carb air bleed ?
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carb air bleed ?

  1. #1
    Senior Member flat screwd's Avatar
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    Default carb air bleed ?

    Now Im sure that Im not asking this question correctly but Ill try the best I can . At what point do you change air bleed dia. and not jet size? For example, say that your lean at 3/4 throttle would it be more helpfull to up jet size or decrees air bleed size? How does this carb tuning balancing act between jet size and air bleed size become right?

    Screwd.

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    058
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    If you are going lean at 3/4 throttle I would 1st use a vacume guage to find out what the manifold vacume is at that point and alter the power valve and/or PVCR before I would change the airbleeds. If the power valve is not opening at the lean point then go up 1 step in P/V calibration, say from a 6.5 to a 7.5 If the engine continues to go lean after the P/V opens then open up the PVCRs by a couple of thou. at a time until its right. Only mess with air bleeds at last resort and use a wide band O2 sensor. Air bleeds perform 2 jobs, 1, to allow a metered amount of air into the emulsion tube to aid in fuel atomization and 2, to act as a "kill bleed" so that fuel does not siphon into the boosters when the throttle is closed. Also you can raise the float level to slightly change the point of circuit overlap, in other words a higher float level will allow fuel pullover into the main circuit slightly sooner and will in effect richen the mixture.
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  4. #3
    gn7
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    To add, if you are not running power valve(s), and you choose not to, your are are going to have to jet up. You''ll most likely be rich at something less than 3/4, but that the price you pay with no power valve.

    Air bleeds have less and less effect as the rpm increase. Changing them will make a bigger change below 3/4 throttle, than above it.

    What makes you think its lean at 3/4, but not WOT? If you can feel it, you can't fix it with air bleeds. You either do as 058n said, or if no P.V.s, fatten it up and deal with the rich at less throttle.



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    Senior Member flat screwd's Avatar
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    OK here's what I have BBC 505 Straub solid roller with a 871 and two QF 950s' without PVs , put it in the car and got it running a few weeks ago, had some issues with pulling fuel too soon from the primaries but solved that by opening the secondaries slightly. Fires up nicely and goes right to idle. wide band says 15.0 at idle but can change that with idle enrichment screws. Right now I cant apply any load to the motor because Im waiting for some trans parts so all I can do is blip the throttle which is useless accept for pissin off the neighbors. I was wanting to arm myself with accurate info because Im thinking this car will be in the street in a few weeks and tuning will be needed. This is the first set of carbs that I have had with the option of changing more than just jets and would like to find out the decision making process to get this right.

    Screwed.




  7. #5
    gn7
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    Funny, last night I got thinking, "is that thing blown"

    OK first things first. You cannot do this sitting still with zero load. But then you knew that.

    I am one that thinks PV in a blown boat is 100% stupid because I have never been able to so much as touch the thottle enough to plane the boat without lowering the manifold vacuum to a point below even the lowest PV. Even a mild cruise will put my engine into a "zero" manifold vac/pressure point, which means the PV is open. Why bother.

    BUT on blown street deal, this is another animal. You really want PVs. Badly! You'll need to manifold reference them, and test drive the car to determine the best PV. But your looking somewhere around a 4.5 for starters.

    Then follow 058's recommendations on determining the PV restricter size by driving it while watching the vacuum gage to determine when the PV opens, like anything below 4.0" vac, and the A/F meter. If the car drives OK, and the A/R is OK with the PV closed, but goes lean when it is open, open the PV restricters .002 at a time. Don't bother with PVs in the secndaries. By the time your in the secondaries, your in boost and the PV is open anyways.
    Bottom line, your enrichment when in boost is the PVs, and when in really heavy boost, heavy load, it is the secondaries.

    You can afford to be a little on the lean side, say as lean as 13-1 while unboosted, and in a vac up to "0" manifold pressure. Then it can go richer. I would consider running the secondaries 6-8 jets larger than the primaries, and jet the secondaries for you wide open A/R. Maybe as rich as 11.5. You really probably won't need to go any lower than that. You won't be able to stay in it long enough to hurt it. Ever driven a blown street deal? Your only in it for a very very short period. Then you going MACH 2, and running out of room. And you can't load it that hard because you blow the back tires off the thing. It sure as hell isn't a boat.

    I take it this is a street machine. If not, and its track only car, forget everything, drop the PVs and jet it square, to WOT, and live with the richness at low R, part throttle. Its just nature of the animal.


    PS, I have seen enough flippin blower motors. Lets see the rest of the car



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    Last edited by gn7; 07-03-2011 at 06:08 PM.

  8. #6
    Senior Member flat screwd's Avatar
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    yea I should have mentioned that this is preaty mucha track car , lexan windows and street crusin dont mix well together.
    The car on the left is mine and the one on the right is Blowen$ums'








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    Wow nice cars!!!!! Let me know the next time you go to the track.

  10. #8
    gn7
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    Very nice! Thanks for posting the pics
    Early Triumph? Looks like the Amal is hanging by the thottle cable, and shit is getting in the engine

    You can mess with the carbs after you getting it moving. But chasing the part throttle A/F on something like that if it isn't in LALA land is something of a waste. You drive to the line, stage and leave WOT. Unless it won't stage, why bother with it to much with it if it isn't crazy lean. You could still follow the PV set up I seriously doubt you'll notice much improvement being able to run a leaner mixture in an unboosted condition.

    Good luck with it. Let us know how it runs on the track.



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  11. #9
    Frank my lord Blown$um's Avatar
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    Default air bleeds?

    who needs air bleeds in a drag motor, hell i would just remove them. just something else to go wrong, i would just add some NO2 to it and call it good. lol, hell thats what i did.
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  12. #10
    gn7
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    Nice.
    I sure hope you meant PV. Removing the air bleeds could make the thing run pretty lean. Everywhere!

    Now if you meant PVs, I agree. If its running a little fat from over rich jetting, just hit the squeeze. It will clean right up



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  13. #11
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    Default clean up time

    hell yes it cleans it up and usually the competition too lol. 14- 1500 hp, its almost 3 hp per ci of displacement. just joking on air bleeds tho, i dont have pv's in my combo, no2 is always fun to play with tho. never popped a blower on it, that would be scary!
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  14. #12
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    Quote Originally Posted by flat screwd View Post
    OK here's what I have BBC 505 Straub solid roller with a 871 and two QF 950s' without PVs , put it in the car and got it running a few weeks ago, had some issues with pulling fuel too soon from the primaries but solved that by opening the secondaries slightly. Fires up nicely and goes right to idle. wide band says 15.0 at idle but can change that with idle enrichment screws. Right now I cant apply any load to the motor because Im waiting for some trans parts so all I can do is blip the throttle which is useless accept for pissin off the neighbors. I was wanting to arm myself with accurate info because Im thinking this car will be in the street in a few weeks and tuning will be needed. This is the first set of carbs that I have had with the option of changing more than just jets and would like to find out the decision making process to get this right.

    Screwed.



    Just wonderin. How do you get the valve covers off with those bars over top of them?

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  15. #13
    gn7
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    i think its an illusion. I think the covers are about as high as the bottom of the carbs or close to it. Look at the other side of the engine. You can see the tube, but you can't see the valve cover.
    On this side, compare where the tube meets the firewall and the distance down to the cover.



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  16. #14
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    Default yep

    yah its optical illusion, he built the entire front end to that car and put adj. struts. he did a lot of cool stuff to that car , it has adj, air springs in the back with if i remember right adj, shocks. 4 link and back half car himself. but i do have a video of that car on a dyno but dont know how to post it. he going to kill me if he knew i posted it lol. ka boooom it went, he doesnt even know i have video of it from the front of the car lol. im glad he went bbc on the next build tho, i used to be all sblock but he is the one who talked me into going bbc on my next build back in 02 and i never built another pos small block again lol. the vids on you tube
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