Backfire on decel is exactly what I would expect with your square transfers and nearly closed mixture screws. And IMO your 15 degrees of Idle-timing is the root cause of it.
Reset the mixture screws to 2.5 turns out, and open your transfers a half a turn. Then, if your idle speed is too high, retard the idle timing.
That's what I would do.
Ok here's the scoop; your low-speed adjusting tools are;
WET fuel level,
Transfer slot fuel,
mixture screw adjustment,
the syncing of the slots to the screws,
the IABs,
Idle-timing,
Engine coolant temp,
Inlet air temp,
the temp of the carb,
and altitude and humidity.
Your low-speed/rpm tuning targets are;
1) to not have a tip-in sag,
2) to not have the transmission bang on engagement
3) to not have any closed-throttle decel-issues such as backfiring.
4) best lean cruising for fuel-economy.
Personally, I don't gibachit what the idle timing is, and neither does your engine, outside of solving the above.
The first time the engine cares about timing, besides the Idle-tune, is at stall. So then, if you have a perceived lack of timing at stall then you have a distributor issue, just fix it. This sounds simple enough, and is, but sometimes sacrifices have to be made in the prevention of detonation; and stall-timing is usually the first to take a hit. But good news, you only go thru it once on the way to somewhere else, and worst case is you have to increase that stall rpm, and in your case, it should be over 2800, probably over 3000, anyway. and your cam wants headers to run right and make power.
Ok end rant.
Once you get it running rich enough on the low-speed circuit, THEN you can try closing the transfers and increasing the Idle-timing; but leave the Mixture screws no more closed than 2.5 turns.
If it still backfires on decel, yur gonna have to look for fresh air getting into the headers* at the flanges by the heads. look for soot on the heads
*.(edit; into the headers or log-manifolds)
If you have headers and a tight lash cam, it is possible, at closed throttle decel, for a cylinder on overlap to draw a partial fuel-air load, straight across the top of the piston while it is dwelling near TDC, and into the primary pipe. If it finds fresh air in there, it can easily catch fire and explode. This can be confused with a lean misfire.
At 60mph = 2800, your cruise-timing will need to be around 48>56 degrees.
You will need to modify your VA to get all you can out of it, around 22 to 24 degrees. That will then require a Power-timing of say ;
54 less 24= 30 degrees at 2800-cruise.
With this tune, it should be able to run pretty lean with headers*.
* EDIT: I hear you don't have headers. With a 270* solid-lifter Cam?
If that's a single-pattern and on a 110LSa
I'ma guessing that cam has around 50* of overlap.... or more. Without headers, that's leaving a lot of power under the table,
cuz when you make it richer down low, it gets richer everywhere else too..
Without headers, another thing happens. You get EGR. The pistons have to physically try and push the last of the exhaust out, and they never succeed. If there is ANY back-pressure at all, in the exhaust system, then cylinder pressure can, when the intake pops open, backflow into the intake manifold. It is a minor thing, and very short-lived event, but it is just enough to, at certain times, to weaken the slow-speed fuel signal, and delay the slug of mixture from coming to the cylinder. You can tell when all this is happening, by the build-up of baked-on soot that occurs in the first inch or so of the intake-port when you pull the manifold off. and, the mixture will be leaner at low speed, than the exact same combo would be, with headers. This makes tuning down there a lil more difficult, Because, the low-speed system never stops flowing until the pistons stop moving.
In this situation;
when you lift off the throttle on decel, at rpm, such as at cruising at 2800 rpm, the pistons are now being driven by the driveshaft. With the throttles closed, this creates a high vacuum in the cylinders. So then, with exhaust pressure higher than cylinder pressure you can get a situation where exhaust is swapping cylinders during blow-down, thru the common logs, when another near-by exhaust valve is still open. When it happens that the near-by cylinder is on overlap, exhaust can be blown/pumped thru that path and into the intake manifold. Theoretically, that exhaust, all burned up, is inert and should not make trouble with the throttle closed.
In reality, rarely is the exhaust all burned up. Therefore, if fire gets into the intake, it will wreak havoc.
Therefore, with a high-overlap cam, and log exhaust manifolds your exhaust system cannot afford to have any back-pressure at all. And, tightening up the lash will only increase overlap, which, theoretically, should make the situation worse.
If you need to, you can measure your back-pressure by inserting and brazing a small pipe, like a brake-line, into the exhaust pipe on the engine side of the muffler, up near the firewall. Then plumb a gauge to the line. When I do that, I bend the installed end downstream; and I make the line long enough on the outside so that I can just pinch it shut when I'm done.
BTW-1;
When the throttles are closed and the engine is in compression braking; the intake vacuum will be very high. This will pull air into the crankcase thru the CC breather, and into the intake manifold thru the PCV valve. This can pull a lot of oil with it, if the valve-cover baffles/separators, are marginal. That oil will accumulate on the floor of the intake, until you get back on the gas. Then it will be washed into the cylinders and be burned up.
But during this event, it can trigger detonation. This is just something to be aware of. You can mitigate this by not closing the primaries up so tight, or by installing a dashpot on the throttle shaft to delay it's closing.
BTW-2
IMO
not a chance would I run your combo without headers and a Freeflowing exhaust system.
BTW-3
As you tighten up the lash, the engine will fight to idle. In compensation you will either,
1) increase the idlespeed or
2) increase the idle-timing.
Both of which are the wrong thing to do.
#1 will upset the low-speed mixture ratio, requiring other fixes, #2 will increase idle-power, which as continued will cause the transmission to clang when being engaged from P/N
BTW-3
how can you know what yur cruise-timing should be?
answer; open the mixture screws to in the range of 2.5 to 3.0 turns; then rev it up to cruise rpm, 2800 in your case. then without regard to reading the timing just advance the timine while simultaneously adjusting the throttle closed to keep the rpm at cruise-rpm. Continue this juggling act until additional timing no longer increases the rpm. Now install the timing lite, and read the timing at cruise rpm. Finally, from whatever number you get, subtract 3 degrees for the no-load situation, and there is your target. BTW, since we're there, this is the time to establish where your mixture screws should likely be. Just rev it back up to cruise-rpm, and fix the throttle there using the fast-idle cam; then commence adjusting the mixture screws lean for the highest rpm. Finally richen it up a quarter turn and return the engine to idle.
If the mixture screws are not in the window of 2 to 3 turns out, something is wrong. My guess is that the transfer slots need adjusting. Put the mixture screws back to what was determined to be optimum at 2800, and then adjust the transfer fuel delivery by using the curb-idle screws.
if it is determined that the engine needs more air, I drill one hole in each throttle valve, on the slot side, near the idle discharge ports, and start over..
If you drill to large, the idlespeed will get to be too high, and you will have to retard the timing. If you get the dreaded tip-in sag, the transfers are too far closed or, the WET fuel-level is too low.
If you get a big bang on the shift from N/P to in gear, the idle power is too high. You can reduce it by retarding the timing, or in your case, by increasing the valve lash. and Yes you can reduce idle power by closing the throttle but that will just screw up all the hard work that was done to this point; don't do it! lol
Listen up. I know I just gave you a weeks worth of work, as a tuning newbe, so read it carefully, and proceed slowly. The BTWs are your lifeline back to normalcy. If I did this carb/timing work in my tiny shop, you would be in and out in an hour, to maybe two, if I ran into something I have not here forseen. This is not a brag, any experienced tuner should be able to match that. And this is not a blot on your skill nor on your person. It just is the way of it.
