idle vs. timing
Thread Starter
Heel & Toe
Joined: Oct 2006
Posts: 17
Likes: 0
idle vs. timing
OK... so I put a new motor in my '81 and have it 99% of the way tuned- but just can't get the last 1%... and I need to pass smog.
When I start the engine cold, the timing is varying slightly between 0 and 4 degrees AFTER TDC and idling at about 1000 rpm... then some magical moment happens as the motor begin to warm up (way before the motor hits the 190-degree marker for the thermostat) and the timing jumps to 6 - 10 degrees before TDC (wavering).. the idle sits at about 900 with occasional wavering (tries to die - then catches). I still get some "jumping / wavering" of the timing if I increase the idle speed a bit. I need to idle at 700 RPM and 6 degrees before TDC for smog (according to the Guy who failed me last weekend)
Questions???-
Is the idle supposed to drop by about 200 rpm when I put the car into gear?...this means I have to idle at 900-1000 in neutral if I want the car to idle at 700 in drive (oh - it has an automatic trans).
Is the can that controls the headlights (vacuum canister) supposed to work when the engine is off?? my headlights go up and down fine with the engine running, but don't move at all with the engine off.. does this indicate a possible vacuum leak leading back to the intake?
I have worked on other cars, but not Corvettes - this seems like it is a vacuum leak - but if so- I can't seem to find it.... Oh - I am not getting a "check engine" light, so i assume the computer thinks the car is running OK.
I have a new motor, new distributor (I did not replace the cap and rotor, but they look ok), and new carb....
any help would be appreciated - Cameron
When I start the engine cold, the timing is varying slightly between 0 and 4 degrees AFTER TDC and idling at about 1000 rpm... then some magical moment happens as the motor begin to warm up (way before the motor hits the 190-degree marker for the thermostat) and the timing jumps to 6 - 10 degrees before TDC (wavering).. the idle sits at about 900 with occasional wavering (tries to die - then catches). I still get some "jumping / wavering" of the timing if I increase the idle speed a bit. I need to idle at 700 RPM and 6 degrees before TDC for smog (according to the Guy who failed me last weekend)
Questions???-
Is the idle supposed to drop by about 200 rpm when I put the car into gear?...this means I have to idle at 900-1000 in neutral if I want the car to idle at 700 in drive (oh - it has an automatic trans).
Is the can that controls the headlights (vacuum canister) supposed to work when the engine is off?? my headlights go up and down fine with the engine running, but don't move at all with the engine off.. does this indicate a possible vacuum leak leading back to the intake?
I have worked on other cars, but not Corvettes - this seems like it is a vacuum leak - but if so- I can't seem to find it.... Oh - I am not getting a "check engine" light, so i assume the computer thinks the car is running OK.
I have a new motor, new distributor (I did not replace the cap and rotor, but they look ok), and new carb....
any help would be appreciated - Cameron
Drifting
Joined: Mar 2006
Posts: 1,526
Likes: 2
From: Oak Ridge NC
If you think you have a vacuum leak in the headlight/wiper vacuum system, pull the vacuum hose off at the intake and plug the intake. If the idle smooths out, you have a leak somewhere in the headlight/wiper system.
Another thread on the subject
http://forums.corvetteforum.com/show....php?t=1557799
Another thread on the subject
http://forums.corvetteforum.com/show....php?t=1557799
Last edited by SmokinBBC; Dec 4, 2006 at 08:12 PM.
Melting Slicks
Joined: Aug 2002
Posts: 3,334
Likes: 4
From: Midland TX
Good advice given.
To answer your question about the vacuum after you shut off the engine. You should have some vacuum to operate your headlights for a little while. If you don't right after you shut off your engine you either have a vacuum leak and/or your check valve is leaking.
Check vacuum lines going to carb as well. I found a loose line that I could have sworn I put on there tight.
Take vacuum line to distributor off and plug to set timing.
To answer your question about the vacuum after you shut off the engine. You should have some vacuum to operate your headlights for a little while. If you don't right after you shut off your engine you either have a vacuum leak and/or your check valve is leaking.
Check vacuum lines going to carb as well. I found a loose line that I could have sworn I put on there tight.
Take vacuum line to distributor off and plug to set timing.
Burning Brakes
Joined: Sep 2005
Posts: 832
Likes: 0
From: Sarasota Florida
Originally Posted by camjohnson69
OK... so I put a new motor in my '81 and have it 99% of the way tuned- but just can't get the last 1%... and I need to pass smog.
When I start the engine cold, the timing is varying slightly between 0 and 4 degrees AFTER TDC and idling at about 1000 rpm... then some magical moment happens as the motor begin to warm up (way before the motor hits the 190-degree marker for the thermostat) and the timing jumps to 6 - 10 degrees before TDC (wavering).. the idle sits at about 900 with occasional wavering (tries to die - then catches). I still get some "jumping / wavering" of the timing if I increase the idle speed a bit. I need to idle at 700 RPM and 6 degrees before TDC for smog (according to the Guy who failed me last weekend)
Questions???-
Is the idle supposed to drop by about 200 rpm when I put the car into gear?...this means I have to idle at 900-1000 in neutral if I want the car to idle at 700 in drive (oh - it has an automatic trans).
Is the can that controls the headlights (vacuum canister) supposed to work when the engine is off?? my headlights go up and down fine with the engine running, but don't move at all with the engine off.. does this indicate a possible vacuum leak leading back to the intake?
I have worked on other cars, but not Corvettes - this seems like it is a vacuum leak - but if so- I can't seem to find it.... Oh - I am not getting a "check engine" light, so i assume the computer thinks the car is running OK.
I have a new motor, new distributor (I did not replace the cap and rotor, but they look ok), and new carb....
any help would be appreciated - Cameron
When I start the engine cold, the timing is varying slightly between 0 and 4 degrees AFTER TDC and idling at about 1000 rpm... then some magical moment happens as the motor begin to warm up (way before the motor hits the 190-degree marker for the thermostat) and the timing jumps to 6 - 10 degrees before TDC (wavering).. the idle sits at about 900 with occasional wavering (tries to die - then catches). I still get some "jumping / wavering" of the timing if I increase the idle speed a bit. I need to idle at 700 RPM and 6 degrees before TDC for smog (according to the Guy who failed me last weekend)
Questions???-
Is the idle supposed to drop by about 200 rpm when I put the car into gear?...this means I have to idle at 900-1000 in neutral if I want the car to idle at 700 in drive (oh - it has an automatic trans).
Is the can that controls the headlights (vacuum canister) supposed to work when the engine is off?? my headlights go up and down fine with the engine running, but don't move at all with the engine off.. does this indicate a possible vacuum leak leading back to the intake?
I have worked on other cars, but not Corvettes - this seems like it is a vacuum leak - but if so- I can't seem to find it.... Oh - I am not getting a "check engine" light, so i assume the computer thinks the car is running OK.
I have a new motor, new distributor (I did not replace the cap and rotor, but they look ok), and new carb....
any help would be appreciated - Cameron
that's what i don't understand...the car is supposed to idle at 1500 in P or N, and at 650-700 when in gear.
Safety Car
Joined: Dec 2004
Posts: 4,071
Likes: 0
From: San Diego CA
1500? Maybe with the choke closed. 
It should be about 900 or so in neutral.
Little note guys, he's got an '81. There's no vacuum advance on the distributor, it's computer controlled, so that leaves on vacuum line out of the mix.
Have you checked your carb gaskets? Sometimes those cause some gnarly vacuum leaks. Also check your pcv valve.
It should be about 900 or so in neutral.
Little note guys, he's got an '81. There's no vacuum advance on the distributor, it's computer controlled, so that leaves on vacuum line out of the mix.
Have you checked your carb gaskets? Sometimes those cause some gnarly vacuum leaks. Also check your pcv valve.
