Not holding ideal

MGMidget · 10-10-2002, 05:25 PM

There are not air leaks, when its humid it ideals at one speed when its cool it ideals at another. Any ideas?

karlInSanDiego · 12-02-2002, 01:21 AM

You say no leaks. Did you check at throttle shafts for air leaks as well? Is dashpot screw in straight? Did you get the proper oil/level in dashpots? Do the dashpots raise and lower smoothly? Is your choke disengaging properly?

When you make a post for help, try to be specific about your setup. What carb do you have, and what type of choke? Midgets came with dual 1-1/4 SU's or a single Zenith-Stromberg. I believe that each of these carbs had a variety of choke methods including mechanical, water, and electric.

MGMidget · 12-02-2002, 04:12 PM

They are SU 1.25, the butterflys have been changed to get ride of the dash pots.

karlInSanDiego · 12-04-2002, 12:58 AM

I'm not familiar with the process that you describe, regarding the elimination of the dashpot. SU carburator function relies on the vaccuum of the intake manifold to drive the dashpot up, thus pulling the needle out of the jet, and enriching the mixture. The dashpot has oil in its center to help slow the progression of this transition.

Here's a web page with a pretty good description of the functions of this carb. Be sure to scroll down to the constant vaccuum ( or constant velocity ) section
http://members.tripod.com/~RoverSD_1/carburation.html

MGMidget · 12-04-2002, 04:56 PM

This is the spring loaded item on the butter fly.

If it does not seat, the carb gets too much air.

karlInSanDiego · 12-05-2002, 12:21 PM

You say that it exhibits 2 different idle speeds in different conditions. Please explain exactly what these idle speeds are. I believe that it's normal behavior for there to be a slight difference.

For example, 0 degrees celcius climate idle 800 rpm and 1100 rpm idle for 22 degrees celcius would be normal. The density of the air in those 2 climates is much different, and the carburetor doesn't have the feedback system that modern cars have to give a precision idle in all climates.

Keep in mind that your engine bay temperature and water temperature can help you understand how much the temperature is varying, but in the end, the air density will have more to do with the air's temperature and humidity.

BTW, the dashpot that I referred to, is the large cylinder that slides up and down in the middle of the carburetor. Your removal of the little spring valve from the butterfly should be a reasonable modification as long as you replaced the butterfly discs in the correct direction (the chamfer on the disc is directional), and plugged the resulting hole in the disc with solder. I've heard you should use something soft like solder in case it ever comes free and gets breathed into the engine. Also you need to align the disc very carefully so that it seats flat when closed.

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