bbc tunnel ram carbs
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bbc tunnel ram carbs

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    AKA JET BOAT DAVE NOT A JET's Avatar
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    Default bbc tunnel ram carbs

    Ok can't get this thing to idle right it is a little over 500 hp with a tunnel ram I called holley they said I could use 2 450s jetted up can't get it to idle under 2 grand no matter what I do I was told I have a vaccume leak was told to spray a little starter fluid around the intake and carbs and if there is a leak it wood rev up nouthing I have tryed several jet sizes I can put my hands over the carbs and block off some of the air and it idles fine what is going on a few people have looked at it they all say vaccume leak does the motor suck to much air to get thease small carbs to idle right I have 64 jets all the way around as big as holley recamonds thanks mike
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    Default Try 76-78's jets first..then ease back if needed

    Quote Originally Posted by NOT A JET View Post
    Ok can't get this thing to idle right it is a little over 500 hp with a tunnel ram I called holley they said I could use 2 450s jetted up can't get it to idle under 2 grand no matter what I do I was told I have a vaccume leak was told to spray a little starter fluid around the intake and carbs and if there is a leak it wood rev up nouthing I have tryed several jet sizes I can put my hands over the carbs and block off some of the air and it idles fine what is going on a few people have looked at it they all say vaccume leak does the motor suck to much air to get thease small carbs to idle right I have 64 jets all the way around as big as holley recamonds thanks mike

    Try 76/78 jets. Back out air bleed screws 1 1/2 turns..to start. Make sure float levels are correct and just level with your site plugs...just so the fuel starts to dribble out...then reinstall site plugs.

    Pump it twice and turn the key partly opening the throttle. It should fire instantly and idle fine. Make sure you timing is at 36 degrees total for good performance.

    If you have a large cam with a bunch of overlap..it won'tcreate a strong enough vaccuum signal with that tunnel ram and those small idle circuits...good luck.

    This is a similar 475" BBC with a bigger hyd flat tappet cam. I went with 78 jets to get it to start and idle perfect...hope this helps.
    Last edited by A handfull; 03-02-2013 at 11:12 AM.

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    gn7
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    Senior Member A handfull's Avatar
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    Default If no vac leaks..and assuming he has cleaned carbs well.

    Quote Originally Posted by gn7 View Post
    Did you just tell the guy to put 76/78 jets on a 450 carb?
    If there is no other issues..say vaccum leaks...gunked circuits....he may just need the extra fuel. Obviously if its stinging his eyes and running extremely rich...he can go smaller. He may have modified carbs and not even know it. They might flow more CFM now...and a simple jet change...maybe even 72's to start.

    I've found that with a large plenum tunnel ram with non-double pumpers larger idle circuits AND a big cam they starve for fuel at idle and run very well going way fatter than you would ever expect.

    Please offer your advice....a jet change takes 10 minutes. Get the thing to run...then back them down so its not rich.

    Not on here to fight...just to offer suggestions that has worked well in past experiences.

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    Senior Member A handfull's Avatar
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    Default Transfer slots...idle screw open so far their uncovered?

    Pull a carb and see if youve opened up your idle screw so far that the idle circuit transfer slots are not effective...and the carb is now in "run" mode per say.

    Fatten the jetting....back off the throttle plate position screws...and check your metering block for clogging and make sure your metering block gaskets are correct AND havent shrank. Spray cleaner in all air bleeds and check cork gaskets around the block screws. Make sure everything is super clean. Also check to make surecarb throttle plate is sealed good. Tighten all phillips screws under the carbs and replace the gaskets if questionable. If base plate is leaking it will not idle at all.

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    gn7
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    Default

    interesting considering that the idle and main are two seperate circuits, and that main jets have no effect idle



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    Default Give the man some advice....I'll bet he's way lean

    Quote Originally Posted by gn7 View Post
    interesting considering that the idle and main are two seperate circuits, and that main jets have no effect idle
    I'm betting he's on the edge of the idle circuit..and it's probably lazy and lean with a quick blip of the throttle..with possible stumble and lean back fire.

    Clean idle circuits, check gaskets, off idle response and restarts seemed way easier being a little fat in my opinion.

    Are the power valves blocked or is he running power valves. Did he know manifold vaccum at idle before he switch to tunnel ram and twin carbs?

    The tunnel rams like lots of fuel...no doubt...to no be finicky.

    Please offer this guy your 2 cents worth on a good baseline set up. I know you have years of carb experience...why don't you help him???

    Post initial timing advance numbers
    Post cubic inch of motor
    Post compression ratio
    Post camshaft lift, duration at .050, and LSA
    Post power valve size and or if they are blocked
    Post if transfer slots are exposed
    Are boosters dripping at idle?
    Eliminate the variables

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    Senior Member eliminator's Avatar
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    Quote Originally Posted by gn7 View Post
    interesting considering that the idle and main are two seperate circuits, and that main jets have no effect idle
    Thank you , I have been telling people this for years . and they still insist on fricking with the jets LOL . Had one last summer that said he fine tuned the idle mixture screws while running wide open down the lake . I bet that was scarey with open headers ...
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    I think the only way the main jets would have any effect on the idle is possably to much transfer slot is showing and it's starting to draw on the main metering system. If you can cover the carbs with your hand and it idles, I'm leaning towards a vacuum leak. If you close off most of the air coming into the carb and it idles, then the right amount of air has to be coming in from some where.

    Tim
    Last edited by River Rat 005; 03-02-2013 at 07:11 PM.

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    AKA JET BOAT DAVE NOT A JET's Avatar
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    I have talked to a few people about this and they said the cam iam running has a 637 lift and may be to big so very little vaccume at start up for thease carbs so what does this mean do I have to go to double pumper carb but like I said I can prity much put my hand on top of the carbs and block some air off and the idle gos down thanks for the help don't no much about carbs
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    gn7
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    Quote Originally Posted by River Rat 005 View Post
    I think the only way the main jets would have any effect on the idle is possably to much transfer slot is showing and it's starting to draw on the main metering system. If you can cover the carbs with your hand and it idles, I'm leaning towards a vacuum leak. If you close off most of the air coming into the carb and it idles, then the right amount of air has to be coming in from some where.

    Tim
    T he transfer slot is feed from the idle well thru the same restricter the idle circuit is The transfer slot is just a off shoot of the idle circuit, much like the power valve is part of the main circuit.
    It is possible to have the idle so screwed up that you pull fuel from the main circuit into the main booster, but that is one screwed up carb. Anybody pulling fuel thru the main circuit at an idle and leave it that way is a hack.
    Quote Originally Posted by NOT A JET View Post
    I have talked to a few people about this and they said the cam iam running has a 637 lift and may be to big so very little vaccume at start up for thease carbs so what does this mean do I have to go to double pumper carb but like I said I can prity much put my hand on top of the carbs and block some air off and the idle gos down thanks for the help don't no much about carbs
    I can't figure out if you're saying it will not run at all below 2000, or you can't get the idle down below 2000.



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  14. #12
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    It will not run at all under 2 grand it shuts off
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    Default

    What list # are the carbs?

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    Default Vacuum

    And the winner is River Rat 005

    The only one to spell VACUUM correctly

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