Carburetor principles

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    smokeonthewater
    US Member - 1 Year
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    #181347

    Hello, I have a 1952 Evinrude fleetwin
    I haven’t run into this problem before, I must be overlooking something. The engine starts on one pull, runs and idles smoothly.
    When I move the lever to the right, rpms increase, but to a medium slow engine speed. The compression has t been checked, but feels strong, the spark looks strong and I cleaned the carb. I plan on cleaning the carb again this weekend, but I have a question about the reed plate. Many of my motors don’t have this, what is it’s purpose and is it possible to have installed it backwards or something and experience the issue I am having? Any thoughts most appreciated!

    frankr
    frankr
    US Member - 1 Year
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    #181348

    So your question is about the purpose of the reed plate? Actually, it is so simple that most of us don’t even give it a second thought, nevertheless it is a worthy question. It all begins with the basic theory of operation of a two-stroke engine. On the piston’s up-stroke, a partial vacuum is formed within the crankcase. That low pressure draws in air and fuel through the carburetor. Then on the piston’s down-stroke, that air/fuel charge is compressed. When the piston nears the bottom of it’s stroke, intake ports open and the compressed air/fuel escapes the crankcase and enters the cylinder. Very soon, the intake ports close again as the piston starts it’s up-stroke and the process begins all over again. Since the question concerns reeds, I am deliberately leaving out the rest of the story,

    OK, so we now see how the air/fuel gets into the crankcase. There is one other thing to understand. The air is being drawn in on the up-stroke and being compressed on the down-stroke. So why doesn’t it simply blow back out from where it came, back through the carburetor? Aha, enter the reed valves. They are simply a one-way flapper valve. They pull open in one direction (vacuum) and slap shut in the other direction (pressure). No, obviously they wouldn’t work if installed backwards if that were even possible.

    The other part of the question concerns why some two-strokes don’t have reeds. Well they may not have reeds, but they do have some sort of valve to allow air to enter the crankcase on the up-stroke, but not blow back out on the down-stroke. Most often, that is accomplished by some sort of rotating valve (rotary valve engines), or a special port or ports opened and closed by the piston skirt (third port engines). Or even a combination of the two, such as the Johnson TD-TN series.

    Reeds were not used on the real old designs, because metal technology didn’t exist yet. Those things are constantly flexing back and forth and would soon break if not made of a metal or other substance able to withstand all the bending. But once invented, reeds pretty much became the standard because of the simplicity, suitability over a wide RPM range, and low cost. An exception might be stuff like chainsaws where high RPM takes a back seat to slow idle. They most often use a third port design for durability, light weight, and low cost.

    Hope this helps.

    frankr
    frankr
    US Member - 1 Year
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    #181349

    BTW, you wrote “The engine starts on one pull, runs and idles smoothly.”. With that in mind, you can rest assured that the reeds are doing their job just fine.

    frankr
    frankr
    US Member - 1 Year
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    #181350

    BTW, BTW, I’ll bet your 1952 Fleetwin won’t rev up because it is running on one cylinder due to old, no-good coils. Almost guaranteed if the original coils are still in there. Have you looked?

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    smokeonthewater
    US Member - 1 Year
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    #181429

    Thanks for the generous reply! Obviously, my reed valves must be working well!
    I had to revisit some of my collection, I suppose my Johnson series ‘A’ accomplishes porting with the crankshaft acting as a ‘blowback’ valve?
    My fleetwin does have the original coils, it would stand to reason if they are known to go bad, I should replace them. It shure runs smooth on one cylinder.
    Anyone know a part number for replacement coils?

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    smokeonthewater
    US Member - 1 Year
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    Topics: 17
    #181431

    https://www.amazon.com/Ignition-Johnson-Evinrude-Cylinders-replaces/dp/B004S6RH1M
    Will thee work on an early fleetwin?

    frankr
    frankr
    US Member - 1 Year
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    #181432

    https://www.amazon.com/Ignition-Johnson-Evinrude-Cylinders-replaces/dp/B004S6RH1M

    Will thee work on an early fleetwin?

    Unfortunately, I lost 9/10 of my part reference books when my computer barfed. I think that is the coils for your motor, but not totally sure. 1952 was a year of rapid change in the OMC magnetos.

    frankr
    frankr
    US Member - 1 Year
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    #181433

    If your magneto looks like this one (note the points & condensers type), then the coils you suggested are correct.

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    Tubs
    Tubs
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    smokeonthewater
    US Member - 1 Year
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    squierka39
    squierka39
    US Member - 2 Years
    Replies: 315
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    #181443

    If your Fleetwin is a ‘52 then it uses the earlier ignition. Coils and points are different. But, the later coils can be adapted to work but swapping them onto the old laminates.

    mercuryman
    mercuryman
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    #181452

    Is the clutch engaged ?

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    smokeonthewater
    US Member - 1 Year
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    #181469

    Nope, she is in full drive…. the timing lever has full travel, and the boat moves foreword ….. just like Wisconsin’s motto!!

    Good idea, though!

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    garry-in-tampa
    Lifetime Member
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    #181553

    This is the ’52 Fleetwin magneto –

    Garry-now -in-Michigan

    frankr
    frankr
    US Member - 1 Year
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    #181555

    No wonder we are confused. If Garry’s picture truly is for the 1952 year, it shows coil # 580118, which is replaced by 580477 available everywhere including e-bay as previously posted. Pretty sure the 1951 coil is # 580040, made of unobtanium and priced accordingly.

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