now fitted an inline filter.

initially the carb spat fuel out of the primary vent tube, but after cleaning the needle and seat everything is now ok with no leaks.

rechecked the valve lash with a friend and found that some valves were off a bit.

with engine running checked the float levels and corrected accordingly.

next procedure will be setting up the idle correctly. at the moment it is very very low and if i take my foot of the accelerator pedale the engine slowly dies. just hope i have time this halloween weekend.

check list
1. distributor cap/rotor/centrifugal and vacuum advance ok
2. spark plug wires and connections ok
3. valve lash ok 0.016" in, 0.023" exh
4. carb adjusted with initial holley settings -- idle mixture screws 1 and 1/2 turns out from seating, idle speed 1 turn in just beginning to move throttle plate, accelerator pump 0.015" play.
5. float levels ok
6.

soon at the end of the tunnel. i can see the light


i tried checking the valve lift in order to see if i have a lt-1 cam. very rough readings i took over .3" and under .4" intake and exhaust . i had trouble installing the micrometer and it's magnetic base.

[Duke W.]

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

update

4. carb adjusted with initial holley settings -- idle mixture screws 1 and 1/2 turns out from seating, idle speed 1 turn in just beginning to move throttle plate, accelerator pump 0.015" play.

I don't understand what you're doing here. Set the idle speed screw to achieve 900 RPM using an accurate test tach.

Then adjust the idle mixture screws from the initial setting to see if the idle speed/vacuum increases.

If it does use the idle speed screw to readjust the idle speed to 900.

Repeat the procedure until adjusting the idle speed screws doesn't increase idle speed.

Duke

[Wolf S.]

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

hi duke,

didn't get that far today. no time to do the "idle part".

as i had taken the carb off for a rebuild, i reset it to its base settings before putting it back on the intale manifold.

wolf

[Wolf S.]

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

well couldn't get much done.
ran out of gas!
as a consequence of the numerous strikes in france the gas stations don't have gas. with my jerry can, i went to 3 today and none was selling high octane (super 98)!
before running out of gas, i fitted an inline holley fuel filter and warmed up the engine (took 15-20min) and got it to idle at around 500-550rpm with the idle set screw turned in 1/2 a turn from seating.
so my idle setup will have to be done on another day.

[Michael F.]

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

just buy couple liters/gallons of regular gas to use while setting stuff up, you are not doing anything but idle and it won't hurt anything

[Wolf S.]

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

today i tried to get something done with the idle.

both idle mixture screws are 1 1/2 turns out.
hooked up a vaccum gauge to port on rear of manifold for healights/wiper door vaccuum canister.
distributor vacuum hose hooked up with carb.
hooked up a tachometer

i got the engine warmed up 20min holding the throttle at 900-1000rpm.
after releasing accelerator pedal, idle went down to 500-550rpm.

then i raised the idle speed a step by step with the idle speed screw to around 700rpm.
as i was pulling the throttle lever past 1500rpm the idle speed wouldn't return right away--it stayed at the 1500rpm level and slowly returned to 700rpm.

vacuum gauge read 5-6 at 700rpm. could it be that i'm on ported vacuum?

i alse tested the idle mixture screws. as i turned one in the engine died. the other side did the same thing. so the idle circuit is functioning correctly.

i tried to set the idle mixture but turning the screw in or out a 1/4 turn did not change the readings on the vacuum gauge or ttha tachometer.

noticed a hissing sound from the primary venturi of the carb. if i pinch the distributor vacuum hose it goes away.

could i have a vacuum leak?

the distributor vacuum advance can? a while back i tested the advance can and it was working. maybe it gets stuck.
centrifugal advance weights don't return properly? they moved freely when i cleaned off all the excess grease.
pvc valve leaking?

well that's all for today

[Duke W.]

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

Hissing is common at idle if you put your ear to the carb air horn because any engine that idles with about 15" or more vacuum has sonic flow velocity past the throttle valve. Less than 15" is subsonic, but will still hiss.

You clearly have issues with either the engine or the test setup.

A properly set up LT-1 with a full time 16 degree VAC should idle with good stability at 900 @ 12" and total idle timing should be 16 plus initial, plus maybe a couple of more if the centrifugal starts below 900, so it should be in the range of 24-32.

Some early emission controlled cars have a damper that prevents the throttle from shutting rapidly, and the revs hang, which causes jerky shifts. I don't think any Corvette engines have this, but one that did was the early RX-7.

The damper was there to prevent the hydrocarbon spike that occurs when the throttle is suddendly closed - like every time you lift to shift. It was an emission control deal, but it was TERRIBLE for driveability.

Duke

[Stuart F.]

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

You get around that by power shifting all the time (ouch!). Duke, you are a very patient and understanding man. We all thank you for that. I have followed this thread and wish I could help, but one almost has to be there in person to see what is really going on with this car.

Stu Fox

[Richard G.]

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

Didn't the car idle correctly before adjusting the valves?
Having this setting absolutely correct before work on the idle is very important. Be confident this is right then work on the idle.

[Terry M.]

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

[QUOTE=Duke Williams (22045);521357]

[Duke W.]

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

I know some GM engines of the era had a so-called "anti-diesel solenoid". For emissions higher idle speeds are desireable, but can cause run-on when you shut off the engine.

I recall that my '72 Vega GT engine had one. The solenoid was supposed to be adjusted to 1200 RPM idle and the "low idle" with the solenoid de-energized was speced at about 700. Turning off the ignition de-engerized the solenoid, which closed the throttle valve further to prevent run-on.

Of course, the high throttle opening for 1200 RPM idle caused the revs to drop too slowly on upshifts resulting in jerky operation.

It wasn't long before I essentially deactivated it either by disconnecting the electrical plug or turning the adjusting screw in to achieve a normal idle speed, but I think I left it installed.

Duke