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Technical Carburetor Jetting The Old School Way

Discussion in 'The Hokey Ass Message Board' started by Truck64, Nov 30, 2016.

  1. Truck64
    Joined: Oct 18, 2015
    Posts: 5,325

    Truck64
    Member
    from Ioway

    Sometimes it's tough for a feller to know what a thing is fully capable of, if he hasn't seen it at its best or doesn't have any baseline to compare with.

    You're right though, most people are happy enough if it starts easy both hot and cold, has good acceleration (no bog or flat spots), and lights the tires, doesn't catch on fire and the rest of it.

    Another thing I've found reading all the different forums online, a lot of people have absolutely NO idea whatsoever how to tune a carb. The problem is, procedures get mangled in the telling and retelling, in translation and, sometimes it does get a little confusing. Sure I'm no expert, but I know where to look, or who to ask. RTFM.

    There's no excuse not to use the resources available. All of this stuff is online now, available for download, for free! I just don't get it.
     
  2. Truck64
    Joined: Oct 18, 2015
    Posts: 5,325

    Truck64
    Member
    from Ioway

    Definitely agree here, have always been a big reader. Inhaled everything I can find on the MC2100. And it's a popular carburetor, the internet forums and archives are my friend here.

    Spent a little time grounding the distributor body direct to the battery, and the heads. I couldn't see any difference in the quality of the spark. It may be a factor on high load situations though. Have been pretty meticulous about grounds and grounding, cables direct to block and frame. Installed 1/0 direct to block, starter, and solenoid last month.

    Spark is pretty fat, whitish-blue. Bubba was saying only solid core wires produce a blue spark, I spent quite a while chasing that rabbit till I read that. I'm running a Pertronix and Flamethrower coil, new wires, cap, rotor etc.

    I may need to go with a cooler plug due the ignition advance, running a lot more than stock. 17° initial, this is one reason I went with heavy battery cables, didn't want any chance of hot starter kickback.

    Also in an unguarded moment last night - somehow, ordered the Innovate LM-2, so stay tuned we'll see how things shake out. I'll try some strategically placed jumper cables at idle and cruise testing and see if that changes the AFR.

    Your point is a good one, any ignition misfire will cause the O2 sensor to indicate a lean condition.

    What's a fair price for a muffler shop to weld in a bung? Figure they can sniff for (unlikely) any exhaust leaks too maybe.
     
  3. What is your total timing and at what rpm does it come in? Vacuum advance? Why so much initial? What is factory timing spec?Your timing curve needs to be right or real close before playing with carb, otherwise you'll never get the tune right.
     
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  4. Truck64
    Joined: Oct 18, 2015
    Posts: 5,325

    Truck64
    Member
    from Ioway

    Around 37 degrees "all in" by 3000, a stock Y block is very low compression and will tolerate lots of advance even with crap gas. Then, as much vacuum advance as it will stand without ping or detonation. Lots of advance will raise plug tip temperatures. For summer freeway use BF-32 would be a better choice.

    Agreed ignition needs to be squared away first before tuning a carb. Here's some background on why I curved the way I did.

    http://m571.com/yblock/distributortuning.htm
     
  5. Truck64
    Joined: Oct 18, 2015
    Posts: 5,325

    Truck64
    Member
    from Ioway

    My local indie shop sez they can weld the bung for me Thursday morning. The one guy used to work in a muffler/exhaust shop so I figure he knows what he's doing - but in case he doesn't - the O2 sensor (bung) should be mounted a couple feet past the collector? Have rams horn manifolds, anyplace after those is OK, mounted between 10 o'clock and 2 o'clock from vertical?

    Dumb question but will the bung plug need anti-sieze? How about a gasket?
     
  6. gimpyshotrods
    Joined: May 20, 2009
    Posts: 23,263

    gimpyshotrods
    ALLIANCE MEMBER

    Have it put as close to where all the tubes merge, and you can stand to see it.

    The sensor has a sealing ring, like a spark plug, and you should put some anti-seize on the threads.

    Between 10 and 2 is best.
     
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  7. Truck64
    Joined: Oct 18, 2015
    Posts: 5,325

    Truck64
    Member
    from Ioway

    Ok. But, when not actually using the O2 sensor, the bung plug gets installed. I'm thinking the heat and corrosion will "weld" that thing in there.

    Found this online:

    "If cast iron manifold(s) or shorty headers are used, install the sensor in the pipe just below the manifold."

    That would work out perfect. It can be mounted horizontally/vertical I take it, the idea is that it is not "upside down" and collecting moisture? The pipe runs straight down for a little bit. I don't care what it looks like. Thanks for the help Gimpy.
     
  8. gimpyshotrods
    Joined: May 20, 2009
    Posts: 23,263

    gimpyshotrods
    ALLIANCE MEMBER

    Yup, not upside down.

    Put anti-seize on the sensor threads AND on the plug threads.

    If you need to re-tune, it would be good if you could get it back out.
     
  9. While he's got it up in the air and the welder out you might think about putting one in the other side too.
     
  10. Truck64
    Joined: Oct 18, 2015
    Posts: 5,325

    Truck64
    Member
    from Ioway

    Hm. That's not a bad plan.
     
  11. I read the article and it was written half way decent, but here is what you need to do optimize your specific combo. First off, get the advance in sooner, like 2000-2500rpm. The only reason not to is if the load on the motor will not tolerate(super heavy 4000+lbs or geared super high). Next, if your butt-dyno is in tune, do a few 2nd or 3rd gear(preferred) pulls at WOT to max rpm you intend to Rev motor. Go up 2 degrees at a time until the truck slows down or detonates, then go back to previous setting. Do this with vacuum advance disconnected to eliminate variable. Always tune for max performance at WOT. Just because it 'feels good' at part throttle does not mean it's right overall. There is way more to this, PM me if you have questions.
     
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  12. Truck64
    Joined: Oct 18, 2015
    Posts: 5,325

    Truck64
    Member
    from Ioway

    Theory is, this shouldn't really be necessary since either bank gets two cylinders from the other side?
     
  13. 19Eddy30
    Joined: Mar 27, 2011
    Posts: 2,314

    19Eddy30
    Member
    from VA

    You want to tune a caburator ,more then just a few turns ,jets & power vale ,, there a lot to do its a big job full of notes, Patients , intsall then remove install so on so on ,drill this restricted that ,( if not adjustable) time , time ,
    It harder & more difficult to tune for street use then Drag (WOT)
    Most are NOT willing to put the time in, that why your car is Slow , & NO mpg
    they just want a bolt on & go :mad:

    http://members.tccoa.com/392bird/tuning.htm

    IMG_0814.PNG IMG_0815.PNG IMG_0816.PNG IMG_0817.PNG
     
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  14. Truck64
    Joined: Oct 18, 2015
    Posts: 5,325

    Truck64
    Member
    from Ioway

    One thing I read on carb tuning in one of the major magazines is that no damage (burned valves, holes in pistons) will result by going too lean with cruise jetting, although surging or lack of power will result.

    I don't know if that's true but a MC2100 is pretty simple, there's no metering block. The accelerator pump shot and power valve handles acceleration requirements, and that is (I guess...?) when excessively lean mixtures do their damage. But there's nothing to drill out (PVCR?) afaik on this type of carburetor. My guess (we'll see) is that it is pig rich when the power valve opens like most carburetors, but I don't really know, and there's nothing much to adjust at least officially.

    The accelerator pump rod has a few slot combinations, mostly setup depending on temperature. More fuel "shot" in cold weather, less in warm weather.

    Since this is a stock motor w/ plenty of engine vacuum a fairly high number power valve can be used, ideally about 2" below the minimum cruise vacuum. Stock is "green", in the manual for my application and altitude, this translates to a 7.5" by my figuring.
     
  15. 19Eddy30
    Joined: Mar 27, 2011
    Posts: 2,314

    19Eddy30
    Member
    from VA

    I know the pics I posted are a Holley , but all carburetor have the same theory on workings .
     
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  16. Truck64
    Joined: Oct 18, 2015
    Posts: 5,325

    Truck64
    Member
    from Ioway

    Right, we'll see what's what (if the eBay seller ever ships) soon enough. Maybe some fine wire could be snaked through the power valve channels if it's too rich, something like that.

    One thing I noticed, is it seemed to run a lot better at 40F than 80F, meaning the size 48 jets are too rich? It sure sounded better anyway. 46 jets seemed too lean, it didn't surge at highway cruise any but maybe a little at part throttle slow cruising around town, thought that was weird.
     
  17. Ok :)
     
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  18. oj
    Joined: Jul 27, 2008
    Posts: 6,454

    oj
    Member

    Probably not a good idea to tune for WOT. Many carbs hit a barrier for air at or before WOT but the fuel will continue to richen and you'll get false readings. Tune the carb for how you'll normally drive it, as long as it is sized close to right for air flow then you'll be able to get good positive results. When tuning people think the changes to be immediate, linear and predictable. They aren't.
     
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  19. Truck64
    Joined: Oct 18, 2015
    Posts: 5,325

    Truck64
    Member
    from Ioway

    LOL, well allrighty then... So let's say one bank is different in the AFR, which side should be believed, or how does one tune for this situation, etc. I can sort of envision idle mixture being compensated for by each idle mixture screw, as far as cruise jetting I'm not sure. Different size jets may be required?
     
  20. Far be it from me to question the theory -
    If all of the engines components and cycles are work in unison to produce the exhaust gasses you'll be monitoring,,,
    let's say you get 1 goofy cylinder throwing the combustion gases off on the side you're monitoring. The natural and logical consequence of such is you will tune for your reading (an average of those 4) and you'll be including a 8 in the tune but only 4 in the monitoring.
    Then you start wondering why the guage is right but the drivability is wrong and the butt dyno tells you the guage is full of shit. At that point you swap the sensor in the other side and confirm. Even if it runs great, it's nice to measure the other side and confirm consistency.
    Only monitoring one side is a lot like doing a compression test on just 1 side - it doesn't tell the whole story.
     
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  21. I don't agree with this, at least from the last 20 years of tuning I have done. Here is why. If you are tuning for performance, WOT is, in theory, your max possible performance. Of course air/fuel needs to be tuned to optimize, but one thing at time. This is the reason to do timing/fuel loops to get the last little bit out of a combo. If you are tuning for part throttle you are tuning around the vacuum advance and part throttle circuits in the carb which is backwards. Engines are supposed to run best at WOT, that is the whole reason to build a hot rod.

     
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  22. oj
    Joined: Jul 27, 2008
    Posts: 6,454

    oj
    Member

    You can only tune is a very general manner, easiest to get a data recorder and graph over a period of time, then make a change. You mentioned an LM2 earlier? try one of them.
     
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  23. Blues4U
    Joined: Oct 1, 2015
    Posts: 7,589

    Blues4U
    Member
    from So Cal

    Trying to test fuel mileage by the seat of your pants is as imperfect as trying to tune a carb without wideband. There are so many things than can influence fuel mileage that controlled tests are the only way you can get close to being accurate, and even then it's a crap shoot. Those claims of an increase in mileage from 12 to 16 mpg, take them with a grain of salt. It's anecdotal information at best. More likely wild imagination.
     
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  24. The best is to tune for both, drivability and WOT and at the end of the day it's a compromise.
    The car should do both well enough that neither is poor nor optimal, that's a compromise.
    If that tastes funny to you there's a reason computers are on cars to get EVERY single possible and conceivable condition at its optimal performance.

    If it's a max effort car that's a different story.
     
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  25. Blues4U
    Joined: Oct 1, 2015
    Posts: 7,589

    Blues4U
    Member
    from So Cal

    An old co-worker / friend of mine that used to race stock cars laughed at the wide open throttle tuners, he said you could even identify by ear them at speed, and you could tell just when they were about to burn a piston. "You tune lean, you tune mean", but chance are you won't finish the race.
     
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  26. oj
    Joined: Jul 27, 2008
    Posts: 6,454

    oj
    Member

    We aren't tuning a race engine, you tune them with egt anyway so you know what each cylinder is doing. O2 at 12.7 could have 3 cyls at 10 & 1 at 20.8, you are reading an average of 4 cylinders in a mixing bowl.
     
  27. gimpyshotrods
    Joined: May 20, 2009
    Posts: 23,263

    gimpyshotrods
    ALLIANCE MEMBER

    If your individual cylinder mixtures are that far off, you might want to pull your intake, remove the dead, stiff squirrel, and try again.
     
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  28. Truck64
    Joined: Oct 18, 2015
    Posts: 5,325

    Truck64
    Member
    from Ioway

    Wintertime is here, sorta. Temps (high) in the teens and twenties this week. Starting to see why carbs were factory tuned on the rich side of things, they had to cover a wide range of possible conditions from scorching deserts to arctic cold.

    Thinking that any jetting and AFR data I come up with won't be any good for summer, if it's tuned down to the gnats ass anyway.
     
  29. 19Eddy30
    Joined: Mar 27, 2011
    Posts: 2,314

    19Eddy30
    Member
    from VA

    I have talked to Holley & Fast about there Efi ,
    I was going to go to dual Throttle bodys , my question to them was if I get 12mpg on my north 550 hp
    Will I see a gain of at least 2 mpg ,because as much as I drive my car it would pay for it self in 1 year , I was told
    No !! If its a well tune carb or carbs that they have had Custmer's complain of less mpg with the EFI ,
    I was all so told one that see 5 to 10 mpg gain that there carb was so out of TUNE , So I stick with carbs !!

    All so if you want to realy get Technical install 8 egt's
    & data logger you will know what each cylinder is doing , just because you tuning your carburetor for summer it's not over !!! spring different tune ,fall different tune ,winter a different tune ,
    it's a chasing game
     
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  30. Doesn't EFI do that for for you? ? ?


    Add on throttle body EFI is a different animal altogether. If you're not controlling the timing you're missing 1/2 the gains.

    there are a couple tricks with GM TBI that really boost both your low/mid range power and your MPG.
     

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