Carb Tuning Issues

Discussion in 'Fox 5.0 Mustang Tech' started by lizardsman, May 9, 2013.


  1. lizardsman

    lizardsman Member

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    I have an 89' lx with a stock block 306, performer 5.0 heads, svo 1.7,1.6 rollers, long tubes,and full msd ignition with a elderbrock 5.0 intake. The problem is with the demon 750. I can tune the car to run great, idle just fine but can't get it to quit running rich. I have an Innovation digtal a/f gauge. 02 sensor is in the driver side on the pipe just behind the collector. Right now the a/r reads 15.2 to 15.5 at an idle. Hit the skinny pedal and it goes to high 9's. Cruising to and from work at 2600 rpm in fifth at 90mph it reads 10.2 to 10.5. My question is shouldn't it lean out when you stab it? and why would it run rich at part throttle? I have an old 650 elderbrock c series sitting in the garage. I've read my 750 might be to big but no idle or power issues. I was running 7.5 lbs of fuel but just turned it down to 6lbs. Just tired of my eyes burning, I know I need to build a scavenger kit for the pcv. I drive my car 94.4 miles a day. Lately I've been using an extra gallon of fuel a day. Just need to make her last a few months till I buy a bike. Thanks for any help.
     
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  2. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    So much for the theory that carbs are easier to work on than the stock EFI that came with the car...
    Did you do the carb conversion or was it already on the car when you bought it?

    The accelerator pump is why the engine doesn't run lean when you suddenly punch the throttle.
    The low fuel economy could be a number of things. Since it seems that the change in fuel consumption is a new thing, it points to wear, damage or misadjustment.

    Fuel presure should be 3-5 PSI. If it is too high, the float will not work properly.
    Check the float level against the manufacturer's specs. If it is too high, you use more fuel.
    Check the power valve to make sure it isn't damaged, leaking or worn and allowing to much fuel to flow through the power circuit when it's not needed.
    Check the choke to make sure it is opening all the way when the engine is warm.
     
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  3. srtthis

    srtthis the guy doing it does every local racers rear end

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    5.5-6lbs of fuel pressure and what size main jets are you running? id go down a lot on the main jet if your running in the 9's and low 10:1 AF range. NA cars mostly like to be in the mid low 13:1 range. and if you dip that low when you go from part throttle to WOT.. id go down a few sizes on the squirters also

    all the fuel pressure does on a carb is open the needle and seat valve to keep the fuel bowls filled. it isnt like a fool injection where you can add fuel with pressure.
     
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  4. srtthis

    srtthis the guy doing it does every local racers rear end

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  5. lizardsman

    lizardsman Member

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    Hate to admit but bought the car that way. I was looking for a low mileage gt with a 5k budget and found this 89lx with 33k original on the clock, heard it run, jued him down on the price and ponied up the cash. So unfortunately I have no clue what size jets. I have the floats set at spec. The carb has no choke, so no worries there. Don't really need one here in Florida. If any one can point me in the direction of a jet kit I'll get on that. And need to do some research on the power valve. My second time dealing with a carb. First time ended up with a blown 504bb in my first car, 73 f100. Hint, don't hook the pcv hose and fuel line up backwards, realize your mistake and switch it, by by piston flanges. Thanks for the advice guys, I'll have plenty of time to work on it this week as my trans blew again today on the way home. So back to the tranny shop Monday. Something hitting the case while in gear. Need to get the tko 600 sooner than later.
     
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  6. lizardsman

    lizardsman Member

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  7. lizardsman

    lizardsman Member

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    New to forums, did reply to your post, just wrong the first time.
     
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  8. srtthis

    srtthis the guy doing it does every local racers rear end

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    we have the holly master kit for carb jets but you wont need that many.... id see what size you have it it currently and get every thing -10 of whats in it now. if its got a .073 is get down to .063 and maybe few larger ones if it ever gets real cold when you run it.
     
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  9. lizardsman

    lizardsman Member

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    Replied to your post, just did it wrong the first time. New to the forum, and agree with your quote, before I build my dream motor I wanna built proof the drivetrain, suspension and cage her up. Then go for my dream of 1500hp, 1000+ f/lbs and still streetable. We can dream right?
     
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  10. lizardsman

    lizardsman Member

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    Ok. Once I take the jet out how do I figure out the size? And not sure what you mean by cold. Florida only gets cold a few days a year. I do have the champion 4 core so she only runs 160. Get real nasty and she'll get to 170, 180. I can drive from my house to Bradenton track, almost 50 miles, head straight to the lanes with out shutting it off and run 3 hot laps and only hit 200.
     
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  11. srtthis

    srtthis the guy doing it does every local racers rear end

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    aint hard to do... somewhere around 427-438" pro charger F1X set of yates Sc1's on E85 it should do it
     
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  12. lizardsman

    lizardsman Member

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    I want a dss racing 427 long block, vortech blower, and some giggle gas just cause. And I wanna put a 5speed behind all that and run tremec true street when they come to town. But with three kids and just changing professions gonna be awhile.
     
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  13. ID89GT

    ID89GT Active Member

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    It should say the jet size on the jet itself. Just pull the bowl off and you will see the main jets and the size should be on the top facing you.
     
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  14. lizardsman

    lizardsman Member

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    Thank you, I was gonna pull it off yesterday, but spent most of the day pulling trans and helping my buddy on a Honda, sorry to use that word, but he lets me use his 10k square foot shop to work on my car, and I just help out for free while I'm there. And it's a personal shop at his house less than a mile from my house so works out great. Got stuck doing some plumbing work for the wife today, so I will stop after work tomorrow and get her off and find out. Thanks again.
     
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  15. lizardsman

    lizardsman Member

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    Ok, finaly got the bowl off, All I can see is 80 on the jet.
     
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  16. srtthis

    srtthis the guy doing it does every local racers rear end

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    I'd get down to at least a 065 jet... Don't think you'll need and thing richer then an 80. We run a 92 in the 275 car during the summer and a 94 during the winter and that thing moves a LOT of air
     
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  17. lizardsman

    lizardsman Member

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    Ok. So any sujestions on where to get the jets?
     
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  18. srtthis

    srtthis the guy doing it does every local racers rear end

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  19. clement

    clement Founding Member

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    does it have a power valve? if not that could be why it has so much jet on the secondary side. Stock jetting in my AED 750 HO modified is 72 on the primary side with a power valve and 84s on the secondary side with no power valve. it could be an air bleed issue too. I would call demon and get their suggestions because I am told they are not like a Holley based carb. Nutzz on corral knows those carbs pretty well. you could get a hold of him. if all else fails, sell it and call AED.

    here is a tuning video


    View: http://www.youtube.com/watch?v=AlbVFU71ff8
     
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  20. lizardsman

    lizardsman Member

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    Ok, I got the .65 jets on the primary. I got the air fuel around 13 at idle and 12.5 cruising. The only problem is if I'm cruising or down shift and punch it it spikes lean, hesitates than takes off. My question is, is the 65 jet to small or is the accelerator jet to big? And the power valve seems fine and it's a 6.5.
     
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