I have a 350 w a MSD streetfire hei vac advance distributor.

I have the base timing set at 12degrees with the vac adv unplugged. problenm is when i plug the advance in and bring the rpms up to 3000 im getting a total advance of 50 degrees. how do i determine how much advance is in the vac advance , and how much is in the mechanical advance, and then get it down to 34 total

Im just not sure how to subtract the timing or which one i need to pull it out of. I thought the total timing was the combination of all 3. I bought a book about ignitions, bt now im more confused then when i started.

 

Unplug the vacuum and bring it up 3000 RPM. Subtract 12 and that's your mechanical advance.

 

ok just went out there. unplugged vac 12 degrees. vac plugged in at idle approx 18. unplugged the vac, brought the rpms up .. it dialed in at 33. minus 12 would be 21 degrees mech timing. but then i plgged the vac in to get a def reading on vac advance , but it was sitting dead on 12. plug it in unplug it the timing doesnt move. the port ids drawing vac, i can feel it on my finger(timed port) and if i suck on the vac line to the canister i can get it to advance.

Am i doing something wrong here?

 

I had the same problem with my MSD distributor the vac advance is adjustable
I had to turn the little allen head screw almost all the way out before it would advance.

 

I have the little sheet with the graphs that came with the dist. but i just dont understand it. im pulling 8in of vac at the canister if that helps. Man ..I was going to buy the msd complete kit from jegs. but i ran out of money. I want to run nitrous one day, but if i cant figure this out.......

 

Ya I didnt understand the graphs ether. I just adjusted it until it advanced all the way.
this might help?
https://www.thirdgen.org/hei

 

ok now im really confused. that article said 50-55 degress is good(in the ballpark)..... I thought i need 34 to 38 . I think i need to take abreak and walk away from this for a minute cause now im just getting frustrated, not to mention i just feel like an idiot

 

I just checked mine its at 31 degrees but mines a 305, every engine is different, I would just play with it until it runs best for you,Thats how I did set mine, if you ask 8 different people you'll probably get 8 different answers.LOL every one I ask about timing tells me to advance it until it pings then back it off till it stops.
As for the vac advance I had to turn mine out about 8 turns until it finally started working properly.

 

Vacuum advance is for part throttle operation. It will change depending on how much manifold vacuum is available regardless of engine rpm. For most people, you don't need to know what the vacuum advance does unless the engine is pinging during part throttle operation.

Set the base timing. Adjust the mechanical advance so that the base plus the mechanical gives full advance by 3000 rpm. Anywhere in the 34-38 range is a good starting point. 1 degree of advance can mean a noticeable amount of HP. How much total depends on what your engine likes. Not all engine are the same so it's a trial and error method to find the best total. Using an engine dyno is the best way.

Once you have the best total advance set, any other tuning you do to the engine such as jets, plug gap or heat range, valve lash etc won't change where the engine likes the timing.