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Technical Are carb jets proportional to engine size?

Discussion in 'The Hokey Ass Message Board' started by StarDiero75, Dec 13, 2018.

  1. StarDiero75
    Joined: Jan 15, 2017
    Posts: 170

    StarDiero75
    Member

    15447392592264112255636320053720.jpg Howdy guys,

    I'm working with my Ford 200 L6 and i got a weber 32/36 on it. It smells kinda gassy at idle.

    I had an extra Holley 5200 (holley version of the weber for those who aren't familiar) and i believe its for the Ford Cologne 170 V6.

    Using proportions, I got pretty much all the same jet sizes i currently have, except my air correctors. i was given the jet sizes that I currently have, i did no calculation whatsoever

    These are the calculated values.
    So should i jump up my air correctors?
    Currently i have 180/160 for the primary/secondary

    Thanks
     
  2. squirrel
    Joined: Sep 23, 2004
    Posts: 55,942

    squirrel
    Member

    jets usually are proportional the the (airflow) size of the carb. The carb doesn't know or care how big the engine is, the ratio of fuel to air needs to be the same for all engines.

    There are some differences due to other factors, though.
     
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  3. StarDiero75
    Joined: Jan 15, 2017
    Posts: 170

    StarDiero75
    Member

    So like for my case, what differences should be for the 170 carb and my 200 carb? So how do I know if my jet sizes are correct-ish without a wideband?
     
  4. Truck64
    Joined: Oct 18, 2015
    Posts: 5,325

    Truck64
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    from Ioway

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  5. squirrel
    Joined: Sep 23, 2004
    Posts: 55,942

    squirrel
    Member

    Adjust the idle mixture screw, play with it enough to get a feel for how the engine sounds, etc when you adjust it slightly rich or lean.

    What I'm trying to tell you is that if you think jetting should depend on engine displacement, you need to do some more reading about how carburetors work, because you don't understand the basic principle of air/fuel ratios, and how carburetors achieve that ratio. It has nothing to do with engine size.
     
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  6. razoo lew
    Joined: Apr 11, 2017
    Posts: 536

    razoo lew
    Member
    from Calgary

    I remember reading posts by @gimpyshotrods on here, he seemed to have a s**tload of information on webers and ford sixes. Search for “weber” posted by gimpyshotrods, lots of information.
     
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  7. Beanscoot
    Joined: May 14, 2008
    Posts: 3,056

    Beanscoot
    Member

    A good start would be to return the carb to the stock configuration, assuming it's been monkeyed with.
    A friend had one of those Webers on a 200 six in a Falcon about 30 years ago, I'll ask him next time I talk to him if he remembers if he changed parts in the carb.
     
  8. gimpyshotrods
    Joined: May 20, 2009
    Posts: 23,264

    gimpyshotrods
    ALLIANCE MEMBER

    I've heard of that guy!

    Also, a funny circular reference, as I believe that he got the jet numbers that he is using from me.
     
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  9. gimpyshotrods
    Joined: May 20, 2009
    Posts: 23,264

    gimpyshotrods
    ALLIANCE MEMBER

    Idle jets (small): https://www.pegasusautoracing.com/productselection.asp?Product=1587
    Idle jets (large): https://www.pegasusautoracing.com/productselection.asp?Product=74409

    I believe that there are two jet body sizes. I would have to check my Holley 5200 spare and see what it has. It might have the fat ones, which, I believe, are the same body size as down/sidedraft Webers.

    Idle instructions: http://www.redlineweber.com/html/Tech/carburetor_set_up_and_lean_best_.htm

    Jer Pac part numbers: http://www.redlineweber.com/html/application_guide/performance_main_and_air_jet_kit.htm
     
  10. StarDiero75
    Joined: Jan 15, 2017
    Posts: 170

    StarDiero75
    Member

    Hey man! What do you know?! Haha.

    Yeah the car runs great and i cant seem to get the idle mixture screw in more than about 3.75-4 turns out. Reading on webers, it says it should be 2ish turns out. Thats a big part of why I think I'm running rich/lean with the jets. Since I'm backed out, either its not getting enough gas or I'm not getting enough air. And this would explain the weird smell i get from the ewww exhaust (either rich or lean)

    Also my speed screw is tightened in a lot more than recommended and im only getting an idle of about 500rpm (i haven't tached it yet) and 12 degrees of timing

    See what I'm trying to get at?
     
    Last edited: Dec 14, 2018
  11. StarDiero75
    Joined: Jan 15, 2017
    Posts: 170

    StarDiero75
    Member

    So let me get this straight, if it has absolutely nothing to do with engine size, then I should be able to take that 170 V6 Holley and slap it on my 200 L6 and have no issues b/c the carb jet sizes have nothing to do with displacement? That can't logically be right. A larger engine requires more fuel and air. A carb will only allow so much to go through it, yes i get that part. But solely playing with the idle mixture screw can't allow the engine's needs to be met. Thats why jet sizes are increased for larger engines and downsized for smaller engines. The jets restrict how much gas goes in, and thus larger jets allows more gas in and vice versa.

    And to being Gimpyshotrod into this, I don't know if it is pure luck, but when you do the proportion math like as I posted, i got pretty much the exact jet sizes he gave me in the first place. That much tells me there, that at least for the most part, jet sizes are mostly proportional to engine size, until the carb can't deliver what's needed. Right?
     
  12. Torana68
    Joined: Jan 28, 2008
    Posts: 1,415

    Torana68
    ALLIANCE MEMBER
    from Australia

    It’s all about air flow, change air flow , change the jets.. more air going in will need more fuel to keep things right. Going by displacement will give you an approximation, but all engines aren’t created equal. The same carb on a stock engine won’t be jetted right for the same engine that’s been modified to get more air in . It’s not as simple as referring to a chart but a chart might get you into the ball park.
     
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  13. Truck64
    Joined: Oct 18, 2015
    Posts: 5,325

    Truck64
    Member
    from Ioway

    Gimpy, those Weber idle adjustment instructions are really good at explaining how (and why) to adjust idle mixture screws. It looks like Weber use different size jets for the idle circuit itself and they emphasize that about 80% of driving is done on the idle circuit (or just off idle) in the transition area of the carb and they have taken a careful approach to get this dialed in closely.
     
    Last edited: Dec 14, 2018
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  14. Truck64
    Joined: Oct 18, 2015
    Posts: 5,325

    Truck64
    Member
    from Ioway

    It's also interesting, it is not universally true that more air in always = bigger jet. More velocity may simply pull more fuel from the well. For example, dual exhaust over a restricted stock system "should" mean bigger jets due to a leaner cylinder charge, better scavenging. This may be necessary, though not always. Give it whatever it wants based on the plugs etc.
     
    Last edited: Dec 14, 2018
  15. Joe H
    Joined: Feb 10, 2008
    Posts: 1,525

    Joe H
    Member

    Wouldn't it be easier to just jet down one size and see how it acts? Numbers in charts are just starting points since all engine are slightly different, altitude, Barometric pressure, humidity, worn engine, fuel type, all play a role in how they run. Start with a jet thats damaged or way to big, solder it shut, drill out smaller then you need and try it. Keep drilling till it runs right, then hunt down a factory made jet with the same size hole you drilled.
     
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  16. Ebbsspeed
    Joined: Nov 11, 2005
    Posts: 6,250

    Ebbsspeed
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    When you take a carburetor from a smaller engine and install it on a larger one, you need to readjust that carb to get the same idle speed. If part of that adjustment involves cranking in the throttle stop screw to increase the idle speed, the relationship of the throttle plates to the fuel transfer slots/ports in the carb throat changes, which changes the A/F ratio. Even running with, without, or a different air cleaner will result in A/F ratio changes. A different engine may also need a different "diet" due to cam timing, overlap, intake manifold differences, etc.

    Simple math may get you close in this instance, but it's not usually the case.
     
  17. LM14
    Joined: Dec 18, 2009
    Posts: 1,936

    LM14
    Member Emeritus
    from Iowa

    Stop for a moment and think about your question. If I had 2 engines, 1 bone stock and the other with everything thrown at it (a roller cam, 13:1 compression, ported heads, etc.), would they take the same carb and same jets for those 2 combinations? Absolutely not. It's about matching air flow and fuel flow with the parts combination inside the engine to get in that specific A/F ratio mentioned above.
    SPark
     
  18. StarDiero75
    Joined: Jan 15, 2017
    Posts: 170

    StarDiero75
    Member

    So my proportion thing im talking about is for stock engine to stock engine. I understand cam/compression/atmosphere/etc all has to do with it. I'm simply trying to get into the ball park using jet sizes from a smaller but similar engine.

    This is what I saw when I asked the same question on the fordsix site and the guy posted about his 170 L6 holley 5200. My air correctors are smaller than his and he's running 10:1 compression on a stock bottom end. So my guess, with the higher compression than mine, but smaller engine size should put me about what I should be at. So i think i should start with either what he has, and work my way up, or start maybe 5 higher on both.

    I'm just trying to get closer to what it should be, and based off what I just saw on the fordsix I think my air correctors are a little small, which explains my gassy smell. (I would think)

    What do you all think? I'll pull my plugs next and take a picture but which one should i pull for the L6? The center ones naturally run rich and the outers naturally run lean. Should i just pull 2 and 5 then since they should be in the middle?
     

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  19. Relic Stew
    Joined: Apr 17, 2005
    Posts: 1,208

    Relic Stew
    Member
    from Wisconsin

    If you're opening the throttle too far at idle, it exposes the transition circuit. The idle mixture screws then have little effect on mixture. The transition circuit supplies fuel for part throttle, and will be over fueled at idle. Figure out why it won't idle with a reasonable throttle adjustment, then come back to mixture adjustment.

    On engines with lumpy cams needing fast idle, one solution is to drill a small hole in the throttle blade near the idle jet to provide the necessary idle air without opening the blade too far.
     
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  20. Use this chart to compare cubic inches to your engine to find a starting point then use your sparkplug readings to make changes if needed. Whoops, this is just the first page, there are 6 more that will not load......
    upload_2018-12-14_17-53-53.png
     
    Last edited: Dec 15, 2018
  21. oj
    Joined: Jul 27, 2008
    Posts: 6,454

    oj
    Member

    Your carb might be new enough that the idle mixture screw is in air circuit, not the fuel and that might be causing you some confusion. We normally think the idle mixture screw affect the gasoline flow, but the EPA 'improved' hoe the idle mixture is adjusted in the '70's.
    The jets are sized the amount of horsepower your engine makes, a 350HP 327 chevy will be jetted very nearly the same as a 350HP 454 chevy. They'll both take the same amount of fuel and oxygen.
     
  22. jimmy six
    Joined: Mar 21, 2006
    Posts: 14,802

    jimmy six
    ALLIANCE MEMBER

    Once the air/fuel ratio is correct, a carb will work on different engine sizes. The smaller carb on the bigger engine will restrict getting the highest HP. In reverse the large carb on the smaller engine will work a very low speed but flood the engine with too much air when opening up.
    Changing compression ratio does more harm to tuning than any other modification. Moving from 8.5-1 to 10.5-1 will definatly need a jet change.
     
  23. 2OLD2FAST
    Joined: Feb 3, 2010
    Posts: 5,217

    2OLD2FAST
    Member
    from illinois

    Not true , the len GTgth of the orifice is as important as the diameter of the orifice , hence you will find 2 jets drilled the same size numbered differently.....check a holley jet chart , also the chamfer of the orifice ma makes a difference
     
  24. StarDiero75
    Joined: Jan 15, 2017
    Posts: 170

    StarDiero75
    Member

    Is there a way you can find a part on that PDF near 3300 cc or 200ci? When i look at the jets for the 32/36 DGV its much smaller all the way around than what I currently have.
     

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