1968 340 timing
It will run at 5 btdc,eith 16 inches of mercury on a vac guage, but it doesnt start smoothly,it long cranks. I am thinking it needs more advance.
While I agree
@RealWing, that 10 to12, or even up to 16 works better, I have found that long crank times are more often than not, related to fueling.
IMO, get your fuel-level stabilized, and your transfer slot exposure set right, and things will work better on the hot re-starts
For cold starts, the bowl has to be full, and your choke needs to be working, otherwise you will have to fill the plenum with pumpshot.
If your pumpgas is a week old,and your daytime temps have been over 85*F in the shade, then most of the easier to light components of the gas have evaporated out of your carb, leaving behind the heavy hard to light ones, that are also slower to burn. Earlier timing can help with that. But the cure is to not let the light molecules evaporate in the first place.
Which is a harder thing to do then it might sound like. The lightest molecules are already boiling at 95*F, never mind just evaporating.The heaviest ones might not boil until 400/450. And in between are about 17 other molecules, each with it's own boiling points.
The usual solution is to use a fuel stabilizer, which somehow binds the molecules together, to slow evaporation. It works for me, and my car is stored outside all year.
What you need to know, is that with a cold engine, the A/F ratio needs to be far far richer than what the carb with no choke can deliver. Even with a choke, a lot of the fuel is gonna either lay on the bottom of the plenum,or stick to the cold surfaces.So only a fraction of what you inject, is actually gonna stay airborn, to enter the chamber. And it will be real tricky to light it if it is fresh, never mind if it is stale.
The proof of this is when you say;
"it doesnt start smoothly"
The rest of that fuel, that is just laying in on the floor, after the engine starts, immediately starts dumping into the chamber, and the A/F ratio is again all wrong, and a lot of that is gonna pass straight thru unburned; partly because the fire started too late,and partly because the heat of compression was not sufficient to light that heavychit up, and partly because the droplets are too big to light up.
As to timing;
Your engine will like a lot of it at idle. All you gotta do to see it, is just tug on the Vcan until the rpm stops rising. When it stops, the pressure to the crank is maximized and more timing will not be better. Now put your timing lite on it and see what you get. I'll guess more than 20, less than 30 at 750rpm. This is the optimum timing for that rpm.
Of course , few engines will accept that, because it is as good as impossible to build a power-timing curve with that number whatever it is. And power-timing always trumps all other timings,else the engine is likely to break due to detonation.
Ok so,unless you have a timing computer, your cranking timing is the same as your idle timing, and can be anywhere in that range of 5 to say 25 degrees, until your starter can no longer fight the compression that is being generated by such early sparking. Typically we see about 20 as the max .
But then you run into these other walls;
1) as
@RealWing mentions, you gotta limit your power timing to what your particular engine wants, so now, you have to recurve that distributor. This HAS to be done every time you select a different idle-timing, unless, during testing, you promise to stay out of WOT.and
TWO) every time you change the idle timing, you will have to reset the idle speed. And when you do, you upset the relationship of the transfer slots to the mixture screws. And that is almost guaranteed to cause tip-in sags, hesitations of even mild bogs. and
3) If you have a V-can; every time you recurve your distributor, the cruise timing and the Part-Throttle timing will change; usually for the better, but not always.
Finally, even if your engine is stock, the factory idle-timing was selected not to favor the engine, but to favor emission controls. So it was never right in the first place. We got away with that in the 60s and early 70s, because we had infinitely better gas. Those earlier engines do not much like today's gas.
As Realwing says;
n general, today’s gasoline burns slower than the good old stuff. As a result, the old engines like more initial advance
You cannot hurt your engine by cranking in more idle timing, unless you forget
However, this usually requires recurving the distributor to ensure total mech advance does not exceed approx 34-36 deg.
Jim
and finally
Just remember about item
TWO above.
