Hi Frank
On venturis The best is to make roughly what Lauri has posted a picture of, with or without the small fuel post, I have several decades of using and testing almost everything venturies/resrtictors. Given the exact same area a true venturie will outperform a restrictor, it gives more power, it seems to prime much easier, and transitions are better.
I use a flared bellshaped mount at the top, going to about a 7 degree angle, then the transition point it changes sharply to a 11to12 degree angle. The change needs to be sharp, and not smooth, this is the point the fuel feed should be, with the hole just below the angle, I try to make the top of the small feed hole just touch the angle, or if you use a fuel post put it there too. We sometimes used R/C needles as fuel post that screwed in for engines that did not have the extra aluminum either front or rear of the venturie to hold the spray bar, or just made them. the OS 2A works pretty well for that. I have used many of them in OS and Evo 36 engines threaded into the venturie itself.
Do not drill the venturies out to change them without cutting the angles back into them, this will ruin them. And do not make the feed hole any larger than the hole in the spraybar.
Randy