The Stovebolt Site Forums The Shop Area Engine Shop Float settings - old carburetors - new fuel valves

One area where even an experienced mechanic can have issues is the setting of the float in a carburetor.

Just had a regular customer on a different make of car call with flooding issues. This guy is an absolute stickler for doing things "by the book".

So when he called, we talked about several possibilities, as the carburetor he had came from a commercial rebuilder, and he thought possibly the floats were incorrect (it was a Carter AFB, and Carter used 4 different floats, each with a different buoyancy in the AFB carbs). He had taken the carb apart, and checked all the settings against the book.

Once we determined that he did indeed have the correct floats, we started discussing other possibilities, and float setting became the topic. His factory shop manual (printed months prior to the manufacture of the vehicle) stated 7/32 float setting. I pulled the current Carter prints, and a float setting of 5/16 was recommended. So started checking various generation prints for this carburetor, and one of them stated TWO different settings:

(1) 7/32 for steel fuel needles
(2) 5/16 for neopreme fuel needles

This particular carburetor was built during the transition from steel to neopreme tipped needles.

Since the float exerts pressure on the fuel valve, the neopreme tip is slightly depressed thus raising the fuel level in the bowl above the desired height, if the specification for the steel needle is used.

It becomes apparant how critical this setting when 3/32 of an inch can cause flooding.

Jon.

Great post. Jeffrey

Jon,

Are you saying that the neoprene tip compresses 3/32" (0.093") due to the buoyancy of the float, thus raising the fuel level by that amount? Wouldn't the tip have to be as soft as a marshmallow for that to happen?

Ray W

Ray - absolutely not. The float measurement is taken at the toe of the float, whereas the neopreme tip is close to the heel.

So the change is .093 2.675 from the fulcrum and the needle is only 0.275 from the fulcrum.

Without figuring the trig, I would estimate the deflection to be just less than 0.01 inch. Just doing a straight comparison 0.275 / 2.675 is 0.102; so multiplying 0.102 by 0.093 gives just a tad less than 0.01. Of course, since the distance would be an integral of a sine because the hinge is a fixed point, this estimate is going to be off a tad, but I am too lazy to do the absolute math. Its "close enough for government work" as Dad used to say; and I used to work for the government.

Jon.

1000 internet points to Jon....for even alluding to trigonometry.

Subtract 100 points for taking the bait.
Jerry

"Without figuring the trig, I would estimate the deflection to be just less than 0.01 inch."

Jon,

Thanks for that explanation. Please verify that I have this right. The neoprene tip compresses 0.010" due the buoyancy of the float. Because the "toe" of the float is 10 times as far from the fulcrum as the needle, the float moves 10 times as much as the needle tip due to needle tip compression. Is that it?

Thanks!

Well, close. The float is pivoting on the float pin; so different portions of the float are moving different amounts. The linear distance at the pivot of course will be zero, with the maximum movement at the toe of the float.

The reason I mentioned trig is that the float is moving in a circular motion, and we are measuring linear motion; and they are not the same (but very close).

Reminds me of a math instructor in college who was trying to explain limits some 50 years ago. He placed a couple of boys against one wall, and a couple of girls against the other wall. He then had them half the distance between them, and then half it again, and then again, etc. After about 10 iterations, he made the comment that, like limits, the distance, when halved, will never be zero, but will be close enough for all practical purposes!

Jon

So when does the flux capacitor come into play? I could join the conversation if you are talking flux capacitors. Now we know when you can use all that math in the real word. Practical stuff like getting a Stovebolt to purr like a kitten. I am glad there are guys like you out there with a passion for this.

It was amusing in a pathetic sort of way, to realize how much math, physics and chemistry I had to teach my auto mechanics students in the process of teaching them to fix engines, transmissions, electricity and air conditioning. It was also amusing to see how frustrated my fellow teachers got when they couldn't solve the problems I gave those students for homework assignments!
Jerry

Jerry,
I think what you were dealing with was the difference between someone using math, physics, and chemistry in a real-world application (your students), versus the same math, physics, and chemistry in a strictly egg head realm (the fellow teachers).
I dreaded it when I had to try to memorize the material long enough to pass a test without any reference to practical application.
I did have a high school course called Applied Math. The teacher was very good about making sure that the lessons matched the title of the course.