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Thread: New owner, G54B, with a couple issues

  1. #1

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    1987 Mitsubishi Mighty Max
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    New owner, G54B, with a couple issues

    Good day all,
    Just snagged a 1987 MM, 2.6L 5sp. Hauled a motorcycle with it the first day, despite it misfiring like crazy above half throttle or so. As I'd expected from reading here, the secondary actuator wasn't holding vacuum, but the linkage was also installed incorrectly so it couldn't turn more than an 1/8th or so not matter what the actuator and interlock were doing. Both those issues are fixed for now, although my diaphragm repair is definitely not for the ages.

    The previous owner did a little bit of maintenance to it: cap, rotor, plugs, and fuel filter were done recently He also disconnected the EGR system in an attempt to get it run better. I also found receipts for rebuilding the carb and setting base timing in the last couple months, but I'm pretty skeptical of both of those being performed properly, if at all. I did wires, air filter, PCV valve, cap, & rotor since I got it, along with new carb base and air filter base gaskets when I put the carb back on. Trimmed a couple vacuum hoses and replaced a couple vacuum port plugs that weren't in good shape.

    It runs a bit better now, although it still misfires when I venture a bit too far up the RPM range. One possible clue is that the fairly new cap had scorch marks to the counterclockwise area of each post. We'll see if it happens to the new cap also. Judging by rotation I'm thinking the spark is sometimes being forced to jump "forward" from rotor to the cap, which would get worse if the timing is advancing with RPM/load. Is my thinking right there? Try disconnecting vacuum advance and see if it gets better as a quick and dirty check?

    Other work I'm planning, in no particular order:
    -set base timing
    -check for vacuum leaks again, and verify hose routing
    -throw new O2 sensor in
    -verify spark strength with inline tester

    If time allows I'm debating putting in a Ford CFI 2 barrel throttle body I snagged off Craig's List a bit ago. Maybe some GM waste spark coils with a crank sensor too, we'll see. For now I'd like to get it running a bit better stock.

    Thanks,
    Mark

  2. #2

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    1989 Dodge Power Ram 50
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    Did the MM have a vacuum operated secondary? I thought those Mikunis were all mechanical.

  3. #3

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    Hi markmakeitso, welcome to mightyram. A non functioning secondary vacuum actuator on the carb would not normally cause misfire. You can swap the dud secondary vac without pulling the carb off but it takes a bit of leverage. The distributor might have a broken counterweight advance spring in it, causing the distributor to over advance. I would personally try dialling the ignition back and tuning it by ear or at least pointing a timing light at it to check the base settings. The Ford carb body injection will make an interesting alternative to the traditional weber 32/36 carb install. They look a lot like the weber too so I bet it will be a relatively easy thing to set up (might be able to use a weber adapter to mount it).

  4. #4

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    Hello gentlemen,
    Andy, yup, vacuum actuated, with mechanical interlock off the primary. I tell ya, will be glad to get back to motorcycle carbs when this is over.

    geezer, I ended up pulling the carb off to get at everything. The incorrectly placed secondary arm hindered getting at the 2 screws holding it on, and even once on my kitchen table I had to use an impact driver and hammer to pop them loose. Replaced those 2 and a couple other screws with new stuff, antiseized also, while I had it off.

    I'll try to play with timing soon, I'm guessing that's my major problem right now. A guy at work recommended both disabling vac advance, and also locking down mechanical advance to test. If it's low on power at RPM but doesn't misfire so hard that could be the problem. Onward and upward?

  5. #5

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    Get the best of both worlds then markmakeitso -

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    A little bike carb insanity just for you...

  6. #6



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    there is a wire you need to ground to set the timing - look in the threads for more info on the color
    Pennyman1
    The best Dodge that Dodge never made
    Living the D-50 lifestyle since 1980

  7. #7

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    Alright geezer, will keep that in mind. I also have a throttle body rack from a GSXR-750 and a 1000 sitting here, hmmm.

    The Ford CFI mounting pattern is much bigger unfortunately, approximately 3.5 x 5.125" center to center of mounting bolts, with each bore around 1 5/8" by measuring the base gasket. The tiny Mitsu gasket in comparison is 1.75 x 3.125", and each bore is 1 5/16" or so. I'm picturing an adapter of 2 layers of aluminum, small pattern with bolts down and larger pattern with bolts or studs running up. I'm a bit worried about fuel mixture, since the injectors seem to favor one side of the throttle plates over the other. Will likely attempt to allow for more mixing volume after the throttle plates with a Ford carb spacer, which is commonly available in a variety of thicknesses. We'll see, will take plenty of pics of whatever I try.

    Thanks for the advice gentlemen, hopefully I can whip that thing into shape soon.

  8. #8

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    If you decide to mess with bike carbs, the build and tune thread on sigma-galant is pretty extensive (McGoogle it and you'll find it quickly). I personally would've spent more time on the inlet manifold and also a design for a plenum to improve torque plus allow for cold air induction. Hot air is bad for bike carbs on a bigger CC engine. I haven't had a good look at the 2 barrel Ford ECI so I was only having a guess at compatibility (Ford used Weber carbs on a number of engines - it was only logical they wouldn't change their design too radically). The Mikuni DIDTA type carbs are not fun to pull down and rebuild but they do an amicable job when they're working correctly.

  9. #9

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    Whelp, dug into that thing again today. Geezer and a coworker had mentioned checking the mechanical advance springs. Pulled the distributor out, everything looked pretty good in there. Reassembled, futzed with getting the alignment back as before. While messing with the vacuum advance line I happened to find half an inch of a small bolt (sans head) pushed up the vacuum line a couple inches. Unbelievable. I knew the previous owner had struggled with fixing it, and that some of the work he farmed out was not done with due care or knowledge, but I didn't plan on outright deception. So, with that out it runs quite a bit better, easily pulls 2nd gear up to 35 MPH, where before I was probably into 4th by that point.

    Should be able to drive it to work this week, plan to swap rear diff and trans oil, change engine oil and filter, change coolant, and take the valve cover off to check on the jet valves. Getting there, slowly.

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