Q-jet float level choice

Bonnewagon

Bonnewagon

Geezer
Sep 18, 2009
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Queens, NY
#1
Let's say I have a 1978 Pontiac 400 but no carburetor, and I take a 1978 Buick 350 Q-jet and build it to the 400's specs. Would I use the Buick's 7/32" float level that is spec'd for that carb number, or would I use the 17/32" float level spec'd for the 400? They are not even close so it is a big decision. What I noticed right off is that at the 17/32" there was not enough fuel in bowl for the accelerator pump. I raised it up to 12/32" and got a decent pump shot but still have a crummy idle. It wants more fuel. So right now I have it apart looking for dirt in the idle passages and found some. But then re-assembling it I realized that the 7/32" is even higher than the 1/4" I usually use on all my Pontiacs. So I set it at 7/32" and I will test as soon as the remains of Florence get done with us. So what is the proper procedure? The float level of the carb or the current application?
 
airboatgreg

airboatgreg

Royal Smart Person
Oct 2, 2016
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#2
Start at 1/4", Use Tee-gauge, hold down the U shaped retainer with one finger and gently seat neeedle with clip attached to float with other finger. Float level usually will not affect idle so there may be other issues
 
pontiacgp

pontiacgp

Canadian Prime Minister
Mar 31, 2006
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#3
Interesting question, are you using the brass float?.. Are you using the inlet size from the Pontiac or the Buick or are they the same. Do you know if the fuel pressure for the 78 buick is the same as the 78 Pontiac, that would affect the float level. I would agree with you to set it at 7/32" and see how the engine likes that.
 
airboatgreg

airboatgreg

Royal Smart Person
Oct 2, 2016
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#4
Use the nitrofill float
 
Bonnewagon

Bonnewagon

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Sep 18, 2009
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#5
It is the plastic float. The carb is totally stock except for jets and rods. Those are 72/45 as per '78 400 specs. I have no problem using 1/4" as that is my
go-to setting. Sometimes it causes flooding and needs to be lowered, but that depends on the engine. It was flooding at first because some idiots plugged the tank return fitting on the pump. I added a "T" fitting and ran the return fuel right back into the supply line. End of flooding. I know it is the carb because I swapped my own '77 TA Q-jet on and it ran superb. That is also a Buick 350 carb I set up with 73/44 and electric choke. So I know there are no vacuum leaks and the ignition is right on. When I place my hand over the carb and choke it she revs up and smooths out. Needs fuel. I found the idle circuit to bit a little clogged so I cleaned that out. Off-idle and WOT are great.
 
Last edited:
spidereyes455

spidereyes455

Greasemonkey
Mar 6, 2013
189
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28
Northeastern PA
#6
If the other carb worked great I would try whatever that float is set at. Where did you get the float settings at? Might be a misprint especially since the "correct" setting didnt even have enough fuel in the bowl for the accelerator pump. Thats an awfully big difference, might also try checking more than one source for the specs. Since logic would seem to dictate that a larger engine usually uses a higher float setting since it generally uses more fuel. Another point to consider is do both applications use the same part number float? If they are different that may account for the large difference in the settings
 
Bonnewagon

Bonnewagon

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Sep 18, 2009
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#7
I got the specs from Chiltons, Motor, and Jiffy Kit sheets. I double checked them all and actually 11/64" - 17/64" is pretty much the range for emissions era carbs. The 7/64" is actually an oddball and it is only for this specific carb # 17058240. Both carbs are for Buick 350's and one is a '78 and the other a '79 so essentially identical. I spent yesterday carefully cleaning the idle passage that goes up from the main jets. It went from almost no carb cleaner spray flow to a nice gurgle. That would maybe explain the poor idle if little fuel could climb the passage up from the bowl and into the idle discharge circuit where it mixes with idle air. This was a Fleabay purchase and it was loaded with charcoal canister granules so perhaps some got trapped in there. Either way I will test it soon. My personal rule is to go with the highest float setting that does not flood so there is plenty of fuel for the accelerator pump. A high float also makes it easy for idle, off-idle, and main circuit fuel to get going. It's raining again so test must wait.
 
Bonnewagon

Bonnewagon

Geezer
Sep 18, 2009
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#8
Rain stopped, Sun came out, carb went on. Result: idle vacuum went from a wheezing 17- 18" to a steady 20- 21". And slowly increasing throttle by the idle speed screw made even more vacuum, no hunting or hiccuping. So the 7/32" float setting was what she wanted. We took it out on the street and let her rip- two long patches of rubber- all you wanted- at will, So the 72/45 jets/rods combo is right on the money too. Idle is still a bit rougher than I like but I have no idea what cam is in this engine so no sense banging my head on the wall until we open it up. The question was- carb float setting or application float setting. I don't have an actual '78 400 carb to try but I used it's 17/32" spec and it was terrible. The 7/32" '78 Buick 350 carb spec kicked ass. So I guess it is the float setting for the actual carb being used that is indicated.
 
pontiacgp

pontiacgp

Canadian Prime Minister
Mar 31, 2006
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Kitchener, Ontario
#9
Thanks for coming back to let us know what you found. That is interesting that the float level is carb specific.
 
Bonnewagon

Bonnewagon

Geezer
Sep 18, 2009
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#10
I think that it is maybe the carb's internals that are specific to the application. Pre-emissions carbs all had high float settings and rich mixture screws. Emissions really threw things out of whack. The '78- '79 TA 400 motor was like the last gasp and they managed 220hp while meeting strict emissions. IIRC they measured pollution-per-mile or something and big motors could not make the grade. Now that I think of it I probably sub-concisely used the carb setting unless I was dealing with a non-stock camshaft that needed more. But 1/4" always worked best with my Pontiac 400.
 
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