Q850 idle tuning help needed

Discussion in 'Other' started by deluxe68, Aug 8, 2015.

  1. deluxe68

    deluxe68 Well-Known Member

    Well I called Holley/ QF little bit ago to see if I would get different advice on why the mixture screws are not operating correctly and ask the question of the picture. They said that it's normal for the screw tip to go into the IFR channel.
    Funny thing is I have read on a couple different sites about this metering block issue QF had. I just don't understand where the punch through would be other then where it's visible now.
    So after going over the normal stuff like t-slot exposure, PV leaking, fuel pressure and bowl height, they have no explanation on where the extra fuel might be coming from. It looks like I'll have to take Larry's advice and send it in and pay the $250.00 + shipping for them to overhaul it and do a flow test.
    Problem is I'd be almost 1/3 the way for a new carburetor at that point so I'm thinking that's not gonna be money well spent. I think it's to much of a race carb for my motor, maybe a standard type 850 would do better?
  2. deluxe68

    deluxe68 Well-Known Member

    After all this tuning trouble I finally have made major improvements to the idle circuit. The "correct" metering blocks for the Q850 is #34-4QFT. These are 4 emulation blocks with .028 jets installed in the second emulation hole from the bottom of the block. Theses blocks also come with .033 IFR's and .055 PVCR's.

    I now have responsive mixture screws and a pretty acceptable a/f mixture at idle. With the engine at operating temperature just sitting parked, I'm running around 13.6-8 in park and when I put it in gear it jumps up to around
    14.2-4 (I'm thinking this is pretty darn good). Light cruising is in the 14's and I was able to do a couple WOT launches before the rain and it's running at 12.8. I still need to look at the highway cruise but all in all I am extremely satisfied with the outcome of the new "untouched" metering blocks with basically the out of the box specs. As long as the highway cruise checks out, I'm leaving this tune alone.

    Currently running,
    Mixture screws are .875 out from seated for 13hg @800 rpm using secondary idle screw +.625 turn from squared.
    (Primary side is squared and untouched)
    Pri jet (72) / Sec jet (80) / IAB (.073) / HSB (.033) / PV (6.5) / Squirters (31) F&R / N&S (.110)
  3. Steve Schlater

    Steve Schlater Well-Known Member

    I haven't read the entire thread but it sounds like you are on the right track. One thing to verify is where the idle jet in the metering block is located. If its above the fuel level, you are going have the problems that you have outlined. Here is a thread on how to relocate the idle jet:


    The owner of this forum is pretty well known in the carburetor industry and his site has a lot of very useful information on it. Hope this helps you out and please report back on your progress.

    Take Care,
  4. deluxe68

    deluxe68 Well-Known Member

    Upcoming racing this weekend has left me wanting to fine tune a little more beforehand.
    From a stop or a roll when you throttle hard it's going way lean and hesitates for a split second and then goes.
    Is this a fuel volume or a duration speed symptom?
    Primary and secondary currently has 0.033 nozzles and pink cams installed.

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