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Thread: Setup recipe for a 390 cfm on a small engine?

  1. #1

    Default Setup recipe for a 390 cfm on a small engine?

    Does anyone have a setup recipe for a 390 cfm on a small displacement engine 165in^3? I've been fiddling with this carb for about 200 miles and can't get it to run right. I have a PRV V6 165in^3 or 2.7L with flowed heads, headers, and a lofty cam. HP is expected to be about 180 HP to 200 HP HP. I'm looking for a starter recipe for jets, etc., to help shorten the trial & error path I'm on, or to use as a confirmation I'm on the right path. I would appreciate help. BTW, no it does not have a wideband, and I'm avoiding putting one in so the car will look stock.

  2. #2
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    What is the list number of your 390? Gary
    Regards, Gary

  3. #3

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    Not sure what you're asking. It's a 4160 390 cfm. I'm looking for feedback from owners that have tuned one.

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    Being a 4160, it must be a vacuum secondary, so it could be a 6299 or a 8007, etc.
    Does it still have the choke tower, the number is stamped on the front?
    I've tuned many Holley carbs, and I build my own custom carbs. Gary
    Regards, Gary

  5. #5

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    It is an 8007. Some Specs:
    Engine is a PRV V6 2664cc (162.57in^3)
    CR = 10.1
    CAM = 292° duration, 108° Lobe Ctr, lift = 11.0 mm

    Port Flow - lift @ 28 in/H20
    0.1in: I=92.5cfm E=41.3cfm
    0.2in: I=135.1cfm E=76cfm
    0.3in: I=158.1cfm E=98.5cfm
    0.4in: I=180.3cfm E=113.2cfm

    Original Setting:
    PV=6.5
    Enrichment screws=2 turns
    cam=orange
    Pump Jet=25
    Main=51
    Secondary Metering Plate= 34R-9716B, 3, 59.

    car idled rich, bucked and popped through the carb at 3000 RPM during driving, hesitated when trounce on throttle and secondary was not opening (paperclip test).
    Vacuum at idle was very difficult to measure but looked to be 10 in. The float level was checked to be correct and timing set to 10° advance.

    Round 1 Change:
    PV=8.5 (was 6.5)
    Enrichment=2 turns
    cam=orange
    pump jets=25
    main=48

    Improved bucking and popping through the carb @ 3000 RPM. I was trying to figure out how the PV works and it appears to smooth the fuel supply between the peak demands in the transition circuit. This is a new device to me.

    Round 2 Change:
    PV=8.5 (was 6.5)
    Enrichment=2 turns
    cam=orange
    pump jets=28 (was 25)
    main=48

    Eliminated hesitation when the throttle was trounced on.

    Round 3 Change:
    PV=8.5 (was 6.5)
    Enrichment=2 turns
    cam=orange
    pump jets=28(was 25)
    main=51 (was 48)
    Vacuum Secondary spring changed to Orange.

    Top end improved and mid-range greatly improved.

    Round 4 Change:
    PV=6.5 (was 8.5)
    Enrichment=2 turns
    cam=orange
    pump jets=28 (was 25)
    main=58 (was 51)
    Vacuum Secondary spring changed to white.

    Better response but some hesitation at 3000 RPM and surge when secondary opened.

    Summary Of the Rounds:
    The car idled rich and would blow sooty exhaust at each tuning round. I would reset the enrichment screws and found 1.75 to 2.0 turns worked best. I had removed the carb several times to verify the primary butterfly was not adjusted to the point of opening up to the transition port (beyond the idle port hole) and it always was. I would reset it and tune it again tweaking the primary and secondary butterfly to get it to idle.

    So this brings us to today. The car still idles very rich and its been too cold to and salty to drive it on the road. I took the carb off again and made some adjustments and verified the primary butterfly was at the idle port (rectangle slot on the port looked like a square at adjustment)and did not touch it. I set the idle with the secondary butterfly and got it to idle but rich. A few days later I fire the car up and it runs terrible and is rich again. I fiddled with it and got it idling again but tweaked both the primary and secondary butterfly to get it to work right. I figured I would reach out to get feedback from people tuning small displacement engines. My guess is if I removed the carb today the primary butterfly is opened to the transition slot again. Also in each case when I pulled the plugs they always look sooty. I reset the carb bits back to stockish and starting again. I could put in a wideband and go to a dyno to sort it but the rolled up cost of doing that would be nearly the same as a smart EFI system. That said however the small displacement engine with only 180-200 HP is out of the parameters the EFI system was designed for so I'm back in the mess again. If that wasn't the case I would have pulled the trigger on that direction already.

    Round 5 Change:
    PV=4.5 (was 6.5)
    Enrichment=2 turns
    cam=orange
    pump jets=25 (was 28)
    main=48 (was 58)
    Vacuum Secondary spring white.
    Added Quick Fuel adjustable secondary metering plate Secondary Jet=54.

  6. #6

    Default One More Point

    I forgot to mention, every single part is brand new.

  7. #7
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    Classic Holley carbs have been a staple for rodders around the world for decades. They're calibrated for use on stock to mildly modified vehicles and have that classic Holley look. Holley's 0-8007 is calibrated and intended for small V8s or 6 cylinder engines as well as 2x4 street tunnel ram style set ups. Plus, it is Gold Dichromate coated for durability and classic Holley looks. Don't trust your performance to just anyone, Holley is the only modular carburetor company that has been in business for over 100 years and has powered every NASCAR champion since the '60s.

    A camshaft like you have needs more timing at idle, so your distributor needs a recurve.
    But for now, can you wind the timing up to 15°?
    The mixture screws are open too far at idle, 2 turns open from seated shows the idle circuit is lean.

    What inlet manifold are you using?
    Your engine may be small, but it breathes like a larger engine because of the cam and compression.
    So it pulls really hard on the idle circuit.

    The engine shows it wants larger mains, which really should be larger PVCRs, and a slightly larger main after lean surge at cruise in found.
    I think you would be happier with a 1848 450 vacuum secondary.
    I've ran a 600 DP on a 3.3 a few years back, ran great, but a DP is not economical. Gary
    Last edited by Gaz64; 02-24-2019 at 07:01 PM.
    Regards, Gary

  8. #8

    Default

    Not sure I follow everything, but then again this carb is new to me. So if anything I would say the 390 carb is too big. Based on the general calcs this engine should be about 255cfm with a 90% Volumetric Efficiency. That seems optimistic to me and even if you compare the rated vs estimated peak vacuum that calcs out to be 332 cfm. Unless I'm missing something I'm thinking 390 is 390 cfm and sufficient.

    I can advance the timing to see if that helps but tweaking timing curve is not a great idea from my perspective unless you are very familiar with the engine, which I'm not. I have one source in the UK that could help but most of the PRV expertise is no longer available.

    I see a reference to a 1848 450 (465? or marine?) carb, but then again that seems to me to be too large of a carb based on the first point. I'm not understanding how that would be better.

    So if a larger carb is the recommendation would it be better to just go to the Sniper EFI system and let the system auto tune itself? What are the thoughts on that?

    I will try advancing the timing at idle and will reach out to my UK buddy in the mean time while I ponder the other recommendations.

    The intake manifold is the standard PRV ant/spider looking type that has been modified to accept a Holley (single plane maybe? - not really sure what that means) The OEM Solex carbs would not be up to the job for the porting and cam so instead of a very expensive set of Weber IDAs the Holley is what some of UK guys have done. To date however no one has modified the engine like I have and has info on the Holley fit up. I am blazing a trail.

  9. #9
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    With a vacuum secondary carb, the secondary barrels will only open sufficiently to meet engine demand.
    So if the engine only "needs" say 300 cfm, the secondaries may only open halfway.
    A 1848 is a 465, sorry for the confusion earlier, I had 450 on my mind when I typed that.
    If your manifold is now modified to single plane configuration, then a 390 is only a 195 two barrel initially.
    Holley rates their carburetors for cfm versus manifold depression, 1.5 inches for 4 barrels, 3 inches for 2 barrels.
    So a 390 4 barrel needs to reach 1.5 inches of manifold vacuum before the carb is flowing 390 cfm.
    You leave power on the table if the carb produces restriction like that.
    A 390 4 barrel is initially smaller than a 300 2 barrel, and should give better all round throttle response and power.
    This 390 list 8007 is brand new, is that correct?
    And the float levels are correct with the engine running? Gary
    Regards, Gary

  10. #10

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    Yes, it is all new, a 8007 and I have checked the float level many times. The manifold looks similar to this Holley Single Plane Mid-Rise Carbureted Intake Manifold GM LS1/LS2/LS6 Cathedral Port, but is a 6 cylinder. This may help.

    This is the first time starting the engine:
    https://www.youtube.com/watch?v=WPLaNYjItnA

    This is how it ran the last time on the road (round 4 settings):
    https://www.youtube.com/watch?v=nGIV_8LbIl8

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