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R2g carb question

FinoCJ

FinoCJ

1970 CJ5
Staff member
2026 Sponsor
Joined
Jul 18, 2013
So most discussions about the amount of fuel in the fuel bowl are focused on float height and drop. But I am wondering how much fuel should be in the fuel bowl. Is there a critical height it must be over to function?
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The main jets and the PV are in the bottom of the bowl, so just about any amount of fuel in the bowl will cover them. The a-pump well has a gravity feed via the aluminum check ball inlet at the bottom. But that will only fill up to the same level as the main bowl.....there is also a slot between the pump well and main bowl to help equilibrate the level in both. How much depth of fuel is needed in the a-pump well for it to fully function through a full pump stroke.
 
A carb is essentially a siphon, fuel is drawn through the jets and up to the venturis by the vacuum going through the carb. The level of fuel has a direct effect on how much fuel gets drawn into the engine, thats why float level spec is important.
 
colojeepguy said:
The level of fuel has a direct effect on how much fuel gets drawn into the engine, thats why float level spec is important.
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Yes ...but that is not the question....I want to know what the equivalent fuel level is for the given float level. In theory, if someone had the float level set correctly, and pulled the top off, where is the fuel level.
 
I'm not sure how you'd determine that on a 2G, since the float is attached to the top. Some carbs (Motorcraft 2BBL comes to mind) have the float pivot in the main body and you could see the level simply by removing the top.
 
The gas level on my Rochester 2G when I pulled the top off was just above the jets, about 1/4”.

Interesting fact, not all had the aluminum check ball for the accelerator well. (Mine does though)

They all have the steel ball with spring under the stake though.
 
This is a tricky question. If the fuel level is 1/4" above the jets with float removed, that only gives one data point.
As soon as the float is installed, that same level will rise as the float displaces some of the fuel.
 
dnb71R2 said:
This is a tricky question. If the fuel level is 1/4" above the jets with float removed, that only gives one data point.
As soon as the float is installed, that same level will rise as the float displaces some of the fuel.
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Yes! Then the level is right at the bottom of the “Slot”
 
dnb71R2 said:
This is a tricky question.
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colojeepguy said:
I'm not sure how you'd determine that on a 2G, since the float is attached to the top.
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that is why I am asking this forum...:D

Beach66Bum said:
Yes! Then the level is right at the bottom of the “Slot”
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That is kind of what I think....There is a minimum height the fuel needs to be in the pump well to get the full amount of fuel 'squirt' from the a-pump. If you drop the flower lower than that, the a-pump won't function to full capacity. Obviously, the a-pump linkage rod also needs to provide for full pump stroke.....is the bottom of the 'slot' sort of an unofficial target fuel level (with float in place)? My testing suggests something at or just below the slot gives full squirt
 
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Beach66Bum said:
The gas level on my Rochester 2G when I pulled the top off was just above the jets, about 1/4”.
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What float is in your 2g? There are a few different floats available for the 2g.....Maybe we have the same float....

Beach66Bum said:
Yes! Then the level is right at the bottom of the “Slot”
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Are you sure.....so with the float level set to spec, I have about 1/4" in the fuel bowl when the top is removed.....but that level (which is higher when the top is put back on) just doesn't seem to provide enough fuel in the a-pump well to get a full squirt. With the float set high, I definitely get more volume of fuel in the squirt, but raising the float level above spec comes with its own set of problems (especially off road).

FWIW - the brass float in my 2g has a 'water line' mark from where it sits in the fuel - presumably from being parked for periods with a full fuel bowl - its hard to get get 'exact' measurements from the underside of the cover to the waterline, but that gives some indication of the fuel level in the bowl when the float is in place. Its low enough I don't think its getting enough fuel column in the a-pump well.

Eventually, I may need to up the main jets one size to help get more fuel - that might also help reduce the volume need from the a-pump.....but trying not too have multiple variables being messed with.
 
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My understanding - the depth of the pool affects the mixture.
Not as important as the size of the jets, but the depth of the fluid affects the pressure entering the jets.
P = rho g h, where rho is the fluid density, g is the gravitational constant, and h is the depth of the fluid.
Does not depend on the shape of the container.
Adding the floats raises the level and increases the pressure.
Thus the level absent the float is only incidental to the performance of the carburetor.
Float setups regulate this pressure and also buffer the carburetor action from the output of the fuel pump.
They maintain constant conditions which you can tune against.

You have brass or nitrophyl floats?
I would expect nitrophyl to sit a little lower.
What's the depth of the gas whne the floats are in place?
Float height is a legit way to tune a carburetor.
Barring Duffer's fenestrum, yu could set the float in the bowl and see where the level rises to.
 
You could also float the floats in some gasoline, measure how much the fluid rises, and add that to the bowl to see your actual fuel depth.
This assumes you currently have a "full" float bowl.
Not exactly the same, since the float pivot supports a little weight.
You could rig something to pivot down into your measuremet, for more accuracy.
 
timgr said:
You could rig something to pivot down into your measuremet, for more accuracy
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You'd have to do something like that, the float pivot on the carb is holding one end down in the fuel bowl. This would be a lot different than simply placing the float in some gasoline.
 
Kinda think you could just hold the float down in the gas.
Not accurate, but would give you an idea of how much the level is rising.

Kinda wonder where this is heading.
Is ther an active situation that James is addressing or are we exploring?
 
You could always drill and tap a hole for a pipe plug in the side of the bowl so you can check the level like a Holley carb.
 
I had the floats too low in my edlebrock not the same but with the level low like the first picture the jeep wouldn't start in the morning... it would give a halfassed squirt and then nothing. raised the float so as to cover that slot by about a 1/4 inch and now starts fine. your results may vary.
 
timgr said:
Kinda think you could just hold the float down in the gas.
Not accurate, but would give you an idea of how much the level is rising.
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this is kind of what I've done...with the cover off, pushed the float down till the fuel level matched the brass waterline - crude yet gives the ballpark idea.

timgr said:
Kinda wonder where this is heading.
Is ther an active situation that James is addressing or are we exploring?
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well, its both. Ever since I replaced the top of the carb last fall (after the original top suffered irreparable damage at the air horn), I just haven't been able to get the carb running like it used to. It used to be FLAWLESS other than always running a bit rich (even with small jets). With the new top cover, it just hasn't been the same. What doesn't make sense is that most all of the fuel passageways/circuits (idle, main metering, power valve and a-pump) are essentially all tied to the lower portion of the carb which was not changed. The things that would be affected by the new top cover are possibly the float position is ever so slightly different than previous cover, and the a-pump pivot arm and height - both could be slightly different just due to casting differences in the two covers, but those small casting variations are minor compared to range I've varied the float height and pump rod adjustment....and yet, nothing really changes. I cannot get rid of the classic 2g lean stumble during hard acceleration from low rpm, despite the a-pump having two great streams of fuel...my best guess is its the timing of the squirt, or rather a slight delay with the pump squirting. This would usually be addressed via the pump rod adjustment....

So that is what started this, and I went through a large range of rod positions from 'lots of lash' to well beyond zero lash. That didn't solve anything, but I realized that if the float level is higher, then the pump stroke will give more fuel (helped reduce hesitation during hard acceleration from mid-rpm range), but it doesn't make the timing of the shot quicker - its still delayed. I can make the condition worse, but I can never make it better. I've been through a large range in float heights and through a large range in setting the pump rod adjustment.....but that started me going through a parametric study of the variables (float height, a-pump rod adjustment, and eventually metering jets) to see if I can eventually get the right mix of variables. I've had the unit apart maybe 30 times in the last few months continually changing one thing, then putting it back together, driving it around and then repeating for the same variable throughout a range. This has all been with the same small jets that have always been in it. Next up will be to switch the jets to a bit larger (and technically more appropriate for the elevation, but it ran well for years on the small jets) and run through the tests again.

So at the moment, I am not really trying to 'solve a problem'....its a challenge with this forum as everyone wants to give a solution....I am not looking for solution yet, just trying to gather information and explore relationships
 
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