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. 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 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.
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. In anycase, it should rest on the lobe or above it but never beside it. 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. 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
http://www.photobucket.com/evinrude_explorer
--- 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@...mate.com>
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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