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Making my own off-road Demon carb, ideas needed.

Discussion in 'The Garage' started by mrk5, Nov 16, 2004.

  1. mrk5

    mrk5 The Sticker Guy Moderator Vendor GMOTM Winner Author

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    Okay first off, this is a carburetor I already have. I want to try and make it work before I go ahead and sell it in favor of some other induction system. I was driving in this morning with the carb sitting on my passenger floor. I had planned to clean it up and post it for sale on ebay, but the more I looked at it I just can't stand to let it go. It is a damn nice piece of machinery.

    It's a 625 Road Demon.

    I had the carb on several trails and the only problem I ever had was a stumble while climbing uphill and hitting hard bumps with moderate throttle.

    So, I want to make this thing into an offroad carb.
    I've read some recent posts and they have confirmed some ideas I had.
    *Use tubing to extend the bowl vents
    *Spring loaded needle/seat
    *Extended jets (what exactly is the benefit)

    I'm wondering what other things there are that make the Qjet and Truck Avenger good offroad carbs.
     
  2. rcamacho

    rcamacho 1/2 ton status

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    I'm rebuilding a Holley carb for ORD with a similar strategy.
    1) Fuel bowl vent baffles
    2) Spring loaded needle/seat valves
    3) Extended fuel bowl vent tubes
    4) Lower fuel level slightly
     
  3. mrk5

    mrk5 The Sticker Guy Moderator Vendor GMOTM Winner Author

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    Your post is what inspired me.

    I was wondering if the vent baffles and extended vent tubes will accomplish the same thing? Especially since I can't get the baffles.
     
  4. rcamacho

    rcamacho 1/2 ton status

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  5. mrk5

    mrk5 The Sticker Guy Moderator Vendor GMOTM Winner Author

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    It's possible, some parts are interchangeable. For example I know the accelerator pump discharge nozzles will swap.
     
  6. rhuarc31

    rhuarc31 Registered Member

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    You can always make your own vent tube extensions that's pretty easy. Heck if you have access to the tubing just pull the old ones and make new ones. If you can't get the baffels try epoxing some 100 mesh metal screen to the bottom of the vent tube, it's not perfect but it does help.
     
  7. mrk5

    mrk5 The Sticker Guy Moderator Vendor GMOTM Winner Author

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    I was planning to make my own tube.

    I'm just wandering if any kind of baffle is necessary? If I still think it is a problem I may to the wire mesh option.
     
  8. folkenheath

    folkenheath Worthless Trash Premium Member GMOTM Winner

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    Jet extensions are used on drag cars with single carbs because when they launch hard all of the fuel goes to the back of the rear bowl and then the jets are in air instead of submerged in fuel. The same thing can happen if you are going up a steep hill. Jet extensions move the rear jets back into the bowl about .750" and keep the jets in the fuel. You also need a notched float. Usually you can get kits to do this for about $20. Such as this:

    http://www.holley.com/HiOctn/ProdLine/Products/FMS/FMSCA/116-10.html

    /forums/images/graemlins/waytogo.gif
     
  9. m j

    m j 1/2 ton status

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    [ QUOTE ]
    *Use tubing to extend the bowl vents
    *Spring loaded needle/seat
    *Extended jets (what exactly is the benefit)

    [/ QUOTE ]


    none of those things will help a Holley based carb's 'flaw'
    you can do all the addons you want and it still will not climb
    the only thing I see in the above posts that will help slightly is reducing the fuel level in the float bowl.
    doing that in a high powered vehicle also puts you in danger of draining the bowl on a long WFO pull, not a good thing.

    rather then throwing parts and modifications at it blindly why not bolt it on and try it.
    then if you find you require any of the above bandaids add it.
    fixing problems that are not present is not productive.

    they cannot climb due to the floatbowl design.
    as soon as you get at an angle where the fuel in the bowl is higher then the main discharge nozzle it will gravity feed the engine.
    that fuel will be pretty much unmetered and will flow as fast as the pump can deliver it.
    there is no addon that can solve that problem that I am aware of.

    the Qjet has the main jets in the floor of a floatbowl and the shape of the bowl does not allow the fuel to easily exceed the hieght of the main discharge nozzle
    that is the only thing a Qjet has going for it
     
  10. mrk5

    mrk5 The Sticker Guy Moderator Vendor GMOTM Winner Author

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    [ QUOTE ]
    none of those things will help a Holley based carb's 'flaw'
    you can do all the addons you want and it still will not climb
    the only thing I see in the above posts that will help slightly is reducing the fuel level in the float bowl.
    doing that in a high powered vehicle also puts you in danger of draining the bowl on a long WFO pull, not a good thing.

    they cannot climb due to the floatbowl design.
    as soon as you get at an angle where the fuel in the bowl is higher then the main discharge nozzle it will gravity feed the engine.
    that fuel will be pretty much unmetered and will flow as fast as the pump can deliver it.
    there is no addon that can solve that problem that I am aware of.

    the Qjet has the main jets in the floor of a floatbowl and the shape of the bowl does not allow the fuel to easily exceed the hieght of the main discharge nozzle
    that is the only thing a Qjet has going for it

    [/ QUOTE ]

    That's information I was looking for. I wondered what it was about the basic design of the Qjet that makes it a better offroad carb. Now I understand. May not be worth the trouble of making the mods. What your describing sounds like what was happening when I had the Demon on the engine and it would bog real bad going up hill and hitting hard bumps.
     
  11. Z3PR

    Z3PR Banned

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    Study the Holley Truck Advenger for some ideas.
     
  12. rcamacho

    rcamacho 1/2 ton status

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    The avenger has some metering block modifications to minimize gravity feed of the boosters. Additionally, it has spring loaded inlet needles and a crossover fuel bowl vent.
     
  13. mrk5

    mrk5 The Sticker Guy Moderator Vendor GMOTM Winner Author

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    [ QUOTE ]
    The avenger has some metering block modifications to minimize gravity feed of the boosters. Additionally, it has spring loaded inlet needles and a crossover fuel bowl vent.

    [/ QUOTE ]

    Ahh, I was wondering if the Avenger had a modification like that after reading m.j.'s post.

    I'm starting to fall back to my original plan, and sell the Demon on ebay and use that money to get a custom built Qjet from SMI. The Demon is awesome for street use, but it just isn't an offroad carb.
     
  14. m j

    m j 1/2 ton status

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    that is total BS
    the avenger has nothing (that I have seen) different then any other Holley
    feel free to prove me wrong as I am a huge fan of Holley carbs
    if Holley has come up with something that allows liquid to defy gravity I am ordering it tomorrow

    I base these statements on the images of side by side regular Holley parts and Truck avenger parts, including close ups of the metering blocks, provided to this site by Twisty IIRC
    I saved them to my HDD
     
  15. rcamacho

    rcamacho 1/2 ton status

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    Hmm, other members including myself might disagree regarding the effectiveness of some of these mods.

    http://coloradok5.com/forums/showflat.php?Cat=0&Board=garage&Number=1242760

    Regarding the TA metering blocks, I'm quoting Holley marketing material. They went so far as to offer cutouts on their blocks. There are a number of threads on the board from members running the TA with effect. Pethaps this is BS, guess the proof is in the mind of the customer.

    "Exclusive New "No Trouble" metering block eliminates fuel spillover through the boosters at extreme angles."

    http://www.rockcrawler.com/techreports/holley_truck_avenger/index.asp
    http://www.rockcrawler.com/techreports/holley_truck_avenger_770/index.asp
    http://www.holley.com/HiOctn/ProdLine/Products/FMS/FMSC/TrkAvngr.html

    With the exception of the side bowls and metering block. The mods discussed in this thread are fairly well documented to contribute in specific areas towards off road drivability. Of course if you are exceeding 40 deg inclines than all bets are off and the discussion is probably irrelevant - read EFI.

    1) Spring loaded needle valves reduce hammering of the viton tip and provide some dampning. Having recently rebuilt a 3310 I can attest to the poor condition of viton after some rigorous bouncing.

    2) Vent whistles/baffles move the vent pickup to the opposite side for center hung fuel bowls so slosh into the vent tubes is minimized. Yeah, this is a temporal effect but it has been well proven on the strip for the last 30 years.

    3) Extende fuel vent tubes help prevent fuel slosh into the carb.

    4) Seems we agree on fuel level provided it's kept reasonable.

    http://www.fourwheeler.com/tipstricks/128_9704_fuel/
     
  16. m j

    m j 1/2 ton status

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    effectiveness?
    I see nothing in the post that claimed the mods cured anything.
    just that they did them.
    exactly what symptom was cured and by which modification?
    I ran a Holley (actually several different ones) on my 4x4 from 1990-2004 with zero modification.
    that isnt exactly true I did try a variation of the vent tube extensions when trying to determine why it wouldnt climb.

    nice links, what are you trying to show?
    there is no picture of the magic gravity defying metering block modification.
    when you find them post them.

    try and list the reasons you did each modification and the effect on the symptom.
    reason your way through it rather then eating the spoonful that is being fed.
     
  17. 84_Chevy_K10

    84_Chevy_K10 Banned

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    Get a damn Q-jet already.
     
  18. rcamacho

    rcamacho 1/2 ton status

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    Having worked with Holley carbs for about 10 years now I'm fairly well experienced as to what works and what does not. I'll drop the TA stuff as you obviously are not swayed from your opinion despite having not driven one - I'm equally unexperience re the TA metering block.

    As to the other mods, there is enough supporting evidence including my own personal experience that I'm done debating the issue. I'd suggest that unless you have direct experience with these mods than perhaps an adjustment of tone is in order.

    Let's agree to disagree. /forums/images/graemlins/angryfire.gif
     
  19. TrcksR4ME

    TrcksR4ME 1/2 ton status

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    [ QUOTE ]
    Get a damn Q-jet already.

    [/ QUOTE ]

    /forums/images/graemlins/rotfl.gif no sh!t !

    You can probably get one for free-$20, do a quick rebuild and it will run better than any "street" Holley off-road. The Q-Jet on my truck was pulled off a motor and run as-is and does way better than my old Carter ever did.
     
  20. m j

    m j 1/2 ton status

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    [ QUOTE ]
    worked with Holley carbs

    [/ QUOTE ]

    what does 'worked with' mean?
    I have owned Holleys since I was 18 years old 1986.
    I have done all kinds of stupid things to them.
    I have done all the 'mods' listed except jet extensions but I have also looked at what each part does.

    spring loaded needle/seat - doesnt prevent gravity feed through the mains at angle
    vent tube and slosh tubes - doesnt prevent gravity feed through the mains at angle
    extending or connecting primary and secondary vent tubes - doesnt prevent gravity feed through the mains at angle
    jey extensions on the primary - this one I would have to test to see if they are long enough to get uncovered at extreme angles and run dry - either way doesnt prevent gravity feed through the mains at angle

    rather then puss out why not prove your point?
    show the magic metering block modifications.
    I would love to run a Holley again on my 4x4
     

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