Tuning by float level?

Discussion in 'The Venerable Q-Jet' started by Electra Sweden, Mar 15, 2023.

  1. Electra Sweden

    Electra Sweden Well-Known Member

    So there is quiet a lot of information online on how to tune-up the Quadrajet by changing the primary and secondary nozzles and metering rods, as well modifying the sizes of certain passages.

    However, I have found none on what to do with the float level. As far as I know for instance all Buick 455 1970 had identical Quadrajets apart from the individual tunings and the part number corresponding to that. As basically every engine model has individual float level specs this must be a tuning parameter of some sort. So how to go about the float level when custom tuning a Quadrajet? If the carburetor is already tweaked, it sounds unreasonable that the float level should be left at the stock level. Trying to asses the overall air fuel ratio and adjust the float level only if it is lean or rich in general? Reasonably, a higher float level would result in a richer mixture overall and vice versa?
     
  2. BQUICK

    BQUICK Gold Level Contributor

    As far as I know the quadrajet is designed to run at a set float level.
    That said an easy way to raise or lower it is with fuel pressure. On my Holley Dominator for racing I run more fuel pressure (like 8.5psi) to raise float on good air days for slightly richer mixture and less on hot humid ones when I might set it as low as 5.5. But...Holley has 2 bowls so maybe more effect.
     
  3. CJay

    CJay Supercar owner Staff Member

    I would go by the casting number on the carb and set it for that. Float level isn't going to make any difference in performance on your Electra. This isn't a max performance racing application.


    I'd be more concerned about air fuel readings at idle and cruise
     
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  4. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

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  5. Electra Sweden

    Electra Sweden Well-Known Member

    That was a neat trick to tune this!

    It got enough power but the current MPG bothers me. Best MPG I had for a longer trip last year was 11 and I know many people get a lot better than that on these engines.

    Hmm. Should maybe buy that book. Have you read this one btw? https://www.amazon.com/Quadrajet-MPG-Secret-Second-system/dp/1075307716

    Still not entirely convinced though that the fuel pressure and needle seats are the only parameters of importance when setting the float height. Looking through the chassis service manual, all 350 engines had a float height of 5/16" and the 455 3/8". Except for the Riviera, all these engine variations had the same fuel pump and similar operating RPMs. And reasonably the same needles and seats in the carburetor, as well as similar fuel feed paths? If Buick then still bothered to go through the trouble of having different float heights already 1970, there must be something more to it?

    https://www.teambuick.com/reference/library/70_chassis/files/64-d.php
     
  6. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member



    Nope, never saw that one.

    OK, let us know what you come up with.:)
     
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  7. Cliff R

    Cliff R Well-Known Member

    If getting 11mpg on a trip is driving all this I'd go back to "basics". Most likely a fundamental issue someplace if fuel economy is that bad for "normal" driving.

    What vehicle? Engine? Transmission? Converter? Rear gear ratio?

    What has been done to the engine? Cam? Compression ratio, etc?

    What distributor? How is it set up? The distributor is equally if not more important than the carburetor when it comes to engine power, efficiency/fuel economy, etc.

    In any case if you want help tell us about your set-up. Moving the float level around in the carb would be a last ditch effort to "adjust" fuel delivery to the engine when the root problem is likely elsewhere.......IMHO.......
     
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  8. Electra Sweden

    Electra Sweden Well-Known Member

    Sure, it is a complete system. What I basically know for sure is that it is a 455 -70. And stuff has been done to it here and there, probably :) I bought a new HEI ignition system for it that I will try to tune in as soon as I have the engine in one piece again. The old ignition system did not have working vacuum advance so I would be a bit disappointed if I didn't see clear improvements from this upgrade. But when that is done it would be nice to tune the carburetor against an AF meter. That shouldn't be a bad thing to do regardless of the rest of the system, I believe. And when I get there I wonder a bit about what tuning options there are.
     
  9. Cliff R

    Cliff R Well-Known Member

    No vacuum advance? No wonder it's a gas-guzzler. It's missing a LOT of timing at light engine load. I wasn't aware they made "new" HEI distributors for Buick engines. Hopefully it's not the same off-shore JUNK they are selling on-line for Chevy, Olds and Pontiac V-8's. None of the ones I've seen are worth two squirts of duck poop right down to the POS drive gear on them that wears out very quickly. Here's a pic of the gear from one that a local guy here bought for his 455 Olds V-8 build. He brought the car here for me to custom tune. I removed the distributor (bought from Summit Racing) and found the gear worn slam out. He had about 200 miles on it.......FWIW.....
    IMG_1164.jpg
     
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  10. Mark Demko

    Mark Demko Well-Known Member

    You’re overthinking this, set the float to the factory specs and move on to other areas of more importance, such as a QUALITY ignition system and time it using Larry’s Power Timing Thread as a guide.
    We need useful info on the engine not just it’s a 70 455 with this and that done to it.
     
  11. Stevem

    Stevem Well-Known Member

    Post 9 nails it, no vacuum advance is the culprit!

    The only mods I have ever done to a Q jets fuel bowls once I had a .136" needle and seat in there is to very very carefully enlarge the fuel bowl for more volume, but only on the sides of the fuel bowl.
     
  12. Electra Sweden

    Electra Sweden Well-Known Member

    Yes, will be nice to see what measurements I get with the new ignition system. Just started to run the engine last saturday since a partial rebuild. Lets see if I end up at an acceptable MPG.

    Apart from that, any measurements from anyone on how float level experiments alone affected A/F ratio and MPG are most interesting :)
     
  13. Electra Sweden

    Electra Sweden Well-Known Member

    Well oh well, I have some hope still and even better also a spare gear, hope it turns out well :)
     
  14. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    No one tunes AFR by float level. The proper float level keeps the bowl full under all throttle conditions. Too high, and the engine runs rich because the carburetor is flooding to one extent or the other. Too low, and the engine runs lean and noses over as it empties the bowl. That is worse. AFR is set up with jets, rods, and a host of other not so easy to adjust parameters.
     
    Last edited: Apr 26, 2023
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  15. Mark Demko

    Mark Demko Well-Known Member

    What Larry said, your not going to blaze any new trails by playing around with the float level, either you’ll set it too high, and you’ll be leaking gas out the top and the engine will flood, OR you’ll set it too low and stall the engine!
    Set it where it belongs and move along :D
     
  16. Electra Sweden

    Electra Sweden Well-Known Member

    This reasoning makes sense to me. In other words the float level is just a fix pre-condition for the carburetor to work properly at all, and tuning is performed using other techniques. Given this reasoning then, if my engine neither stalls nor floods I can probably just leave it be then?
     
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  17. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    If your engine is completely stock, then yes. The engineers did a lot of research in order to get everything right. That was back in 1970. Not sure what fuel you are using in Sweden, but if you want to tune the carburetor for proper AFR, you need to start with a book like this,

    https://cliffshighperformance.com/product/how-to-rebuild-and-modify-rochester-quadrajet-carburetors


    And then get a wide band O2 sensor gauge or tailpipe sniffer.

    https://www.aemelectronics.com/products/wideband-uego-air-fuel-controllers

    https://www.innovatemotorsports.com/mtx-l-plus-digital-wideband-air-fuel-ratio-gauge-kit-8-ft.html

    https://www.innovatemotorsports.com/shop-all.html?cat=5&display_type=12
     
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  18. Mark Demko

    Mark Demko Well-Known Member

    Listen to Larry and head his advice, I do!
     
  19. Electra Sweden

    Electra Sweden Well-Known Member

    I have been looking at AF meters for some time. I am thinking, maybe it would be alright to use one of these cheaper AF meters for panel mounting and use as a hand held tool. Don't think I want an AF meter in the car permanently.

    I just realized something, is it always obvious when the carburetor is flooding? I rebuilt my carb last year and the car smells of some unburnt fuel at idle, which I cannot remember it doing before. I set the float height to stock, but on the other hand I also changed out the needle and seat to some aftermarket variant that looked slightly different. Haven't been able to tune it out. Ignition advance is around 11 degree at idle and tried to tune the idle mixture screws against a vacuum gauge.
     
  20. CJay

    CJay Supercar owner Staff Member

    What do the plugs look like?
     

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