Initial Timing question

[Doug Burgoyne]

Running points distributor and non feedback carb in my 2.8 ranger 4x4. My vacuum canister is shot, so currently no vac advance. Very hard to find new or working one...so looking for best way to set it up until I can a working canister installed. Can't test drive until c5 transmission problem solved. If I set initial timing at 12° btdc at idle it maxes out around 20° when I rev it up...all from mechanical advance of course. Being mostly ignorant on timing dynamics, I'd think setting initial timing higher, without vac, might be better. Any suggestions? Thanks!

 

[RonD]

Pretty hard to solve that, only saying that because NO ONE ever has solved it, lol
Vacuum advance is for engine Load spark timing
Pointless distributors(electronic) used engine computer to calculate Load advance


The issue is the air/fuel mix "Burn Time"
The 14.7:1 air/fuel mix takes xxxx time from ignition by spark to FULL Explosive force to push down the piston, its not instant, but it is the same xxxx time as long as its 14.7:1 ratio, its a known fixed burn time

You, of course, must get Full explosive force AFTER TDC, or engine would stop and usually break something, lol

At idle(750rpm) with standard air/fuel mix if spark happens at 12deg BTDC then full explosion occurs at say 10deg ATDC so engine starts and runs
The time it takes for this air/fuel mix to burn is fixed
If you increase RPMs the time it takes for engine to move from 12deg BTDC to 10deg ATDC is much faster, but the burn time is the same
so at 2,000rpm a spark at 12deg BTDC wouldn't get full explosive force until 20deg ATDC which would add no power to crank

This is where Mechanical spark advance comes in, RPM advance
It causes spark to happen at 22deg BTDC so full explosion is back at 10deg ATDC
Problem solved.........................but there is another monkey in the wrench

Richer air/fuel mix burns FASTER than regular air/fuel mix
And THIS is why we need Vacuum Advance(Load advance)
If you "step on the gas" RPMs increase but the now Richer air/fuel burns much faster
So full explosion happens sooner, so there will be a hesitation when explosion happens TOO CLOSE to TDC, and this can cause engine damage
It very hard on pistons and connecting rods if explosion happens at 5deg ATDC or less

Load advance
What most car maker distributors do is to pre-load vacuum advance
So you set base timing at 12deg BTDC, with vacuum advance disconnected(SPOUT unplugged)
Then after setting it you plug in vacuum advance and you will see spark timing move to 20deg BTDC, at idle or so

So what this does is to reduce Mechanical Advance as RPMs go up and Vacuum goes down, i.e. accelerating, too compensate for faster burning air/fuel mix
When you "step on the gas", vacuum drops reducing spark advance as Mechanical advance goes up, faster burning Rich mix is sparked at the right time
Then once at cruising speed and Vacuum is back up with regular air/fuel mix, Mechanical Advance is all that is needed


There is no substitute, engines that use spark plugs and that need to accelerate quickly need some kind of added Load spark system
For now you can just accelerate slowly, expect the hesitation and drive around that way until the part is found

 

[Doug Burgoyne]

Thanks RonD,
All makes sense...thanks for detailed explanation. Same stuff I've been reading up on, but you made it a bit easier for my head to get around. Since this isn't a daily driver...sounds like my best bet is to probably wait til I get a good canister, to avoid possible damage to the engine. The nice gentleman on this forum that sold me the distributor (as is, not tested or guaranteed) had 4 of these points types. He was nice enough to check the other 3 to see if any of the canisters held vacuum. Sadly none did. Which makes me wonder...how long were the cars they were pulled from (his junk yard finds) driven without any vac advance? I would imagine many older cars on the road have either bad vac lines or canister diaphragm without even knowing it. They probably just know their car has bad gas mileage, hesitations, etc., without knowing why.

 

[RonD]

Its mostly from sitting around that the rubber inside drys out and rots, when in use there is moisture and oil vapors in the intake that keeps the vacuum diaphragm "moisturized", lol