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Carb float questions. If set incorrectly, and float level be somewhat corrected by adj.

October 5 2017 at 7:24 PM

Mike U.  (Login mtrain)
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I've been building some old carburetors from my fathers shop.

Some of these were carbs made in the 50's, and early 60's. I can't seem to find accurate float adjustment measurements.

As most of you know, the floats on most carburetors aren't externally adjustable like on an 1850 Holley. The floats on most carburetors have to be set inside the carb by bending the tangs, then measuring the floats to a certain level, and drop.

Well, I have checked the specs on some of the old 4Jet carbs I'm working on, and the adjustments are all over the place. It would seem that float adjustments with the same carb, can vary for different vehicles.

The only thing I can figure out is that these vehicles have different fuel pressure settings.

So, in having said that, could the float settings be corrected by simply adding an external adjustable fuel regulator?

The reason I'm asking is that I'm going to be putting these carbs up for sale, so I want to make SURE I can give the correct advice if there is a problem with them as far as the float level is concerned.

Also, just for you guys here is an old Motorcraft 4bbl carb I just finished for a 1974 big block Ford. The float level settings on this carb were pretty close to each other vs the Rochester 4 Jets.

Well Hell Imgur is just too slow tonight. Pictures tomorrow.

 
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Drew Pojedinec
(Login DeepRoots78)
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Re: Carb float questions. If set incorrectly, and float level be somewhat corrected by adj.

October 5 2017, 9:08 PM 

Pressure is different, sure, but also remember needles are different as well.

Lets take two common needle/seat combos

.110 and .097
Lets say we have a constant of 7psi fuel pressure.
To keep the same exact level in the bowl with the same fuel pressure
a .110 needle and seat requires .77 lbs of force to seal
a .097 needle and seat requires .679 lbs of force to seal

Another difference is float leverage. A side hung float provides different leverage compared to a center hung, and obviously a lot of variation with different makes and models.

To answer your question, yes a regulator can be used to set a float level provided it's "in the neighborhood"
I think on Holleys the spec is 1psi= 1/16 of an inch of fuel level in the bowls with side hung. (roughly, it's info from an old manual)


    
This message has been edited by DeepRoots78 on Oct 5, 2017 9:57 PM


 
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Mike U.
(Login mtrain)
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Yes Drew, I was trying to say, but failing, that its "in the neighborhood", or close. If

October 6 2017, 2:43 PM 

Where the float just thrown into the carb, as in not knowing where its sitting then I wouldn't chance regulating fuel pressure would help much, if at all.

Thanks for breaking it down for me.

 
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eric
(Login wwyin62)
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if it is a tang float,

October 6 2017, 5:52 AM 

you should not have to do any adjusting, it was set at the factory. with a new needle & seat it could change, but not enough to cause a problem. With a new float, yeah, it will need to be set. I bench test them with a jug of gas & a small electric fuel pump & pressurize the carb with fuel with the top off (if that is possible) and look at the level & check for flooding & accelerator pump action.

 
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Mike U.
(Login mtrain)
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Good idea Eric I wll give it a try with mineral sprits as I can't put gasoline..........

October 6 2017, 2:46 PM 

I'm afraid to put gasoline into the carb for fear of a buyer accusing me of selling a used/already ran carb. I know that sounds silly, but we all know how some people are.

Thanks again.

 
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Gerry
(Login gerryproctor)
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Beyond slight mechanical differences mentioned...

October 6 2017, 7:02 AM 

Fuel level has an affect on A/F ratio. A higher fuel bowl level makes the carburetor run richer. A lower level runs leaner. It has to do with the weight of the fuel...darn gravity. This is rarely a consideration by most folks since very slight differences don't seem to matter much, but it does have an affect. You see this in carburetors with a long service life used in many vehicle applications and why rebuild kits contain a laundry list for vehicle-specific float adjustments for what is essentially the same carburetor. The slight difference account for different carburetor calibrations for different vehicles.

A long time ago, an old timer said; "If you want to understand why it's important to get your tune up right, work on a single or twin cylinder motorcycle." These words are true. I own a 1973 Honda 450. Float settings seeming minute have a dramatic impact on how rich or lean the carbs run.

 
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Drew Pojedinec
(Login DeepRoots78)
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Re: Beyond slight mechanical differences mentioned...

October 6 2017, 7:38 AM 

On a Holley or Autolite the fuel level has a direct impact on when the main boosters start to flow as well.

 
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Mike U.
(Login mtrain)
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Thanks Gerry, that explains the differences. I know on my turbo charged engines with a car

October 6 2017, 2:48 PM 

I know on my carbureted, turbocharged engines, I keep the floats set a little on the high side. [along with other things/adjustments]

 
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