I finally got started on the throttle body adapter!
Using the gasket that came with the intake as a pattern, I cut a piece of 3/16″ plate to shape and cut the holes in it.
I struggled for a couple of hours trying to fit existing tubing and a couple different O-rings to the base of the throttle body. I tried using exhaust reducers. They came the closest to fitting and I guess it was worth trying, but the only combination that seems likely to work will be to gently stretch a 2″ fitting so that the smaller diameter end is very slightly larger than it is now. It will not be easy to do that and keep it round and O-ring-sealing smooth.
Luckily, I may not need to. After noticing that my hatless short sleeve work uniform was inadequately warm for the falling temperatures in the carport, I came inside and found that a best offer I’d submitted on a pair of actual EX650 intake boots was accepted.
There is a slim chance that I may have to remake the plate for these intakes, but it will be worth it to not have to deal with the precision required to seal and attach the throttle bodies to the intake manifold!
Since I’m waiting for parts now, I thought this might be a good time to apply some thinking out loud time to X-Tau acceleration enrichment.
In short, this is an enrichment that compensates for how much of the injected fuel sticks to the walls of the manifold. This is another one of those things that I’m so very glad that someone else had to figure out. As astonishingly brilliant as I am, I’m not sure it would have occurred to me. Anyway, the problem is that a percentage of the fuel going into the manifold sticks to the walls. This fuel eventually evaporates and makes it to the cylinder, only to be replaced by more injected fuel. In a steady state load, the sticking and evaporating fuel reach an equilibrium, but it can become a significant factor with changing loads, making the mixture unexpectedly lean during acceleration and unexpectedly rich during deceleration.
It would seem to me that intake tract length could be a large factor in this phenomenon, and in my configuration will in fact have a relatively long intake tract.
On Buzz, the intake tract was about 5 inches between the injector and the intake valve and there just isn’t much manifold wall to stick to. That doesn’t mean that it’s completely not a factor, only that its a small factor.
On the VW, in my configuration at least, there is a good couple of feet of intake tract between the injector and the intake valves. Additionally, the intake tends to be cooler near the injector and warmer near the valve. This temperature differential is why most VW manifolds have tubes to circulate hot exhaust gases through the manifold, so that the temperatures along the intake tract will be more consistent. A cold manifold will condense vaporized fuel, exacerbating the manifold wall wetting problem.
Honestly, getting the engine running will not require much attention to X-tau compensation, but tuning it for best performance will.