Q-JET 4MV missing parts
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500, 472, 425, 368 Discussion, Q-JET 4MV missing parts in Cadillac Engine Technical Discussion; Hi, have a 1970 Eldo I pulled the carb to have a check up. It had a sticker saying it ...
  1. #1
    Captain wow's Avatar
    Captain wow is offline Cadillac Owners Member
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    Question Q-JET 4MV missing parts

    Hi, have a 1970 Eldo
    I pulled the carb to have a check up. It had a sticker saying it was an original but rebuilt at some point. The Number is: 7040230 2259, which I believe is correct for that year. Its A 4MV. Fairly clean in and out.
    I cross referenced it with my 1970 caddy workshop manual and noticed parts were missing...

    Most noticeably there was no Power Piston Spring!
    But also i have no sign of the hot idle compensator, or needle retaining clip that hangs on the float arm.

    Do I need the hot idle compensator?

    I haven't been able to drive the car wot much as Im waiting for suspension parts but the performance seems to be fairly ok...i cant really tell till I take it out properly. I do know that if I floor-it the engine pings /knocks /rattles could this be due to the missing power piston spring?

    The float is a brass one, I read that it should really be a NitroFill type one.

    Can anyone point me in the direction of a separate parts supplier, I've had a rebuild kit before but they dont have these parts in them.

    Thanks

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  3. #2
    Captain wow's Avatar
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    Question Re: Q-JET 4MV missing parts

    Hi, here is a diagram i have marked the parts that are missing which i need.
    I noticed a few more things missing.
    plus the checkball screw is seized in there, so i want to replace
    that.


  4. #3
    Submariner409's Avatar
    Submariner409 is offline If it won't run, chrome it
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    Re: Q-JET 4MV missing parts

    If the body casting is not cut and drilled for the compensator, it never had one - some QJ applications don't.

    For individual parts your best bet is junkyards or swap meets - pick up a few QJ' and disassemble them.

    If you really want only those parts, try the guys at:

    Automotive Rebuilders, Bensalem, PA - 215 639 6511
    Jet Performance - www.jetchip.com (They build QJ's for my marine Olds 455 engines)
    Allstate Carb & Fuel Injection - www.allcarbs.com , Islip (Long Island) NY

    ............but you'll pay parts prices and shipping.

  5. #4
    Captain wow's Avatar
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    Re: Q-JET 4MV missing parts 1970 eldo

    Hi, thanks,

    im trying out jetchip but my internets gone slow for the evening.
    What about the part marked 23 - (early dashpot assembly).
    and part - 19 (idle vent valve). Could it be that they are not required?

    So im now wondering that all I really need is a power piston spring, a needle retaining clip and a NitroFill float.



    I'm in the UK so taking the carb to a knowledgeable re-builder is a no-go so ive got to figure this out myself, which parts need to be in there to get smooth results.

  6. #5
    steelybill is offline Cadillac Owners Enthusiast
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    Re: Q-JET 4MV missing parts

    The brass float is always better than a plastic float. Over time, the plastic floats absorb fuel, and sink a bit, making the float/fuel level too high. If it ran OK with the metal float, it would be a good idea to keep it.

  7. #6
    deVille33 is offline Cadillac Owners Connoisseur
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    Re: Q-JET 4MV missing parts

    I think your diagram is an "in general" application illustration. Note the notations (early) and (standard). Some of the parts mentioned aren't always used in different units. Note #44 - Pull clip Float Needle The pull clip was used on the Nitro-fil float. It is not a good illustration, but seeing that you had the brass tank type, your pull mechanism may have been the front of the float bar located in the needle nail. I theorize that the previous owner rebuilt this carb and installed the parts he wanted to enhance performance.
    Doug Roe published a booklet with a lot of info in it regarding setting up of the Q-Jet for performance. PO may have set this carb up for his own requirements. You may find that the needles have been changed out also.
    This may explain the pinging at WOT. Perhaps the needle changes allowed enough fuel dump to compensate for the accelleration pump spring (power piston). You may also find the secondaries are activating earlier than neccessary.
    I also think that you have part #8 and parts #11 - #15 are earlier design parts which aren't neccessary when #8 is used.
    Parts # 18, 19 ,20 may not be present if your air horn doesn't have a vent in this area. Earlier horns had vents, with the advent of emissions controls, the fuel bowl vents were redirected to vent to the area above the carb throat under the air cleaner.
    I don't recall, parts #42 & #43 may not be required. They mounted into a threaded recess in the floor of the reservoir channeled to the power piston. It was a check ball which ensured directional flow through the channel. This may explain why you did not have a power piston return spring.

  8. #7
    Captain wow's Avatar
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    Re: Q-JET 4MV missing parts

    Ah ha, this gets more interesting/confusing by the day.
    The diagrams in my original 1970 cadillac workshop manual mentions all the parts seen in this diagram.

    Do you think i should really ignore my original manual and work with my apperently stripped-back carb. If so i would still like the powerpiston spring back in there working so as to maintain some atttempt at saving fuel at idle...which i do a lot when driving round town.

    Does anyone know which primary needles/jets I should have installed?
    Also i have a feeling that the pin on the bottom of the powerpiston has been trimmed ..does this really matter?

    Thanks everyone so far.

  9. #8
    Captain wow's Avatar
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    Re: Q-JET 4MV missing parts

    Actually i just had another thought,
    im in the uk and getting parts for this is a real bitch over here. so..
    If my carb is now not original and has had various mechanisms removed or plugged then i wonder if i should just go ahead a stick a new holley or edelbrock on it, that way its a new easier serviceable/tuneable part. Or are the quadrajets really that much better with economy?

  10. #9
    Submariner409's Avatar
    Submariner409 is offline If it won't run, chrome it
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    Re: Q-JET 4MV missing parts

    The change from QJ to either Holley/Edelbrock spreadbore or Carter AFB (square bore) involves a lot of tuning and jet/needle work which may be hard to find parts for. My personal opinion is that a well set up QJ is the better carburetor.

    Get in touch with Allstate Carb and have them ship you a rebuilt unit for your engine. Yes, more expensive, but it sounds like you're trying to rebuild a question mark..........

    The QJ primary and secondary metering rods have two styles depending on the year and carb model. Very hard to tell the two apart and, to complicate matters there are several different tapers of primary rods and about 100+ variations on the multiple-taper secondary rods (which are hung on one of 5 different "hangers"). Returning your present carb to "stock" sounds like hunting the proverbial needle in a haystack.

    Find the HP Publications book on Rochester/Quadrajets by Doug Roe.

  11. #10
    deVille33 is offline Cadillac Owners Connoisseur
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    Re: Q-JET 4MV missing parts

    Due to the fact that there is a question here as to the originallity of your carb set up, I ask, what is the number of your carb? Carb ID number is found on the back upright web of the carb body (driver's side). This will at least determine if you have the right carb body.

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