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Nice guy eddie
 
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Carburetor float level question:

I have a general question about float level or fuel levels in float bowls of carbs; I know that if the fuel level is too low in the extreme, the engine will starve for fuel under full throttle acceleration and if it's too high in the extreme, it will overflow and go places it's not supposed to go(?)

My question is what difference does it make high/low in between those extremes? IOW, some carbs have a fairly precise float level adjustment-what difference does it actually make if it's off a little but not to one of the extremes?

Most of us are through w messing w carbs, I still have some old cars and bikes that have them. My reason for asking is the Zenith carbs on my old Mercedes, which are supposedly very sensitive to float level but the question is general in nature for my edification. Would you ideally want the level on the low side of acceptable but just above *fuel starvation* level? TIA.
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Old 09-29-2019, 12:00 AM
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On most of the carbs that I work with the float level has a range of plus or minus about a half millimeter, maybe 1 mm at the outside. Just set the level at the correct height and be done with it. You don’t need to add any more variables when tuning a carburetor.
Old 09-29-2019, 04:00 AM
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I replaced the jets and needle in my Honda XR last winter because it came from altitude in Montana. Prior to doing this, fuel regularly overflowed onto the ground if the tank petcock was left open. I fiddled with the float a bit while I had the carb apart and after reading and watching YT it occurred to me that I would want the fuel bowl to be as full as possible without overflowing which no longer occurs. I cannot think of a down-side to having the bowl full of fuel but as you stated, there is a potential down-side (lean running) to having it closer to empty.
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Old 09-29-2019, 05:28 AM
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Here’s the issue. The fuel level does affect how much fuel is drawn into the carb and how quickly. The jets are selected with the fuel level where the carburetor designer intended it to be. If you change the fuel level, then the jets are likely to be no longer the optimum size. That gives you another variable that you now have to screw with. It’s actually a little bit more involved than that, and I don’t want to get too deep into the weeds.

If you think you know more than the guy that designed the carburetor, go for it.
Old 09-29-2019, 05:48 AM
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Quote:
Originally Posted by javadog View Post
On most of the carbs that I work with the float level has a range of plus or minus about a half millimeter, maybe 1 mm at the outside. Just set the level at the correct height and be done with it. You donít need to add any more variables when tuning a carburetor.
I agree w you 100% but the Zeniths have a fairly wide spec and it's adjusted by using different sized washers under the needle jet. Think of the sealing washer when you screw it into the casting, they actually have MB part#s for .05/1.0/1.5/2.0 mm. I raised the float level w a thinner washer when I was having another issue which I had misdiagnosed at the time, (honestly, I was just chasing my tail but that's another story). I'm thinking that I made it too high because while I solved the original issue, now the car is not running optimally and I think I'm overfilling the carbs, even though the current washer is about 1mm.

The range is about 2mm according to spec sheet, (of float height). It's hard to measure w great precision because of the design and the upside down float is not level when resting but I can get close. Thanks for your input.
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Old 09-29-2019, 10:29 AM
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Originally Posted by afterburn 549 View Post
Hip shot-
Venturi affects, AFAIK you want the level in spec because as you say it will lean or rich.
With my AFR meter hooked up, you can readily demonstrate this on a HSR flat slide.
I would guess-just a guess the further up the jets the fuel has to be pulled (pushed) the leaner the mix will be.
Pretty good guess, I think you are correct. I've always thought that you wanted the level on the low side of operational acceptability, if that makes sense.
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Old 09-29-2019, 10:31 AM
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Originally Posted by speeder View Post
Pretty good guess, I think you are correct. I've always thought that you wanted the level on the low side of operational acceptability, if that makes sense.
Nine times out of 10, you always want to shoot for the spec that is the design dimension.

I’ve never screwed with a set of Zeniths for an old Mercedes, most of my work involves modern motorcycles, but I usually measure the float level with the carburetor on it side. If you don’t have a float level gauge, you can make one pretty quickly out of a piece of scrap metal. It’s hard to give you any more advice, without even knowing what one of these damn things looks like.
Old 09-29-2019, 10:47 AM
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To answer your OP. Simply put - carbs are designed to deliver a metered flow through-out the RPM rand and load of the engine it's feeding. The capacity of the float bowl is really the key to keeping fuel supplied under all conditions. The float level in my experience can be slightly above the spec or below and the carb will still function correctly.

When setting any float - again my experience - I set just below the spec. I want to make sure the float bowl is not over filled. This prevents fuel spillage out of the vent during hard acceleration or cornering. What ever the application - you can bet the fuel pump and pressure line size is more than adequate to provide both flow and pressure to the carb as it was all designed. So the float could be set on the low side of volume - (less fuel in the bowl) but you'd never see any difference as long as everything else is functioning as designed.
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Old 09-30-2019, 08:20 AM
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Getting a little deeper into the weeds, the fuel level height in the float bowl affects full throttle/low rpm and, also, richness or leanness at cruise/low rpm. You may or may not care about that, as the car doesn't make enough power that this might be noticeable.
Old 09-30-2019, 08:32 AM
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I cannot comment specifically about Zeniths but only in general. Think of float level (actually the gas level in the float chamber) as water over the spillway of a dam. Gas is sucked into the venturi by the velocity = low pressure theory. Throttle response coming off idle is best when the fuel is just level with the top of the dam. Too low and you may have a stumble, which in many cases is handled by the accelerator pump. Too high and you have a flooding situation. This happens mainly coming off idle as opposed to the operation while driving but if the float level is out of whack by enough margin you could experience starving or a rich running condition throughout the range of throttle position.

Using the dam example, a wind blowing water over the spillway is the draw of the venturi. If the water is low behind the dam, it takes more wind to get some water over the dam. Obviously, if the water is high, it runs over by itself with no demand.

Oversimplified.
Old 09-30-2019, 09:01 AM
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To add to that, basically the air rushing through the gap between the throttle plate and the holes in the bottom of the carburetor throat creates a vacuum for a given airflow, the greater the airflow the greater the vacuum. I say vacuum, it’s more proper to say the atmospheric pressure is less in the carburetor throat than in the float bowl. That forces fuel up through the various jets and passages, and 34mm into the throat of the carb. The lower the fuel level, the longer this takes. It’s not instantaneous, which is not why the accelerator pump is there. The purpose of the accelerator pump is to deal with quick increases in the throttle position, which affect the vacuum signal and reduce the fuel flow as a result.

Anyway, here’s a link that might help:

Mercedes Benz Zenith Carb Manual – JaimeKop.com
Old 09-30-2019, 09:46 AM
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Originally Posted by javadog View Post
To add to that, basically the air rushing through the gap between the throttle plate and the holes in the bottom of the carburetor throat creates a vacuum for a given airflow, the greater the airflow the greater the vacuum. I say vacuum, itís more proper to say the atmospheric pressure is less in the carburetor throat than in the float bowl. That forces fuel up through the various jets and passages, and 34mm into the throat of the carb. The lower the fuel level, the longer this takes. Itís not instantaneous, which is not why the accelerator pump is there. The purpose of the accelerator pump is to deal with quick increases in the throttle position, which affect the vacuum signal and reduce the fuel flow as a result.

Anyway, hereís a link that might help:

Mercedes Benz Zenith Carb Manual Ė JaimeKop.com
I've been using that manual and it's excellent but as you can see, it lists a fairly wide range of float level spec.
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Old 09-30-2019, 09:52 AM
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I didn’t read it, all I can tell you is I shoot for the design spec. If I’m going to go to the trouble of adjusting it, it might as well be right on the money as opposed to “close enough.”

One thing I’ll share about my experience with carburetor work. If I’m going to go to the trouble of removing a carburetor, cleaning it and rebuilding it, I’m going to go through the entire cotton picking thing and adjust everything as perfectly as I can. I only want to screw with it once and once I get it back on I don’t want to do anything other than set the idle mixture, the idle speed and do any synchronization that’s required on multiple carburetors.
Old 09-30-2019, 09:57 AM
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Quote:
Originally Posted by javadog View Post
To add to that, basically the air rushing through the gap between the throttle plate and the holes in the bottom of the carburetor throat creates a vacuum for a given airflow, the greater the airflow the greater the vacuum. I say vacuum, itís more proper to say the atmospheric pressure is less in the carburetor throat than in the float bowl. That forces fuel up through the various jets and passages, and 34mm into the throat of the carb. The lower the fuel level, the longer this takes. Itís not instantaneous, which is not why the accelerator pump is there. The purpose of the accelerator pump is to deal with quick increases in the throttle position, which affect the vacuum signal and reduce the fuel flow as a result.

Anyway, hereís a link that might help:

Mercedes Benz Zenith Carb Manual Ė JaimeKop.com
Also the fuel/air ratio is lower on first acceleration, like 9:1. WOT is more like 13:1.So, yes, a lot going on and it's easy to over simplify. A lot of folks working on carbs know how to fix them w/o understanding them. It takes a George Bignotti.
Old 09-30-2019, 12:56 PM
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Originally Posted by speeder View Post
I've been using that manual and it's excellent but as you can see, it lists a fairly wide range of float level spec.
you might want to find a forum of MB Zenith carb owners & see if they have figured out which end of the design spec. is best (or best for what conditions...)
Old 09-30-2019, 02:39 PM
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Shoot for the middle of the spec. I’ve never seen a float height spec that wasn’t using the middle of the range as the preferred setting.
Old 09-30-2019, 02:41 PM
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Quote:
Originally Posted by speeder View Post
I have a general question about float level or fuel levels in float bowls of carbs; I know that if the fuel level is too low in the extreme, the engine will [B]starve for fuel under full throttle acceleration and if it's too high in the extreme, it will overflow and go places it's not supposed to go(?)
(I'm talking out of my layman arse here...but I was under the impression: )

There is a float with a cutoff needle fill valve which always keeps the bowl level within range specs.
-The air-filled floats will crack and sink which allow the bowl to fill up completely.
-Fuel pumps usually have an internal delivery pressure regulator, but if the delivery pressure is too high it will overpower the needle valve cutoff and also flood the engine.


The main jet mostly regulates the fuel flow into the venturi where suction pulls it into the air stream to swirl and particalize.
-More swirl and turbulence makes a better mixture when warm, at a a loss of top end.
-More swirl and turbulence makes a worse mixture, when cold when it gels to the cold sides and carburetor heat is needed.


Under WOT/full acceleration, an extra long squirt of gas is pumped directly into the middle of the carb by the accelerator pump.
More gas=rich mixture=less chance of lean detonation under full load.


Idle -vs- under normal operation -vs- under WOT/full load are different things to a carburator.
A bad idle doesn't necessary mean bad operation, unless there are other mechanical faults of the engine.

Idle is controlled by the butterfly valve closing completely, where a separate bypass keeps the air moving faster and the fuel pulled into that limited but very fast-moving airflow.
-A notch in the butterfly allows some bypass air.
-A smaller separate bypass channel is controlled by an idle-air+idle-mixture screw.
-Those passages are smaller, so EGR waste and/or particles will block them up easily.

Last edited by john70t; 09-30-2019 at 06:48 PM..
Old 09-30-2019, 06:44 PM
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