I'm going to have to (within a very narrow range of use) disagree with going to a mega light flywheel.

Here's the why. Being lighter there is less mass to accelerate (in neutral with the engine free revving) so a lighter flywheel WILL improve throttle response in neutral. Once the car is in gear and the clutch is out, you have the weight of the gears in the gearbox, the tailshaft and the diff gears to spin/accelerate (and to overcome the resistance of the car (at several hundred kg) to accelerating. Meaning that in practice, the change in flywheel mass is only _very_ small in terms of the overall picture of what there is to accelerate.

If we look at it back to front, in first gear (obviously the lowest gearing, highest numerical ratio) it increases the torque applied to teh wheels, the diff ratio also increases it. You could almost compare it the other way round, if torque is being multiplied by the gearbox and diff gearing, then the 'resistance' to the car being accelerated away is 'reduced'. so if the first gear is 3.2:1 and the diff gears are 4.11:1 then the torque multiplication overall is just over 13:1 and so the engine will perform (in contrast to how well it would in 4th gear) as if the car is only 1/13th of its actual weight. Since the mass of the car (as far as the engine knows) is reduced to 1/13th, then dropping a kg or two off the flywheel _will_ actually increase the acceleration (albeit mildly) in first gear. It'll do it less so in 2nd gear, and by the time you get to 3rd or 4th gear, the difference in flywheel weight is practically negligible and has no impact on acceleration. So basically 'at best' it'll improve acceleration in first gear a little bit, and less and less as you move through the gears.

But having a really light flywheel has drawbacks. That slightly heavier flywheel has more inertia, and so it actually helps to smooth out the idle quality (since the force from the pistons on the crank is one at a time, 4 'shunts' evey 2 full rotations, meaning with no flywheel weight the idle rpm can effectively go up a fraction with each combustion stroke, then die off a fraction until the next cylider fires and so on. The heavier flywheel will help to keep momentum up and smooth out each pulse. This can have a good effect on idle smoothness (including just how low you can get it to idle without stalling). In practice, esp for street driving, that light flywheel will also allow the engine to rev up quickly, and this can make for more 'attention' every time you take off from a set of traffic lights, as it'll rev higher. You could 'feather' the throttle and only let it lift the revs a tiny amount before lifting off the clutch, but with less flywheel weight, it's easier for the engine to bog down, and be a 'pig' to get off from the standing start at said traffic lights, so it's sometimes a case of either having to feather the clutch across some % of the intersection, or having to lift the rpms higher and take off like that.

It ends up being a pain (potentially) for a streeter. Some tractors and other equipment often have particularly heavy flywheels, so that the motors can run at lower rpm, and if caught in terrain where the engine will bog down (potentially) this heavier flywheel and its inertia helps prevent it stalling even at quite low rpms.

Don't get me wrong, if you are racing, esp with diff gearing that produces a very low overall gear ratio with the gearbox in 1st, it's certainly the way to go (and don't forget that within reason, you don't have to constantly get the car off the start line, you do that once, and so even if it was less than optimal at lower rpm, in competition the engine never or rarely sees those rpms/conditions. So certainly good for those applications.

To further clarify, certainly I think that going to an a12 flywheel is worthwhile, I'd be reluctant to take that a12 flywheel and drastically lighten it if it's a streeter. The a12 flywheel is heavy enough to avoid the negatives (for a streeter) but light enough to improve throttle response. Lightening (if done at all) has to be done 'right' and done right, the result is actually a safer flywheel at higher rpm, but done wrong (and that can be a result of it not being properly centred, or the tool and angle/direction of the tool to machine it away) and it is actually far more likely to come apart at higher rpm.

The bike carbs are a doable option for sure, but just be sure to take on board that custom making an intake manifold is straightforward enough BUT it's after that that the real work begins. It'll take a while to optimise the carb a/f curve for all driving conditions (and the flow/rpm rates for an a-series are dramatically different to any big motorbike engnie). You could do it on a dyno, which is good for finding peak output, but be well aware it can take a _long_ time to get the fuelling good for full power at high throttle openings, amd for part throttle cruising/idle etc. You can do that on a dyno, but the amount of time involved, you'd want to be good friends with the dyno operator, or that process will cost a fortune. Another option (and one I'd recommend) is getting one of the wideband ego sensor/reader/logger kits. There's a few brands and I haven't heard anything bad about the wbO2 and innovate (spelling) one. Using one of these you can find a quiet enough factory area or whatever and set it up, logging the a/f mixture and slowly cruising around through all throttle positions (obviously if it starts going lean at higher rpms stop before it can do any harm, and alter the settings (possibly jet, possibly air corrector, or in teh case of motorbike carbs jet/needle profile/needle height setting) and continue. Driving around and logging like that will allow you to get right in on the ideal settings, and whilst the wideband kits aren't dirt cheap, in the long run they wil actually end up saving you money, since you don't need anywhere near as much time on a dyno, within reason you won't spend as much time/cost of parts, because you have a more accurate picture of the a/f curve with any particular setting and know which to change and by how much, and then ongoing savings because you are also able to tune the part throttle fuelling for reasonable economy.