Tillotson help please

Has anybody ever measured the pop off in a Tillotson carburetor straight from the factory? Or any other carburetor for that matter! I remember it being very high, why do you suppose they do that? Surely they know that most people are going to great pains to lower the pop off to whatever. And yet they keep building carburetors with high pop off, makes me wonder! You need some pop off to keep the engine from flooding while sitting in the pits, so I always set mine at 10 psi, just because everybody else was doing it. It didn't seem to hurt anything, but I can't say it helped either. I knew a guy who insisted that, on a KT, the best pop off was 3 psi, and he won a lot of Sprint sit-up races, proof of his expertise, maybe?
The one time we raced against him, with the pop-off set at 10 psi, we led him on every lap but one. Unfortunately, our driver misread the white flag and thought it was the checkered flag. Actually, he saw the checkered flag for the group that was in front of them (that group was lapping his group) and thought it was for his class.
 
Al , It's been around 10 years since we had this debate about pop-off on Bob's and your still saying you just need "some". I'm here to disagree again, some won't get it done. It depends on the carb, whether it's alky or gas, venturi size. for example , the Walbro WA55 with it's .437 venturi runs a high 20 lbs pop-off bolted up to a KT be cause of it's signal, a Tilly, alky limited mod carb needs 8 to 10 at the most. Switch those pop numbers around see how the limited runs on 20 and the wa55 handles 8 on a kt, the limited would never start, the wa55 would be untunable with the needles barely cracked. If you have been reading all posts there's a great example of adjusting the pop-off. I'll repeat it for you . The engine was hard to start at 13 lbs pop, he dropped the pop to 7.5 and cured the problem, a 4.5lb difference changed the fuel flow enough to get it started . 13 is some , 7.5 is some. It's a little more complicated than you let on.
 
Is the pop-off basically the Tilly equivalent of the float height....too high and the jets run out of fuel and run an "apparent" lean condidition (engine's still pulling air, but jets are starved), but if it's too low, engine can access the fuel vaporization at lower throat pressures? I'd think the "ideal" would be where each jet is optimized for the pressure it's seeing at the RPM range that occurs most often -- ie. you can tune the carby for high rpm or low rpm optimized, or a middle range where top and bottom both suffer a little, but throttle response across the ranges is better?
 
Hey Ted, I thick fulcrum arm height is more the equivalent of float height than pop-off. For one they are both related to as heights, but most importantly both have a huge effect on the amount of available fuel. Obviously float height sets the level of fuel in the bowl, fulcrum height is not as obvious but the lower the arm the leaner the setting, I guess because the demand diaphragm has further to move. In a Tilly arm height and pop-off work together to insure there is adequate fuel supply in the carb.
 
Jon Brogden said:
Hey Ted, I thick fulcrum arm height is more the equivalent of float height than pop-off. For one they are both related to as heights, but most importantly both have a huge effect on the amount of available fuel. Obviously float height sets the level of fuel in the bowl, fulcrum height is not as obvious but the lower the arm the leaner the setting, I guess because the demand diaphragm has further to move. In a Tilly arm height and pop-off work together to insure there is adequate fuel supply in the carb.
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Great info Jon.
Thanks for the knowledge .
 
What type of kart stand do you use, my stand picks up from the rear, when I started using it I had starting problems. I figured out the fuel was draining out of my case , I just carry a baggie over the air filter, it starts fine
 
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