Re: [OMC-Boats] carb operation:

From: jd <jdood@...>
Date: Thu, 10 Sep 2009 10:10:11 -0700

wow - thanks for this very clear explanation. now I feel like i know
how it works atleast. see my responses further.....

On Sep 10, 2009, at 6:17 AM, BLDFW wrote:

> The upper flapper is your choke flap. When closed, it reduces air
> which in turn enriches the fuel. It's useful when the engine is
> cold and needs richer fuel. Without it the boat might be hard to
> start or to stay running until it's wamred up. If the flapper is
> closed too far, not enough air is flowing and the engine will under
> perform or stall. It's on a temperature affected spring that holds
> the flapper closed when cold and eventually eases it open as the
> choke mechanism warms up.

ok this part i need to dig into deeper. So as I'm moving the flapper
mechanism with my finger and it doesn't seem to want to return all the
way closed, it's because the spring operates by temp. Makes
sense, but what doesn't make sense is if that flapper works all on
it's own by temp, why is it connect with that rod down to the cam and
arm near the idle screw? On the back side of the cam there's a
little arm that sticks out. It looks like it's suppose to intersect
with the tang on the idle lever. On my carb neither of the two ever
touch. They look like their supposed to touch and push against each
other, but then again they don't seem to line up enough to even bend
them to touch. I tried a little and any resulting operation would
have jammed. So why the rod down to that cam? why the arm sticking
off the cam and coming about 1/4" from the tang on the throttle? This
is the part that's still confusing me.

> The heat source for the choke mechanism is the tube that comes up
> from the intake manifold. The choke will stay closed or close to it
> at idle enriching the fuel but as the throttle is opened the choke
> plate will open on it's own

[ on it's own ] - right? So again, the rod that runs from the
flapper lever down to the cam piece is for what then? It just looks
like the choke flapper is supposed to work with the throttle via that
cam piece somehow. In fact in the manual they say to adjust
something by bending that tang, implying it's supposed to be pushing
on something. The only thing near it to push on is that arm from the
cam. But unless mine has been screwed up by someone, i don't see
how that would ever be possible - the arm and the tang can't quite be
bent enough to touch smoothly. hope that's making sense.

> to allow more air to flow. If the choke plate is held closed for
> any reason, as mentioned not enough air will flow causing bad
> performance.

well i believe you, but see below as to why it's not making sense
> The idle adjustment screw on your's is a lot bigger than the one on
> mine and has clearly been replaced with a non standard one.

yes, i had to change mine out.

> In anycase, it should rest on the lobe or above it but never beside
> it.

no, it's UNDER the cam, not beside it. It lines up perfectly and
isn't bent or anything. So when I came back to the dock (and while
out in the water fiddling) after that nice 1/2 hr run - the screw was
always sitting in line with the cam but UNDER it - as shown in
photo. And when I put it back in that same position, it requires
the flapper to shut to be in that position. Which means i was
successfully driving around at WOT with the flapper shut. But
according to your explanation above I should've noticed problems due
to over-enriched fuel. I distinctly remember the position of that
screw because the idle wouldn't drop below 1500 and i was worried
about shifting that high. So i was back there checking it out - both
while out on the water a couple times, and back at the dock. I
distinctly remember seeing that it wasn't resting on the cam and
thinking "that don't seem right". But it was working fine that
way, i was clueless, and so i always left it in that same
postion. Most of those times however i IDIDN'T have the spark
arrestor off to even see the position of the flapper anyway. Only
saw the flapper when I took off the SA (and lost the little brass
screw down the gullet).

So this is my dilema - Other than the flapper issue, maybe this carb
is fine, or maybe it's completely screwed up. It worked before.
But I'm 99% sure with that screw under the cam. So I'm tempted to
bolt this thing back on that exact same way and test her. Even
thought it's apparently the wrong way.

> Not sure how that can be unless someone messed up the trajectory of
> the screw when they replaced it or the lobe is loose allowing the
> screw to slip off the lobe. That is likely the culprit of the high
> idle problems you have referenced. If the screw is resting against
> the side of the lobe, it can affect the choke plates ability to open
> fully on it's own which in turn can affect lower speed operation by
> causing the engine to run rich. As mentioned above, with higher
> throttle RPMs the screw is not resting on the lobe and thus the
> choke plate is free to open as more air flows through.

ahhhh - ok so at WOT the flapper/cam moves where it wants. Then at
low throttle the screw comes back and rests on the cam. So if your
at WOT for a few minutes, in theory the temp would be causing that
flapper to open up. then you suddenly drop to idle, the screw is
gonna come back and rest against the cam in whatever position the temp
has it. And then if the temp starts dropping, the flapper wants to
close, but then the screw is gonna be holding it open isn't it?

> If the screw slips off the lobe, in theory, it would cause the RPMs
> to be minimal at best unless perhaps it's causing the choke plate to
> close which might cause the RPMs to increase due to the enriched fuel.
> -Bill
> Dallas, TX
> 1970 Evinrude Explorer - 155 Buick V6 - OMC Sterndrive
> --- On Thu, 9/10/09, jd <jdood@...> wrote:
> From: jd <jdood@...>
> Subject: [OMC-Boats] carb operation:
> To: "Evinrude & Johnson Boats of the 1960's and 70's" <omc-boats@...
> >
> Date: Thursday, September 10, 2009, 3:14 AM
> so now on to the important questions. This is how things were
> set when everything was working great. Only issue was when
> returning to dock, idle would sort of stick up around 1500 when in
> N. Which is why I took off the spark arrestor and started fiddling
> with the choke levers and idle screw stuff in the first place. Just
> to see how it was supposed to work. Note idle screw stops/rests
> UNDER that cam lobe. This is how it has been and has been working
> fine. The top flapper can't really move when it's like this in an
> almost closed position but it didn't seem to affect how the motor
> ran anyway:
> Impossible position to get to with screw resting under cam.
> Although the manual seems to show this position as sort of the
> "normal" position. So this means that when I was having those
> successful runs, I guess the flapper was never really open, or
> being used. And, unless I am missing something here, It
> doesn't seem to matter. So my main question is can I just put
> the screw back to where it was under that cam (as in 2nd photo) and
> call it good til the end of the season? What does that flapper do
> anyway? Should the engine run well with that flapper closed
> almost all the way, or am I just imagining something that couldn't
> have possibly happened? It seems like that whole flapper
> mechanism can be disregarded and bypassed. Esp since it isn't
> smooth operating and jams up anyway. Hope I can just bypass
> it. Sorry for the basic questions, but I really don't
> understand carbs.
> I am ordering a new carb in a matter of days, but It's foolish to
> rush the shipping. So by the time I get it the season will
> probably be over. So this is all just to allow me a noise test,
> and if the noise is gone, get me by for the next 2-3 weeks. Which
> this carb WAS in fact doing before all this rattling noise stuff
> happened.
> thanks! Jeff D
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Received on Thursday, 10 September 2009

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