Re: [OMC-Boats] carb operation:

From: jd <jdood@...>
Date: Thu, 10 Sep 2009 11:51:07 -0700

On Sep 10, 2009, at 10:54 AM, BLDFW wrote:

> If I'm understanding your questions, the flapper is controlled by
> the spring housed inside the round box mounted to the left of the
> flapper when looking at it with the linkage to the right. The rod
> that comes down to the lobe/cam controls the cam. When the choke is
> closed, the rod pushes the cam down to the degree that the idle
> adjustment screw rests on it allowing the RPMs to be increased while
> the choke is on.

ahh ok, so it's designed so that the cam can just move up or down
even if the tip of the screw is resting on it. So that screw tip
style is important and I need to get an original screw then. There's
too much friction with the one i have. I'll grease it but still....

and i see where the flapper is binding up now (in part). on the rod
where it bends in and L shape and goes into the hole in the cam,
there is a fin cast on the bottom. That fin is scraping paint where
it's rubbing. I might just dremel that off. And/or grease.
> IF the choke is working properly, when the engine is hot, the choke
> is open the cam is up far enough that the idle adjustment screws
> will not contact it but rather will rest under it allowing the
> least level of RPMs. This is normal.

I have my flapper all the way open right now on my desk. the cam
ends up at it's halfway point. So the screw is resting on the
highest point of the cam when flapper is fully open. Not correct?

> As the engine cools sufficiently (like overnight), the choke springs
> begins to apply pressure to the choke plate to close. When the
> engine was shut down warm, the idle adjustment screw should be under
> the cam.

just like it was.......that explains why it was always appearing under
the cam - i had been out using it and it was hot.

> As the spring applies pressure, the cam will come into contact with
> the screw but obviously can not continue until the throttle is
> manhandled (ie;d pumping) and the screw is pulled back allowing the
> cam to drop down as the choke plate closes. With that, the idle
> will be raised while the engine is warming up. When sufficiently
> warm, if everything is working, as the choke plate opens, the cam
> will rotate up out of the way allowing the idle adjustment screw to
> once again rest under the came thus allowing the RPMs to drop to
> their lowest setting.

everything you are describing makes sense however seems to be the
reverse of my setup. When my flapper is opening, the cam is
dropping (rotating) downward, not upward as you're saying. Did you
mean down? When my choke plate drops down (closed) it rotates the cam
upwards (CCW). Do i have something backwards? Note these two



is something wrong with mine?

> It sounds maybe like the cam is occasionally not rotating completely
> out of the way and the screw is catching the end of the cam not
> allowing the RPMs to drop down. That might be due to the size of
> the screw you have in use. It looks bigger than the original one on
> my carb.

the tip isn't tapered like the original. Im gonna see if i can get
one today and see if that makes a difference.
> I don't rightly know the purpose of the tab sticking out the back
> side of the came. Mine has one too. I daresay it's likely a "stop"
> that is utilized on a carb version other than the ones we have.
> Those tabs are pretty universal across a multitude of Rochesters.
> The setup on these carbs are pretty simplistic so likely it has no
> use on these particular carbs.

ok - very coincidental that the tab sticks out on the back of the
cam, very close to a tang on the back of the throttle. and the
manual then says to bend the tang to adjust. Sheesh......this stuff
is mind boggling sometimes.

> Now, if the screw is not resting on anything and your RPMs are still
> high, it could be the adjustment on the throttle housing. Not sure
> about your setup, but on mine there is an idle adjustment screw for
> the throttle lever itself. It might be preventing the throttle from
> being pulled all the way back to the lowest RPM setting. In other
> words, it's holding the carb throttle open.

yep - i have that same screw.....formerly known as the mystery
screw. But on mine, it adjusts the top end stop of the throttle,
not the low end stop.

> -Bill
> Dallas, TX
> 1970 Evinrude Explorer - 155 Buick V6 - OMC Sterndrive
> --- On Thu, 9/10/09, jd <jdood@...> wrote:
> From: jd <jdood@...>
> Subject: Re: [OMC-Boats] carb operation:
> To: "Evinrude & Johnson Boats of the 1960's and 70's" <omc-boats@...
> >
> Date: Thursday, September 10, 2009, 12:10 PM
> 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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