I have a 1977 350 sbc I rebuilt this fall and winter (mild performance cam change as well as an intake change), and my concerns started out with a poor idle. The advice I got was to set up 36* mechanical advance and the turned into that the distributor could not get up 36* at an initial base timing set of 12*. Ok, so I put on some lighter springs, no change. Took off the springs, no change. So there is no way I am going to get 24* mechanical advance from my HEI. What is the issue that I am having? Should it not peg out at 36* without spring at a 1500 or so RPMs. I am certainly not getting that much advance with stock or light springs at 2800 RPMs. What should I try? Another issue is that now that I have pulled the distributor to get a closer look, I noticed a lot of play (1/8" or so) at the end of the shaft between the housing and the gear. Logic tells me that this could cause some timing issues with the gear potentially moving up and down as the engine ran. Is there a fix for that or is the distributor need to be replaced. The distributor looks clean and the like with little wear, but I am not sure what my next step is with this thing.
Machanical advance does not work at idle, it`s only use is off idle, centrifical force is what advances it. Connect the vacuum advance to a full time manifold vacuum port, one that pulls vacuum even at idle. Since you already had 12 degrees base timing, the vac advance will add another 10 to 12 degrees depending. After you connect it, recheck the base timing, it should be 20-24 degrees.
The springs don't regulate how much timing you get, they regulate at what RPM the timing advances. You need to use smaller pins or longer slots in the mechanical advance to get more timing. The vaccum advance should only come in in high vaccum situations such as cruising on the highway, usually to give you better gas mileage.
Here we go again. Many ask why do you need vacuum advance at idle?
I used to ask that same question.
You need it because idle and off idle mixtures are lean, leaner mixtures burn slower, and when the mixture burns slow it needs more spark advance to compensate. Manifold vacuum works off idle just the same as ported vacuum. However, since the idle mixture is lean, it needs that extra spark advance when coming off idle to the main circuits, if not the engine will sometimes sputter. You have to remember, the power valve doesn`t kick in until the vacuum drops under a certain vacuum limit, during regular driving you won`t open the throttle enough to make the vacuum drop low enough for the power valve to enrichen the circuit so the mixture is still lean. Take any chevy vehicle from the mid 70`s to early 80`s. The catalyst says "timing set with vacuum advance disconnected and plugged" if it`s connected to a ported source there is no need to disconnect it, because it doesn`t pull vacuum at idle. When I ran my small block to a ported source, a mild 9:1 compression 350 with headers, small RV cam, intake and carb with 3.23 gears, it wouldn`t even spin a tire from take off, it was a dead player. For a long time I couldn`t figure out why it was so sluggish, until a member of this site told me to connect my vac advance to a manifold source, I did as he asked and the car felt like it picked up 50 horses. It had great bottom end torque and throttle sensitivity.
set your base timing by posotion of the distributor
use vacuum advance if you need more advance at higher rpm
it uses a ported vacuum source on the carb
mechanical advance also works at higher rpm
alot of times i dont use vac advance, i dont even connect it, especially on stock cam engines,
as they run perfectly fine without it and get plenty enough advance from the mech weights already
to run fine.
overkilling ignition advance will cause problems quicker and worse than not having enough .
due to detonation/pinging from over advanced timing
use a timing gun to figure out your base timing and to see how much your advancing at what rpm levels
too much of it is the worst thing to have.,,
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