if you have the initial advance set at 16*
and a total mechanical of 34* BTDC are you happy that it needs 34 * total
an efficient engine may not
I can run mine with total advance of 28* and see no appreciable difference other than noise and a slight roughness if i go higher. i'm not saying mine is efficient it just doesn't seem to like more than 28 you need just enough...more is not better and sticking with a rule of "thumb" max of 34 is best might not be the best for your motor.
the vacuum can will pull in a specific no. of degrees based on the level of vacuum
but only get to the total it can possibly add, if you can achieve the level of vacuum necessary so you need a can tuned to your max available vacuum.
the no of degrees it adds is a function of the can and lever/rod this is sometimes stamped on the can or its Rod, 8 would mean 8* at the distributor which equates to 16* timing that can potentially be added if you have enough vacuum.
and that is added on top of initial timing and mechanical timing appropriate to the RPM at the time based on level of vacuum
the vacuum level the can starts to work at, is a function of the spring inside that presses on the centre of the diaphragm. which may be adjustable via an allen bolt down the pipe, tighten bolt and compress spring, it is then harder to move diaphragm
you can in theory set up the vacuum can, to have no effect....becasue you never achieve the vacuum necessary to overcome the compressed spring
kinda pointless..but hey it is a starting point....
The vacuum can must be connected to the ported vacuum connector. the one above the throttle plates. it should see no vacuum at idle and add no timing at idle.
it should see and full vacuum with the throttle plate just passing the hole that leads to the tube for ported vacuum. but the level of that vaccum is dictated by RPM and engine load
On low rpm to high rpm acceleration this has some effect, the throttle sweeps past reasonably fast but it provides engine load based advance for that lower speed driving. you don't necessarily want mechanical advance here this is where vaccum advance has its forte providing load based advance.
mechanical advance is basic RPM based advance basic and a bit rubbish like a throttle position sensor is rubbish and a MAP or MAF is better
vacuum advance is clever it is engine load based advance for 2 purposes its a basic sensor style input that is emulated by MAP or MAF on FI cars
Its used to pile in the advance on pull away or acceleration from idle based on vacuum at the carb, which indicative of engine loading. not RPM
It pulls in a huge quantity of advance at High RPM cruise to deal with massive lean, low mass of air/fuel mix per cylinder. i.e vacuum disrupted running at high engine speed and nearly closed throttle
on cruise this has maximum effect. the throttle plate is held in the "just past the hole" position with the full "suck" of 8 cylinders turning over at say 2500 ++ rpm with the throttle blocking off much of the normal flow through the carb...you are running mainly on the idle circuit with the vacuum stretching the mixture to a thin and horribly lean state hence a huge bunch more advance is nice to set it off.... its helping to light what is essentially a wet log with wet matches in a room with little oxygen
if you connect this pipe to the inlet or carb below the throttle plates it will see idle vacuum and just mess with your initial timing.
its a bodge to get some cars to start and idle with a dizzy that has way too much mechanical advance, that needs to be run with a very small initial advance to compensate. don't go there its a mugs game.
anyway i have rambled
vacuum gauge plugged into the ported vacuum
and friend to read it while you drive??
lift off the throttle and cruise at various RPMs and see what max reading you get.
if you see some at idle then is the throttle held open too wide on the stop screw? are you idling on the mains?
idle vacuum is pretty pointless here as the vacuum can, should not see idle vacuum when plugged into ported vacuum port
do you have a miti-vac type brake bleeder?
if so you can use it on your vacuum can pipe to see what vacuum you need to activate it fully by watching the rod as you pump the air out of the can.
so
34 degrees total? too much in too early? with potentially 16 -25 extra degrees of vacuum advance on top for a split second or 2 as you press the throttle gently, to accelerate away.
16+25 is 41 might be ok..... 34+ 25 = 59 oooooooooooooooooooo000 oooo oooo might be ok at 4000 rpm with the throttle nearly closed and a weak thin mixture, not too good at 1300 rpm with the accelerator pumps in full-on squirting mode that might make a big flame shoot out of the carb. do you need heavier advance springs to keep the mechanical advance off while you transition through the area where vacuum is higher?
that is, as far as i know how it is supposed to work.
Hence the "all in as fast as possible" advance curve is perfect for racing engines used as such,
with no vacuum advance.... but you are not doing that. you want to drive it and have load-based ignition timing, designed for the street. maybe heavier springs? maybe not 34* total
34* Is touted as the best total mechanical figure for lots of engines but it really depends....you need just enough...maybe that's too much for yours
mine likes 28 dunno why just does...and it was a joy to find this out....
or
34 degrees total in at just the right time, yay.... but vacuum advance plugged into NONE ported vacuum so is adding 16-25 extra degrees on top of your 16 initial at idle and as you press the throttle gently to accelerate way 16+25 = 41 at 800 -1000 rpm...... slightly smaller oooooo etc
its needs to be on the ported vacuum
more experimentation needed
dave
Food for thought in there for sure - but as always, I have some questions...