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Carb question


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I pulled the carb off my A15 last weekend and cleaned it up then made new gaskets and remounted it.

I also replaced all the vac caps as some were starting to leak.

I believe the main problem was found when I removed the jet needle screw and spring.

They were all gunky with tumorous looking growths.

I cleaned them and reinstalled everything and it went from running like total crap to running smooth and steady.

My gas mileage had dropped to the low 20's and it would either idle at 2000 rpm or die.

After the remount and cleanup of the carb it now idles and runs correctly and the gas mileage is at 30 mpg.

This is after a week of driving back and forth the 15 miles to work on the freeway.


Here's the question (finally):


On the primary there was a tube extending down into the manifold about 1.25" long.

What was this tube for?

I left it out, and it seems to run great.

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Some of the 720 have them too. My theory is that it's to prevent the warmed air and fuel from rubbing against the cold manifold on cold start up and run. External air is run past the heated exhaust manifold and then through the carb. Warmed air prevents carb icing and helps vaporize the fuel. The tube prevents the air from swirling around the cold base of the carb. It reduces cold start emissions is all and will run fine without.



The two on the left use an electric heater to warm the air fro reduced cold start-up emissions. The two far right ones direct air flow to the runners inside the manifold. The runners are water heated.




Maybe there's another reason for them?




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