wallifishrmn

Have a lt-1 engine with solid lifters and original NOS cam that was just rebuilt and am trying to set the timing. The car starts and runs (on jack stands) but the timing makes no sense. With the vacuum cannister hooked up, air cleaner off and a vacuum gauage attached these are the reading I get.

Vaccum: 12
timing at idle: 33 degrees BTDC

When I try to adjust the timing (retard) to 9-12 degrees it dies. The engine is warm and the idle is about 750 rpms. How can the timing light read 33 degrees at idle. It is a timing light that can be advanced or retarded so when it goes to TDC the timing light reads 33 degrees????? Makes no sense. Help--what am I doing wrong.

TimAT

Set your timing about 10*-12* with it warm and the vacuum advance disconnected and plugged. Set idle speed to specs and recheck the timing. Then WITHOUT hooking up the vacuum advance, run the RPM to 3000 and see what the total mechanical advance is.

Tonio

Wow,that's close to all-in timing, at idle! Did your engine rebuilder mark true 0 degree advance on the harmonic balancer with the piston at TDC compression stroke? Many harmonic balancer outer rings slip out of calibration after all these years.

'75

18 from vacuum plus 12 initial, 33 doesn't seem all that far off. Follow Tims advise to get it dialed in.

calwldlife

yup
disconnect the vacuum advance.
dist weights come in if idle rpm too high so follow the spec.
you say you made a LT1.
did you find a distributor with proper weights curve and proper vacuum can?
there are differences.
don't forget the idle mixture screws.
depending on what the carb came off, power valve and
jetting will come in.
LT1 is a temperamental beast and to run pump gas, you will need to tinker.
idle stop screw will no doubt need careful adjust.
good luck

wallifishrmn

I didnt make an LT-1--it is a 71 LT-1, Will try timing it again next week and report back.

7T1vette

GM defined the 'timing at idle' specifications for THEIR benefit...so timing would be retarded, power would be less, and warranty costs would be lower.

I suggest that you set timing at idle (with vacuum advance disconnected and line plugged) to about 12 deg. BTDC. It will run better and perform better. You can maximize timing adjustment for performance by following instructions from Lars Grimsrud's tech paper Use SEARCH function to find it.

64Scout

Have you verified that you are at 0° TDC when the timing mark says you are?

lars

With vacuum advance hooked up, those numbers are correct and normal. That's about what the timing should be running at idle. Nothing wrong at all.

Lars

F4Gary

My 72 LT-1 is at 16 degrees btdc with the vacuum advance disconnected and plugged. I'm using manifold vacuum.

calwldlife

any chance you have a cam setup for a different firing order?

lars

...and with the vacuum hooked up to the vacuum advance, which will pull in about 16 or 17 degrees in most cases, your timing at idle with everything hooked up is about 32 to 33 degrees, which is exactly the same as the OP posted initially. Nothing wrong with those numbers - right where they should be. Normal timing numbers at idle for a performance engine.

Lars

wallifishrmn

My timing light seems to fire intermittently so Im having a hard time reading the light so Im going to replace te plug wires, the rotor, and the distributor cap. I will report back. Thanks for all the input so far.

lars

An intermittent firing timing light is usually caused by the timing light mag hookup probe. I wouldn't replace all the engine ignition components due to a bad timing light mag clamp probe... unless you actually have verified that the engine is misfiring. My timing light frequently misfires, but I don't replace the plug wires on the engine to fix it.

Lars

Big2Bird

I'll ask. How does it run?

wallifishrmn

I can't tell you how it runs because it's 3 degrees out and we have a foot of snow. Won't be able to drive the car for several months--unfortunately.

CheezMoe

Lars' math is sound. My built 383:

18* Initial
12* Vacuum Advance (manifold)
30* Total @Idle (900RPM)
36* Total advance @3000rpm

You seem to be in that ballpark. Did you do a total advance test?