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Thread: Mikuni PHH adjustment question

  1. #1
    another classic car guy EricB's Avatar
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    Default Mikuni PHH adjustment question

    So I finally got my own synchrometer (SKtype) and after having warmed up the engine and disconnected the throttle arms from the throttle shafts I took some baseline readings...

    The gauge on the synchrometer goes from 0 to 30.
    Both barrels on the first carb (the one closest to the front of the car) showed a value of 6 on the gauge, while the remaining four barrels on the other two carbs all registered at 3.25 to 3.5

    The synchrometer manual/leaflet indicated that before any tuning could be done all carbs need to be within one gradation of one another... Obviously this is not the case here...

    On the other hand though my stock NGK BP6ES plugs look fine, I've got an O2 sensor plumbed just rearward of the header collector which is showing barely a tick over stoichometric so that's good too... Am getting great power and torque and even better MPG on the freeway...

    The Mikunis don't have that many adjustment features unlike Weber DCOEs for example and all three carbs have their idle mixture adjustment screws set at 2.5 turns out from seated as indicated in the Mikuni manual.

    So what can I adjust?
    Where would this discrepancy come from?
    Any ideas or suggestions?

    Thanks all & it's good to be back - yeah!

    -e

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    Registered User gramercyjam's Avatar
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    You wouldn't be adjusting mixture with a syncronmeter, just syncronizing the throttle plates for even air flow across the carbs.
    --John B
    '73 FP 240Z

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    another classic car guy EricB's Avatar
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    Yes of course I realize that... I was just stating that I've got the PHH setup as indicated by the factory manual and wondering out loud HOW to adjust airflow on a carb which doesn't really have many provisions for adjustment... know what I mean?

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    Several different Mikuni PHH series have different throttle stop arrangements for idle speed adjustment. None are cast in the carb assembly, but fastened to the sides with screws and limits the rest point of the throttle arm. If you have none, do you have turn buckle style throttle pull rods? those "can" be used to balance the idle speed of each carb, but it's not ideal as the main rod will twists and not always comes to rest at the same point.
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    Eric, Found a diagram showing the throttle stop screw and fittings of a 44 PHH, of the "Late" series (single screw jet cover)
    44 PHH
    Notice items 37 ~ 41
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    Quote Originally Posted by EricB
    Yes of course I realize that...
    Sorry, I wasn't sure since there was mention of plugs and an O2 sensor. My own experience with syncrometers hasn't been good. I prefer to measure the amount each throttle plate is open at idle and listen to the carbs.
    Last edited by gramercyjam; 07-20-2004 at 03:01 PM.
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    another classic car guy EricB's Avatar
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    Hmm... ok... I'll spend some more time on them this weekend and report back...

    Victor that picture you posted leads me to another question, in the exploded view check out part #85 which is listed as "Cooling Body Assembly"... What is that? Mine came with it, I've got nozzles plugged off because I didn't know what they were... but "cooling assembly"???? For cooling the fuel in the bowl because the carb sits above the header??? What would one route through there to cool the fuel???

    -e

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    Eric, I have them too on my set of 44's on the roadster. I have them open, attached to nothing.
    Last edited by Victor Laury; 07-21-2004 at 12:45 PM.
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    Default A/F adjustment

    Was just wondering if any of you can tell me how I would go about adjusting my fuel pilot screw on my 44PHH mikuni's?

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    Turn them all in until they hit the stops (gently, don't mash them into the stops). Then back them all out 1.5 turns. You're now adjusted. If it's too lean, get a bigger pilot jet. Too rich, get a smaller pilot jet.

    Some people go 1.25 turns with the above method. Either way, the idea is that the pilot has a function over a fairly large rpm range and cranking the screw open to change the performance at idle leaves you with a lean spot a bit higher up.
    Jon

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    My twin 44's had slightly different readings similar to this as well. The adjustment needed was at the idle stop plate adjustment screw. I don't know if this is what you need just trying to help. They needed to have the same idle stop placement before the individual adjustment screws were synced.

    I think the first two ended up being around 1 1/2 turns and the rear two were at 2 turns.
    Last edited by five&dime; 05-22-2009 at 12:23 PM.

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    Quote Originally Posted by five&dime View Post
    My twin 44's had slightly different readings similar to this as well. The adjustment needed was at the idle stop plate adjustment screw. I don't know if this is what you need just trying to help. They needed to have the same idle stop placement before the individual adjustment screws were synced.

    I think the first two ended up being around 1 1/2 turns and the rear two were at 2 turns.
    Are you answering the question from today or 2004?
    Jon

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    Hahaha. 04. Sorry didn't realize

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    Default Pilot jet sizeing?

    hey jmortesen...did I spell that right? lol, well I've got an L28 with stock cams and N42 head. I ended up turning the pilot screws 2.5 turns out from seated position. I think the turns depend on what engine the carbs are on.

    But before I did this, I bought the car with it idling relatively rough and had black smoke coming from the exhaust. When I was done adjusting the pilot screws I had no more black smoke, idle really quiet, but it tends to sputter more. Just a little during idle but more so when I take my foot off the throttle. I read in the Mikuni manual that black smoke is from the car being to rich; the pilot jet being to large and needs a smaller jet. This would also explain the sputtering right?

    How am i suppose to know what size pilot jets I already have in the car. Are numbers stamped on them?
    Last edited by Tyrone; 05-22-2009 at 04:27 PM.

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    Quote Originally Posted by Tyrone View Post
    hey jmortesen...did I spell that right?
    No. It's right there. You could copy and paste, or just write Jon if that's easier.

    Quote Originally Posted by Tyrone View Post
    lol, well I've got an L28 with stock cams and N42 head. I ended up turning the pilot screws 2.5 turns out from seated position. I think the turns depend on what engine the carbs are on.
    You CAN adjust them, but you SHOULDN'T.

    Quote Originally Posted by Tyrone View Post
    But before I did this, I bought the car with it idling relatively rough and had black smoke coming from the exhaust. When I was done adjusting the pilot screws I had no more black smoke, idle really quiet, but it tends to sputter more. Just a little during idle but more so when I take my foot off the throttle. I read in the Mikuni manual that black smoke is from the car being to rich; the pilot jet being to large and needs a smaller jet. This would also explain the sputtering right?

    How am i suppose to know what size pilot jets I already have in the car. Are numbers stamped on them?
    The numbers are stamped on the pilots. Buy the How to Modify Your Nissan/Datsun OHC Engine by Honsowetz. It has a good although brief section on tuning Mikunis, has mathematical formulas to figure out where to start with jetting. You can also download the Mikuni tuning manual from Hybrid Z: http://forums.hybridz.org/showthread.php?t=122217

    I would also suggest installing an O2 sensor and either a gauge or a voltmeter to read it. You can go crazy with a wideband if you want, I had pretty good luck with a $30 narrowband Bosch O2 and a $6 voltmeter. I've heard several people who bought the Autometer A/F mixture gauge say the sensor that comes with it is crap. The gauge works fine when you put a decent sensor in it though, apparently.

    With an O2 installed you're no longer guessing, trying to read plugs (which just about everybody does wrong, including me), or smelling the exhaust. You will KNOW what is happening with the O2. It's really worth the hassle to install one.

    My take on Mikuni tuning is that it is mostly about the pilot jet. If you have a stumble when you punch it, pilot jet (and venturis). If you're getting bad mileage, pilot jet, if you are running lean at cruise, pilot jet. For whatever reason it just doesn't seem to be that hard to get these things to run good at WOT. It's getting the right pilot for cruising around town and getting rid of the stumble going from cruise to WOT that seems to be the problem. You almost never want to mess with the pump nozzles, and main air and fuel are easy to dial in with the O2 sensor.
    Jon

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    Quote Originally Posted by jmortensen View Post
    My take on Mikuni tuning is that it is mostly about the pilot jet. If you have a stumble when you punch it, pilot jet (and venturis). If you're getting bad mileage, pilot jet, if you are running lean at cruise, pilot jet. For whatever reason it just doesn't seem to be that hard to get these things to run good at WOT. It's getting the right pilot for cruising around town and getting rid of the stumble going from cruise to WOT that seems to be the problem. You almost never want to mess with the pump nozzles, and main air and fuel are easy to dial in with the O2 sensor.
    gotta deadthread props for the best tuning tip ^ i could find on google

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