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Exactly what does the choke lever do?

R

rmcderm313

Regular Contributor
Hi All,


I'm trying to reconcile my understanding of the concept of "choke" with how my outboard ('87 25HP) is working. Please refer to this diagram of the carburetor in question.


http://www.boats.net/parts/search/M...0/CARBURETOR (MERC 25)(MARINER 25)/parts.html



So I have always understood "choke" to mean choking off air from the carb thereby making the gas/air mix richer. It may also include adding more gas in a priming effort but primarily it implied choking off air.


On this carburetor I can't see any impact pulling out the choke knob would have on air flow. What I'm seeing is that when the choke knob is pulled:


1) It pushes down on the primer assembly (Primer level part #24 on the diagram) on the side of the bowl, which I think by way of the little bellows part inside the primer forces some gas up from the bowl through the main jet into the carb body (venturi?). It only does this once when the choke lever is first pulled out. That makes sense in that it ensures some gas is in the carb, but it doesn't appear to do anything ongoing once the lever is pulled.


2) By way of the idle wire (part #25) it advances the throttle a very small amount which opens the throttle valve (part #2) a bit and sets the carb up to pull a bit more gas through the system.


But again, I don't see any change in air flow based on the use of the choke knob. Am I operating with an older definition of "choke"? Does choke on this motor only have to do with the amount of gas set to be delivered? Airflow doesn't seem to be restricted in this carb at all as there is only one valve, the throttle valve and apparently no air valve at the opposite end of the venturi.


Am I on the right track in thinking that this choke has nothing to do with air flow or is there something else going on here?


Thanks,
Rob
 
First off you are not pulling out a choke knob and there is no choke lever. It is an "ACTUATOR, PRIMER" #26 and a "LEVER, PRIMER" #24. Hence it is a primer not a choke. Item #18 is an "ELECTRIC CHOKE" which is used on some models which operates following you're understanding of "Choke" but it chokes off an air orifice in the idle mixture circuit.
 
Ahhh, good point. I made an incorrect assumption that was a choke lever. So given that the only choke on this motor is an electric choke, does that mean that for a manual start model (and hence no battery) there really is no choke at all?

The reason I'm trying to understand all of this is that I'm having trouble keeping the motor running after pushing in the primer knob (notice I didn't call it a choke knob). It runs OK while it's out but stalls when I push it in. Understanding everything this lever is doing will help in troubleshooting.

Thank you,
Rob
 
You pretty much have it - when you pull the "choke" on a rope start portable (25 horse and under) it pushes "extra" fuel into the carb throat and opens the butterfly a little bit.

So that gives you a fuel rich mix to get it started (cold) and allows it to draw extra fuel, without advancing the throttle while it's warming up.

That "choke knob" has a range between fast and slow idle and depending on how you have it set (clockwise or counter-clockwise twist) it will give you a little more or less "throttle" at the carb.

Each time you pull out that knob it "squirts" a shot of gas into the throat - so in very cold weather or if the motor has been sitting for a bit you can pull it out, push it in, pull it out again and then start (with 2 shots of fuel into the intake)...

(should have added, it also moves the timing a little bit - there is a (rod) wire that goes to the timer base that is activated by the choke knob as well)
 
Last edited:
..."it pushes "extra" fuel into the carb throat and opens the butterfly a little bit.
"

Aw contraire, mon aime! It "opens" the spark advance a bit, that does very little on these motors.

Many times I had to pop the cowl off on mine and manually open the butterfly to get a flooded motor going. Bad design.

Jeff
 
This is a primer and fuel enrichment system,. IMHO not a choke. When the primer actuator is pulled all the way out ( there is no knob to turn clockwise or counter clockwise on this model) the primer portion of the carb injects fuel into the intake manifold not the carb throat and opens a check valve that allows the vacuum in the intake manifold to draw extra fuel through the primer discharge port. fastjeff is correct, a link is provided to advance the ignition timing. If the engine will not idle properly without the primer system delivering additional fuel I would first try enriching the idle mixture with the idle mixture screw.
 
An idle mixture screw would be found on a carburetor.----Look the carburetor over carefully and see if you can find a screw with a spring on it.----The spring puts tension on it so it does not go out of adjustment by itself.
 
If you look at this picture, it looks like the screw with the spring right over the main air intake. Look right? Any recommendations for where to set it at the start? Just a turn or so until it stays running? I guess I can just play with it.

I'm pretty sure that is my issue though because I screwed it all the way in when I cleaned the carb.

Thanks!

Merc 25 87 Carb.jpg
 
..."
I'm pretty sure that is my issue though because I screwed it all the way in when I cleaned the carb."

Ah, that'll do it.

Jeff
 
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