Choke problem on old Merc 25

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MikeA57

MikeA57

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Got my boat ready for the recent Labor Day weekend by installing a brand new MK trolling motor and then I checked out the 1980 25 hp Mercury and although hard to start to begin with, once I got it started it ran fine for about 45 minutes. I stopped and started it several times and it started on the 1st pull every time.

I got to the lake and tried to start it and it wouldn't start for anything. I ended up using my new TM to get me from the ramp to the cabin. I didn't have a lot of time to spend trying to figure out why it wouldn't run over the weekend, but once back home I dug into it. What I found was that the choke is not opening the butterfly at all. If I manually open it, the motor fires and runs but as soon as I let go of the throttle the butterfly closes and it dies. Once the motor is warm though it starts and runs OK.

The choke on this engine is a rotating knob that has several detents for increasing the idle speed to keep it running while it is cold. Once it warms up you can set the idle back to normal. The last position on the rotating knob is full choke. My question is: What is supposed to make the butterfly open when the knob is rotated all the way around to the choke position?

Thanks!

Mike
 
I have a mid-90's Merc 25, and I believe most of these engines were very similar so here goes...

Mine has the same rotating knob, and it is used to increase/decrease idle speed, however this knob can also be pulled out for choke. Can yours be pulled out? I typically don't ever twist, just pull to choke, then push in once warmed up.

You can always reference the schematics on iboats to see if something is missing or broken.
 
I've got an '84 Mariner 20, I believe the line was mostly the same from back when thru the late 90's whether you had a merc or a mariner, same knob setup etc. - but at least on my motor there isn't a connection from the choke to the butterfly. When I rebuilt the carb I noticed the choke is actually a secondary gas feed to the carb that is filled by pulling the choke handle out which depresses a sprung valve and allows gas into the chamber. You hear a hiss of gas when you pull the choke, one is all you need, any more and you'll flood it - all throttle control is done via the tiller handle.
 
With your S/N you should be able to find a parts diagram here https://www.marineengine.com/parts/mercury-outboard-parts/

it will show if there is a connection
 
Thanks Nate. No, mine doesn't pull out, only rotate. I have noticed that there is a rubber plunger that the choke rod connects to (#35 in the diagram) that closes down over the enrichener valve (#2 Carb diagram) as the choke is activated. That plunger assembly is not shown on any of the diagrams I've looked at. Attached to the back of that plunger is another, thinner rod that connects from that plunger to one of the linkages (the timing linkage on Throttle & Shift Linkage diagram, if I'm reading it correctly).

DIAGRAMS
Carb:
Merc25Carbdiagram_zps563d7bbe.png

Throttle & Shift Linkage:
Merc25ThrottleampShiftLinkagediagram_zps2020ba5d.png


And when I go home this afternoon I'll take some pictures of my setup and show what I'm seeing. I think that will help tremendously.
 
The plunger you're describing and the enrichment valve are what I was trying to describe before, though the layout on yours is slightly different. By activating the choke you press that valve and it allows extra fuel to enter the carb - but there isn't a linkage to the throttle shutter apart from the throttle cable.

This isn't truly "Choking" the engine, where you block the engine's ability to use its vacuum to draw in air so that it has a richer mixture to start on. Instead you're enriching the mixture initially by supplying it with an additional shot of fuel beyond what the carb would otherwise be supplying.

Your starting issues were probably a result of flooding the engine - next time follow this sequence to start:

1.) Prime your fuel line bulb
2.) Turn the choke knob to engage the enrichment valve (you may hear a slight hiss as pressurized fuel fills the enrichment chamber)
3.) Turn the choke knob back to its original position
4.) Set the throttle to the correct starting position (~1/2 open on mine)

This will prevent you from leaving the enrichment valve open while you attempt to start the engine which can allow too much fuel to pass through if the engine doesn't fire in the first couple revolutions of the starter/pull cord.
 
[url=https://www.tinboats.net/forum/viewtopic.php?p=365377#p365377 said:
onthewater102 » 37 minutes ago[/url]"]The plunger you're describing and the enrichment valve are what I was trying to describe before, though the layout on yours is slightly different. By activating the choke you press that valve and it allows extra fuel to enter the carb - but there isn't a linkage to the throttle shutter apart from the throttle cable.

This isn't truly "Choking" the engine, where you block the engine's ability to use its vacuum to draw in air so that it has a richer mixture to start on. Instead you're enriching the mixture initially by supplying it with an additional shot of fuel beyond what the carb would otherwise be supplying.

Your starting issues were probably a result of flooding the engine - next time follow this sequence to start:

1.) Prime your fuel line bulb
2.) Turn the choke knob to engage the enrichment valve (you may hear a slight hiss as pressurized fuel fills the enrichment chamber)
3.) Turn the choke knob back to its original position
4.) Set the throttle to the correct starting position (~1/2 open on mine)

This will prevent you from leaving the enrichment valve open while you attempt to start the engine which can allow too much fuel to pass through if the engine doesn't fire in the first couple revolutions of the starter/pull cord.
Click to expand...

Agree 100%. I've owned several mercs over the past few years, and all of them were very easy to flood. I've always engaged the choke for the first pull only. And I never use the electric start with choke on. Been there, done that, learned my lesson, sort of thing. I imagine the choke is used more in cold weather, like duck hunting, but for warm weather, I almost never use mine for more than one pull of the start rope.

P.S. I like the schematics here better. https://www.crowleymarine.com/mercury-outboard/parts/676_7.cfm You can zoom in for a better close up and that really is useful if your vision isn't any better than mine. :oops:
 
I don't even leave it choked for the first pull - i open then close it and it's worked fine on 35 degree mornings and 75 degree mornings, though in the summer you're right, it really doesn't need it.
 
Pics have been uploaded of issue with the choke. They are numbered so you can follow what happens as the choke is rotated from OFF to ON. The wire leading to what I suppose is the timing linkage is not shown on any of the diagrams I have found. Neither is the plunger that closes down on the Enrichener valve.

https://s240.photobucket.com/user/MikeA57/library/25 hp Mercury

But there's still no physical connection from the choke to the butterfly. Why???

Also, I checked the operator's manual again and this choke only rotates, there is NO pull setting for it. I also found that the motor is a 1983 model.
 
[url=https://www.tinboats.net/forum/viewtopic.php?p=365502#p365502 said:
MikeA57 » 05 Sep 2014, 10:04[/url]"]...But there's still no physical connection from the choke to the butterfly. Why???
Click to expand...

There is no need because of the addition of the enrichment valve. You don't need to cut air to richen up the mix when you are instead adding extra fuel.
 
Thanks OTW, but the motor starts when I use the old timey method and open the butterfly. It doesn't when I use the choke and the new fangled Enrichment Valve.

One other thing I noticed is that last week when I was testing it out before I went to the lake, the bulb would firm up after a few pumps and I couldn't pump it anymore. Now though, the bulb won't do that. I pump and pump and it just keeps pumping fuel which I can smell after a few pumps.

Is this starting to sound like maybe I need to clean the carb???

Thanks!
 
Definitely. The bulb would get hard to pump when the carb float closed the inlet needle, if that's not happening you're flooding it before you ever get to try and start it. The only other cause for this that I can think of is the check valve in the bulb could be bad. Assuming you have clear fuel lines on the motor, or a see through filter housing, with the cover off the motor you would see this if you pumped the bulb your fuel would head toward the motor then fall back into the fuel line.

Some float needles have a rubber tip on them which gets eaten by the **** alcohol in the gas, otherwise it's probably just gunked up and not moving freely and stuck open despite the best effort of the tiny amount of pressure applied by the float. Pulling it apart and cleaning it sounds like your next step.

By chance, you didn't replace the quick connectors did you? I erroneously picked up a fuel line fitting which looked just like my tank side fitting and ended up with two opposing check valves blocking fuel from getting to the motor.

Once you get it back together you'll still need to use the throttle to open the butterfly - but you'll be opting to give it an extra shot of gas via the valve rather than involuntarily pumping gas through it.
 
So just to make this plain...you do NOT have a choke on this and many many other outboards....you have a fuel enrichment valve...equals the same thing(you can reduce air or supply more gas, it does the same thing ENRICHENS the mixture for starting)...this is a great system that has been used for many years on millions of outboards(on larger mercs/yamahas people call it push to choke but uses a solenoid to activate the enrichment circuit on carb by ignition key push). You may or may not need some carb cleaning/work I didn't understand if you ever to follow the steps he laid out for you....by closing butterfly you choked the engine and that worked so I would try using valve and then pull plug after two start pulls(or key turns) and see if wet.
 
Mercury confuses the issue by referring to the components with terms that include "choke" (such as the choke knob) when there isn't a traditional choke mechanism at all. I'm not an antique engine guru, though I've rebuilt my share of them - but his could have been one of the earlier models to use enrichment, and thus the lingo might not yet have been fully differentiated.

It is all a way of adding more fuel than air to the initial mix from what the engine would otherwise require to run. Choke the engine you reduce the air volume while leaving the fuel volume the same resulting in a richer mix (like choking someone's throat you cut air to their system), or enrich the system by giving it the means to add an extra shot of fuel on startup while leaving the air volume constant also results in the same extra fuel:air combination...if you're trying to close the butterfly on an enrichment system you're double dipping. In this case more isn't better - you'd be causing it to flood.

Though if you follow my instructions and it still floods and won't run then give it a cleaning. Double check you have spark too - too many times we chase down the issue we expect only to find it was a different issue the whole time.
 

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