Igntion Timing 70 LT-1

[wolf]

hi everyone,

i've been trying to fix some vacuum leaks with the carb. seems i have a gasket sealing issue between the carb and the intake manifold. rpm lingers around 1500 and slowly returns to an unstable idle at 700 or lower.

the engine runs perfectly with a stable idle between 800-900rpm with no problem going into the higher rpm ranges but sometimes it does the above mentioned problem.

in the meantime i decided to do something about the setting of the ignition advance. i'm a real novice as regards ignition timing. i've read the different docs available on the site.

i took a timing light reading with engine idling at 850rpm and found the balancer timing mark lined up with the 0° on the tab!

well i noted down some ignition advance numbers as this is my next step in setting up the engine.
vacuum advance to distributor plugged.
only centrifugal advance working.
distributor is a points type with the lectric breakerless ignition.

what to make of these numbers?
numbers are 'about' as it was quite difficult to hold the rpm and the timing light at the same time.

500rpm 0°
800rpm 0°
900rpm 0°
1000rpm
1100rpm
1200rpm 6°
1300rpm
1400rpm 8°
1500rpm
1700rpm 15°
1800rpm
1900rpm
2000rpm

vette is a 70 lt-1

thanks

 

[wolf]

i'll try to answer my own question.
anyway i realized that the engine didn't have any ignition advance! i've been driving without initial ignition all this time!
with the lowest idle rpm possible and the vacuum advance to the distributor blocked, the engine had 0°.
i first tried with my dail timing light an advance of 4° by turning the distributor and the engine ran much better.
then set it to 8° at which it is now. engine runs a little rough to my taste but probably because i haven't reset the idle mixture screws or rechecked the float levels.

curb idle is now about 850-900rpm

what is the correct initial advance is for the 1970 lt-1. on the emission sticker it is marked 14°/750rpm. in some of my literature it is 8° or even 4°.

i'll take the vette for a test drive as soon as the weather clears up here in paris.

 

[Carb Doc]

I picked my 70 LT- 1 up new, I run +16' at 850 rpm, timing set with the vacuum unplugged and hose sealed while setting, you really should run the vacuum advance once timing is set, as it will idle a bit better and run cleaner. I always use premium fuel (91 octane ), that is the best we have here in Eastern Canada, would like a bit higher but you have to run what you can get. If your motor is the old 1970 factory type set up you may want to set your idle up around 1000 -1100, the car will be more drivable as the old factory cam is fairly long on duration and will tend to stumble at low rpm's and off traffic lights. If it is idling rough , richen up your air screws a bit, this will help.

 

[wolf]

well i set initial timing a 8° for now. idle is at 850-900rpm.
the engine runs fine with very little movement. no more backfiring or carb spitting.
next, as soon as the spring weather comes by, i'll try to setup the advance curve with all in at 2500-3000rpm by changing the centrifugal advance springs.