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roch Q jet carb gurus please help, primary rods jets power piston issue thanks

Discussion in 'The Garage' started by R72K5, Aug 6, 2005.

  1. R72K5

    R72K5 Banned

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    hi all, i am trying to get part throttle to not be so lean in a 17080203 carb i am trying to get going

    the carb is a stock 3/4 ton 1 ton 84 carb and is mounted on a stock untouched '88 LG4 305

    i finally got the accelerator pump system to work good

    but now it has lean out problem when you have throttle down a little ways past off idle, when are driving

    its not pulling/getting enough fuel beyond idle and pump shot time

    so i know i have a power piston/primary rods jets issue



    has 45 primary rods and 73 primary jets in it, currently i am trying to find and gather up all the rods and jets i can find around here, cuz apparently i need to swap them for larger ones, and apparently a weaker spring too,

    i know that ytou have to keep the jets and rods around 30 sizes apart give or take 1 or 2 sizes,

    it is just too lean its not getting the fuel it needs to get going past around or 10mph, it doesnt die- it just leans out- stays low RPM- cant increase RPM under load

    it idles fine and revs up fine

    what jets rods would you recommend ? larger or smaller?

    and weaker power piston spring or stronger one ?

    is there a vacuum passage in bototm of the power piston/spring well ?

    is this how the piston is pulled against force of the spring ?? idle vacuum keeps it all pulled downwards right ?? and then when you get on the throttle it drps vaccum sucking donw the piston and lets the spring under it raise it and pull rods out of jets and richen the part throttle mix up right ??
    no ?

    im trying ot understand how this works in these carbs,
    i hope i got it right ?

    please elaborate

    thanks for anything, i appreciate
     
    Last edited: Aug 6, 2005
  2. diesel4me

    diesel4me 1 ton status Premium Member

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    vacuum power piston..

    The way you decribe the power piston's operation is correct--it is held shut,against the springs tension,by vacuum--as the manfold vacuum drops as the throttle is opened,the spring pushes the power piston open,and allows the metering rods to open and pass fuel thru their jets...using the wrong gasket between the throttle base and the carb body can block the vacuum passage,and it will cause flooding and loading up at idle,if no vacuum gets to the power piston passage..

    I'n not very well educated on jetting and power piston sizes,so I'll let somebody who knows answer that question..I've been lucky I guess,I never had to change any in Q-jets..all the ones I had ran well after rebuilding them and adjusting all the linkages to specs..had to "tweak" the accelerator pump on a few to get rid of hesitation,and my brother showed me how you can bend the metering rod holders to allow them to open further,rather tham change them..(but I never had to mess with any)..

    Maybe your "off idle" woes are related to that "Adjustible Part Throttle" screw your not supposed to touch?--(If your Q-jet has one!)...I had a few carbs that acted lean at cruising speeds and caused surging,all I had to do was drill out the plug to get at the "forbidden" screw,and back it out about 1/2-3/4 of a turn,and it cured that problem...never could get the carb to stop leaking after I drilled the plug out though,despite using a lead "sinker" from my tackle box tapped in with a hammer,and locktite...probably why they say "dont touch this screw!" :doah: :crazy:
     
  3. dyeager535

    dyeager535 1 ton status Premium Member

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    A problem as severe as you are describing (unable to accelerate essentially?) sounds more like a hardware problem than mismatched components. GM obviously used tons of different pieces in the Q-jets, but one Q-jet on any engine, from what I've experienced so far, is going to be pretty darn close, and perform better thanyou describe.

    One problem I've run into twice (barely applies in your case) is on carbs that came with EGR end up having a part throttle lean condition, where the vehicle surges as you hold vehiclke speed steady. In both cases stepping up the jet size by 1 eliminated both problems. Engines were also larger, so I had guessed that the larger primary jet wasn't going to hurt matters anyway.

    There are different accelerator pump piston assemblies, be aware of that, and don't forget about the two holes on the accelerator pump lever, that typically doesn't help matters to be in the wrong one. :)
     
  4. R72K5

    R72K5 Banned

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    no it revs, out of gear without load

    but when you are dirving nd want to go faster than about 10mph then good luck as it wont go no more, its so leaned out its not getting fuel

    i gave up on it and rebuilt a 1705 series carb real quick this morning and i mean REAL quick i did it in an hour before work and got done just in time to go and all i did was yank it apart and blast it out with can of carb cleaner and rodded out all the jets and tubes and slapped it back together(wth a new air horn gasket and new float and needle and seat parts i have laying around) and set the float level to around 3/8" and its running like a total dream it has soem good power and no stall or bog or hesitation or anything like that at all,

    BUT its a heat spring and rod (mounted on intake manifold type) of carb and well this car has no spot on intake for the spring its an electric choke car, so ill have to look for another electric or hot air choke carb later on down the road, as this damn 17080203 carb just wont give any fuel undr load past idle

    i dont understand it

    however i did notice that the APT screw in there was threaded ALL the way down into the bowl casting, which is not good, that would be leanest, i did unscrew it a few turns but it didnt help any at all,

    and i couldnt rod the main jets/tubes in the bowl casting they seem to be blocked REALLY bad

    so maybe thats th whole problem, the tubes are blocked super bad i bet, the carb was full of dried crusty white crap REALLy bad, and i know i didnt get it all out,

    the carb may be a loss, may have no hope for this one

    which is sad because it was the PERFECT one for this year make model and tranny..

    dammit.,,.


    thanks
     
  5. pauly383

    pauly383 Daddy383 Staff Member Moderator

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    Somethings wrong for sure inside , rebuild time , maybe send it off to the Carb Shop or something if you want to reuse that exact model carb .

    The APT , at least on my application makes a TON of difference , emissions and driving . 2 out is the norm for most , from what I found online . I know mine passes at 1/2 turn out , runs real awesome at 3 turns or more , and has a nice compromise at 2.5 turns out . But thats on my particular app , just saying its supposed to make that much difference .
     
  6. R72K5

    R72K5 Banned

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    thanks :d

    yeah soemthing bad is wrong, id say thees two problems with it

    the APT setting issue and also there HAS to be major blockage of the main jets/ vertical tubes in the bowl casting

    i couldnt get a rod down them cet for about halfway, seem to be badly blocked, both of them..


    the APT setting was all way down tight, and the last time i took air horn off the carb to fiddle with it i unscrewed the APT stop screw about 1 or 1.5 turns

    but of course it did not help anything AT ALL

    same problem/behaviour as before, 100%

    so hmm..


    so that carb is now not on an engine any longer,
    and instead is on the backburner
    and i quickly rebuilt(slapjobbed) an old 1705 series M4M Q jet yesterday morning and that went very well no problem it runs GREAT but its wrong choke type and such for the car year and engine, but oh well dont need a choke yet its hot as hell out hopefully ill find a good electric or hot air choke 80s carb by the time cold weather arrives,.., (M4MC or M4ME)



    thanks :D
     

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