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Thread: Mikuni reinstall - reving high at idle

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    Default Mikuni reinstall - reving high at idle

    I just rebuilt my triple Mikuni PHH 44s reinstalled and set the pilot screw to it's original setting of 1.25 turns out from bottom. The butter fly valves are completely closed and it will idle around 1000 RPM(ish) then it will idle right up to 2.5 to 3K RPM after a minute or so. That's with the starters turned off. If I play with the butter fly valve opening (idle set screw) and turn in the pilot screw to about 3/4 it seems to run but its rough. I haven't balanced it out yet. I'm trying to understand what's going on so a little direction on my misunderstandings would be great. My understanding is that the pilot screw adjusts the amount of air added to the mixture. Turn it in - less air. Turn it out - more air. So the part that confuses me is if I turn it in to 3/4 and open the butter fly valves it seems to run better. I can understand that the air is metered differently through the pilot screw adjustment than the butter fly valve but that would be at various RPMs. Shouldn't it be the same at idle?

    I rebuilt the carbs completely but I was told not to mess with the throttle plates and shafts so I never replaced the shaft seals. I'm assuming air could be leaking past the shaft but it never did before and I would think it would have to be a significant leak to go from 1000 RPM to 3000 RPM.

    Thanks guys any help is always appreciated! I'm dying to get this back on the road. The plan was to have it ready at the begining of the summer but I kept doing the "while I'm at it" lol.
    "Rachel" - 1972 Datsun 240Z
    "Kayla II" - 1983 Datsun 280ZX
    "Unnamed" - 1983 Datsun 280ZX Turbo
    "Unnamed" - 1993 Nissan 300ZX Convertible
    "Zathina" - 2009 Nissan 370Z 6MT PB Touring Sport

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    Registered User steve91tt's Avatar
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    As I understand it, the pilot screws adjust the amount of fuel that is added at idle. The pilot jet meters the amount of fuel for idle+progression ports (the progression ports are the little holes that are uncovered directly after throttle tip in. Ideally, these should be covered at idle). Therefore, adjusting the pilot screws changes the air to fuel ratio at idle. However if you adjust the position of the butterfly with the idle adjustment screw enough that you uncover the progression ports then you will start to meter fuel through them and this can cause a rich condition. If you don't have your carbs balanced at idle you can have progression ports on some cylinders open at idle and some closed.
    Steve

    1973 240Z (daily driver)
    1971 240Z (track car)

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    Registered User madkaw's Avatar
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    It might be helpful to know more info as far as what engine and what pilot sizes and jets you have in the carbs now.
    Do you own a unisync to try and sync the carbs? Just one carb out of adjustment will send the idle up.
    So the engine idles up like this without you touching the throttle at all?
    maybe one of the starters is letting in air?
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Thanks guys - the wheels are turning now. I think the jetting is fine - it ran fine before I pulled the carbs off for a rebuild. The starters sound like they have the potiential to let significant air in to bring the RPMs up that high. I think a leaky throttle shaft wouldn't be that significant. I was pretty carefull with the linkage but I think I'll disconnect it all to take it out of the equation. I also left the booster hooked up on #6 so I'll disconnect that as well. I don't have a manonmeter but I did build my own with 6 MAP sensors like they use on the Mega Squirt ECUs. I works fine on the bench but when I hooked it up there are too many pulsations on the intake so I need to some averaging to dampen it out a bit.

    "Rachel" - 1972 Datsun 240Z
    "Kayla II" - 1983 Datsun 280ZX
    "Unnamed" - 1983 Datsun 280ZX Turbo
    "Unnamed" - 1993 Nissan 300ZX Convertible
    "Zathina" - 2009 Nissan 370Z 6MT PB Touring Sport

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    Registered User steve91tt's Avatar
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    Do you have a wide band to go with that MAP sensor dealy-bob.
    Steve

    1973 240Z (daily driver)
    1971 240Z (track car)

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    Registered User Travel'n Man's Avatar
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    Engine info and current jetting setting would help. AFR is a must for triples these days. How many sets of Mikuni's have you rebuilt in the past? I have recently discussed with Todd at Wolf Creek about the basic settings and he did agree that the 1 or 1.25 turn setting is BS - if you need to go beyond 2-2.5 turns then you may want to revisit jetting but there is no basic setting per past threads.
    Life's a journey; enjoy the ride!

    Mitchell
    L28 - N42 Block w/Flat tops - N42 Rebello Head & Cam - Triple 40 PHH Mikuni's - Headers - Recaro Seats -
    R180 3.9 Diff - Close Ratio 5 Speed - Toyota Vented Brake Upgrade w/ Porterfield High Performance Pads & Shoes

    1972 Datsun 240z
    HLS30-75040


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    I picked up another WB02 yesterday (didn't want to pull the one out of my 280ZXT MS3 setup). Haven't installed it yet but... I found the problem. The starters have a spring on them between the top plate and bottom plate and a circlip that keeps them from coming up off the shaft under the spring pressure. I made sure to align the tooth that seats onto the inner portion of the starter but the spring was pushing the top of the starter up too much and the tooth was disengaging. I'm getting a solid idle now - thanks guys! Bang on MadKaw! Now to tuning
    "Rachel" - 1972 Datsun 240Z
    "Kayla II" - 1983 Datsun 280ZX
    "Unnamed" - 1983 Datsun 280ZX Turbo
    "Unnamed" - 1993 Nissan 300ZX Convertible
    "Zathina" - 2009 Nissan 370Z 6MT PB Touring Sport

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    Registered User madkaw's Avatar
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    Glad I could help-I like the troubleshooting game!!
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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