i have a 98' vortec that is carbureted. new carb, new coil in cap distributor with vacuum 2ndary. with the vacuum secondary connected to the ported carb vacuum barb the car will start and run. I want to check my initial timing so i disconnect vac secondary from distributor plug and cap the vacuum line and barb and the motor wont start. plug it back in motor starts... any ideas???????

 

Let me think on that one.

 

I believe you are describing the vacuum advance, not the vacuum secondary.

 

yea vacuum advance not secondary.

Sounds to me like your timing is retarded and your hooking it up to full manifold vacuum not ported. which is advancing the timing and allowing it to run.

Recheck your timing with the vacuum advance disconnected.

 

yeah sorry about that i did mean advance not secondary sorry for the newbie error.
from what i have read the vacume advance only advance the timing 20* or so is this correct? I rotated the distributor counter clockwise quite a bit trying to start it at different points but i could not get it to fire, maybe i need to rotate the distributor more but im sure i rotated it at least 20* or so, maybe more. If i move the dist back to the original location and hook up the vacuum advance it starts right up.
maybe the springs in the dist are the wrong springs, i dont know im kinda confused at this point

 

the vacuum advance won't do a thing until it actually starts- unless you have one hell of a starter on that thing that can crank it to 500rpm or something..

set the initial timing at about 18 degrees with the advance unhooked, then hook it up to a port that gets full manifold vacuum. you will see something like 45 degrees of timing at this point- that's ok, you want that.. then back off the idle speed screw until it wants to die and then adjust the idle mixture screws for max vacuum and idle speed.. back the idle speed screw off to lower the rpm and tweak the idle mixture screws again... keep doing this until the mixture screws don't make it any better. if it's an auto, have someone that you trust hold the car in gear while you go thru the mixture screws and idle speed screw one last time to dial it in under load.
i guarantee that if you do this it will start better and just run better in every situation.

 

That is incorrect and easily proven. Put a vacuum gauge on an engine while it is cranking.

 

And you'll see that the couple of inches of vacuum you get isn't enough to provide any advance.

 

incorrect. Try it yourself.

 

Believe me, i've done a few hundred cranking vacuum tests. You might get 5 inHg or so out of it. A vacuum advance can for a street application won't start advancing until around 8 inHg or so, often more. Some applications from the salad days before emissions controls would start around 5 inHg, and a very few hi-perf applications would start as low as 3. You can't even get an aftermarket adjustable can to go that low these days.

 

Unhook the vacuum hose and then verify that you do or don't have spark at one of the spark plugs. If it starts with the hose plugged in, you obviously have spark then. If you unplug and don't have spark, something is wrong in the wiring and moving the hose is exposing that problem. It may have absolutely nothing to do with the vacuum itself.

 

Ok so two things: 1) I put a vacuum gauge on a full manifold vacuum port while cranking a fully charged battery... it pulls 3-4 inches of vacuum while cranking.
2) I unplugged the vacume advance and pluged the line and the port again this time i played alot more with the distributor and only moved it a tiny bit at a time. finally i got it to fire without the advance hooked up. so was it my that my initial timing was off that caused this problem? where should i set my initial timing (with advance disconnected)? Also when i turn the car off it keeps running and running and running and running and i mean running strong not missing a beat. have to pull dist power wire to shut off motor

 

set the initial timing to 16-18 BTDC to start.

regarding the way it keeps on running: sounds like you need a new ignition switch or the switch needs to be adjusted.

 

Does it have an MSD or similar ignition box? They are known to cause run on problems. Also, if the ignition is advanced a ton, the engine will want to run on.

 

The dreaded run on, lets get that repaired.