Originally Posted by bumpstart

you can modify the dizzy's mech ( centrifugal advance ) curve to suit various modified engines needs

bridgeport and peripheral port engines usually have a "locked" dizzy
- racers worked out the engine basically sat at full load and revs all the time
and would weld up the mechanical advance mech to not operate
- and hence be able to tune the car easily for full load tuning without risk of the mech curve taking things too far
( and the losses to idle and off idle quality inconsequencial in a car with low gearing kept at high revs 90% time )

other tricks more often used in street cars include changing the response rate of the mech curve
- its springs and bobs working against centripetal force making centrifugal motion
its all as easy as changing the weights, or changing the spring rates
--but even easier, on the mazda dizzy , is to cheat, and just bend in the posts that hold the springs
this will speed things up, full mech timing will come in sooner at less revs
( keep enough spring tension to return dizzy to home position when idling )

lastly, you may find the need to shorten the mech advance curve
( but not lock it )

EG
extend ports, and LPG cars like more initial timing to idle well, and respond well with large carbs
they don't particularly like too much load timing more than stock though
( though may enjoy it "all in" earlier, as prescribed above )

for this neat trick, welding up the travel distance of the mech advance will limit timing
( with careful estimations and adjustments )
to what it may have been close too stock
( but allow you to set base timing aggressively )
remember , whatever happens at the dizzy, happens DOUBLE at the crank
- numbers quoted are always CRANK numbers in degrees unless stated
what is 30 degrees crank, is actually a 15 degree swing of arc in a dizzy

- heres some math

most electronic dizzies will add 25- 30 degrees mech timing to the base timing
( disregarding anything the vac / boost retard pots do )
[ early 13b 6 port dizzies have the larger curve over 12a ones ]

EG
- 13b 6 port, stock , 5 ADTC idle , 25 BTDC endpoint at revs ( sans vac signals )
[ 30 degree mech curve ]

- 12a s2 stock , 0 TDC idle , 25 BTDC at rev endpoint ( sans vac signals )
[ 25 degree mech curve ]

if, for example
i am trying things on my extend port LPG 13b
- currently modified , the dizzy has 40% of its curve trimmed
this allows me to work things thus ( mech numbers only )

40% x 25 = 10 degrees removed = 15 degree curve

set at 10 BTC degrees idle , my max timing without vac input would be 25 BTDC
( about what most mad extend port petrol engines would enjoy )

i think i want more boost
and knowing LPG likes high initial advance, but same or similar endpoints to petrol for optimum power
( rather than ping limited )

i would like to try things with 22- 23 degree endpoint
( allowing another 2 to 4 psi safety ceiling )
BUT without dropping my aggressive initial timing

- so
50% x25 = 12.5 degrees advance
so, 10 idle, 22.5 max

here-


the dizzy


a pile of indiscrimate screws, actually they all have a place, remember what they are
everything comes out, a screwdriver is all you need
( don't forget the one down centre of shaft , that ones hard )
the trailing pot arm is also fixed in by screw type fitting
remove the pots and the igniter modules
( mine are dummies for bridging through the BIM igniters )





its the weights that are the magic
bending in those spring posts has altered the spring rate, speeding it up
( you can get at them with thin screwdriver via top of assembled dizzy to adjust later )

the little nubs on top ride in our advance slots

- it really is very simple under there



our old advance slots, you can see i had previously altered them permanently, i wanna try a little more
( if you just use a washer cut into moons, it will be easily reversed on the belt sander )



OK, things at 50 % now, using the washer moon method


touch her up on the belt sander , you don't have much room to leave any tac weld high
touch her up more, and i can undo my move !



slam it together
( if gumby, mark things first before disassemble , but you find things more or less line up without much thought )

to add to the tricks, my LPG car is only using the dizzy to pulse a pair of leading coils in DFI , wastespark mode
( no trailing ignition, or any dizzy HT leads )
as such, i can use the trailing pickup to send signals to fire my leading only coils
- principally to use the trailing boost retard pot
its further sweeping in its range of movement than the leading pot
( i have more vac advance, and more boost retard )
and best of all, i can adjust the timing by pulling on the trailing pot !!!
( the old split adjuster )
bloody handy when FC and Dizzy with PS bracket making dizzy base hard to change !

Originally Posted by vxturboxv

Bringing back an old post, but couldn't you just double up the springs or use a stronger spring? This would still give you "some" advance, but not full adv?

Originally Posted by Megabridge

Quit trying to reinvent the wheel. Get out the wire feed and tack in some weld. Grind it down and reinstall.