DaveNC beat me to it. If you want us to start another thread for this discussion, we'll move on. I don't want to hijack your thread either. At this point, we're just discussing theory that sort of pertains to some of what you're doing. Just say the word and we'll start a new thread.
Hi Dave:
Where are you in NC? We might be next door neighbors. I'm on the southern border of NC in central VA.
Here's the essesnce of the battle being fought by many of us who are trying to tame high HP engines to drive them on the street with good manners at 1 - 2% throttle opening.
You've most likely got a very responsive engine that doesn't need much throttle opening to make the few horsepower it takes to move it down the street (or highway). You've probably got a carb with large throttle plates. Larger throttle plates = smaller opening angle to achieve a given airflow. Many times, this small throttle opening puts your throttle plates smack-dab in the transition slots. Note also that your plates most likely aren't open enough to get the main circuit flowing much, if any at all. So you're cruising down the road operating almost solely on a circuit that is dependent of manifold vacuum, not airflow.
Compounding this problem is the fact that you perhaps have a cam that produces low idle vacuum. So, you get out your box of drill bits and brass set screws and get the Idle Feed Restrictions (IFRs) and Idle Air Bleeds (IABs) sized just right. You've got that sucker idling great with 1 1/2 turns out on the mixture screws; the holy grail according to everybody. Life is good.
Problem is, out on the highway, you're tooling along running pig rich at 20 or 22" hg. Open up the throttle and the carb seems to behave just fine, but you want to drive this car on trips, and this simply won't do.
The problem here is twofold. One, you're driving on the t-slots and not the main circuit. Two, the transition circuit is rich because orfices that are propely sized for 8" hg are too large at 20" hg.
This is why the factory embraced spreadbore carbs when emissions laws tightened up. The smaller primaries allowed greater throttle openings, which ensured that the main circuit was working.
OK, on to your question. Fuel and air for the transition and idle circuits comes from the IAB and the IFR, as you know. After the fuel and air are mixed, there is a "fork in the road". The mixture flows unrestricted to the t-slots, while it flows to the curb idle discharge ports after being restricted by the idle mixture screws.
To limit the transition flow, use the smallest IFR/largest IAB possible and still get a decent idle. Your idle screws will most likely be more than 1 - 1/2 turns out, but as long as you still have adjustability and the idle doesn't suffer compared with what you have today, it's a compromise that we must endure. Once you reach the point where the idle screws have no response, you've gone as far down that path as you can.
Further, any effort that achieves greater idle vacuum allows you to further restrict the idle circuit (IFRs and IABs), thereby restricting the transition circuit. Note that you are still cruising on the transition circuit, but at least you've found a way to lean it out somewhat.
In a perfect world, we'd be able to adjust the length and width of our t-slots. Alas, our world is not perfect.
You can also make and install t-slot restrictors is the main body. This sometimes helps when engines have low idle vacuum/high cruise vacuum, but there are problems inherent with them that usually rear their heads before the problem is totally fixed. I'll elaborate later, but this post is long already.
Back to Y07_A66's results: he's got a very good starting point. He has a carb with annular boosters which is helping the mains get started at lower airflow. Y07_A66, when you get a chance, post your idle vacuum, cruise vacuum, and number, position, and size (if known) of your emulsion bleeds. Based on your data, you don't have any rich spots at light throttle/high vacuum. I'm curious how high your "high vacuum" gets.
