Testing an electric choke?

Just to make sure my electric choke is functioning properly, can I just watch it while the engine is warming up and visually see it opening up?

'01 25 four stroke Yamaha.

Most electric outboard chokes I've seen only closed while you have the key depressed or button in some cases. But I've got zero experience with 4 strokes so maybe they operate differently. Automatic chokes on vehicles are the only ones I know of that open gradually as the engine warms up, but that may be the way 4 strokes work for all I know.

With the key on and pushed in it should function. What motor is it? The merc 20 I had the installer used the wrong pigtail, should have been a 9 pin they used a 6 pin. Something to that effect. Thats how I discovered the problem. The choke never worked. I ended up using a piece of stainless wire fed through a rubber plug and manually operated it.

25 yamaha 4 stroke, and many others, do not have electric "choke". They have prime start. The way it works is very similar to an ATV carburetor's prime start (aka choke). A passageway in the body of the carburetor is closed off by the plunger built into the prime start assembly. A cold prime start, the plunger is "up" (sucked into the prime start body). This opens up the passages to allow extra air and fuel into a cold engine. Once the engine is running, the generator supplies voltage to the 2 wires of the prime start, which warms it up. As it warms up, the plunger "drops" back down into the carb body, which closes off the enrichment passages.

To test it: Remove it, and remove it from the two wires. Stick it in the freezer for an hour or so. Then pull it out, and quickly measure the length of the plunger, measured from the base of the prime start body to the end of the plunger (not the needle). Then put the plunger out in the sun for a while. It should extend. How much? There really isn't a specification. It just needs to extend. I've never seen one fail, so I can't say how they "usually" die.

Most commonly, the engine gets hard to start when cold, but once hot, it fires up just fine. That is usually a function of the passageway inside the bowl of the carburetor becoming plugged. Same passageway also feeds the accelerator pump, so a quick blip of the throttle will usually result in a bog or sometimes a backfire. If the bowl passage is plugged, it won't allow the prime start to enrich the mixture, effectively "not choking" it-but the air passage will be open so then it's hard to start. The passage in the bowl has a restriction in it. If you flip the carb upside down and study it, you'll see where the restriction is. Follow the drilled passages, at the end of it you'll find a brass "ball" pressed into the bowl. It has to be drilled out to get to the restriction and the passages that need to be clean. Once clean, you'll have to devise a way for the passage to be sealed back up. A 8mm long 5mm flange bolt with a 5mm aluminum sealing washer works well, but it also entails tapping the threads into the bowl. If you get a crew that's too long, it'll block the intersecting passage and your "choke" and accel pump will not work. Cleaning the passage is tedious. My eyes are not good enough to do it anymore without aid of a magnifying glass or something similar.

Don't want to go through that? Replace the bowl. Its the only other way to do it, and honestly is the best way to do it. But it's not cheap. Be advised that the bowl of the manual start motors is different than the electric start motors, so be armed with that info if you have to replace it. Easier to just use the motor's model number (eg F25ESHA or F25MSA, etc)

If you have to go into the bowl, replace the bowl gasket (old one gets flat and will not seal properly). Also replace the float valve assembly as the new ones are more resistent to todays garbage fuel. Keep in mind that when replacing the float valve, you'll also have to replace the little plastic "pin" that's pressed into the float valve itself, as it's not reusable from the old one. It just pushes in with finger pressure but make sure to lightly lubricate it before pushing it in.

2001 may have had the old style carb, which was non-adjustable. When servicing one of these, under the top cover there is a tiny jet with o-ring that tend to fall out. They are about the size of a ball point pen's tip, tiny, and easy to not see them. If you blow it out and the jet/o-ring fly away, they are gone forever, and you can figure on a $20 mistake. Please don't ask me how I know..

If you had to replace the carb, get one for a 2005 or newer as they have adjustable idle mixture screw. The earlier ones did not. The newer ones, the screw can be removed (cap has to be drilled out to gain access to it), and then that passage can be cleaned. And the main benefit is obviously that the idle can be tuned MUCH better instead of just a little too lean.

turbotodd said:

This opens up the passages to allow extra fuel into a cold engine.

Click to expand...

Fixed that for ya. :wink:

While I have zero experience with 4 stroke outboards, I do have experience with quite a variety of carburetors. As far as I' aware of, there are 3 different methods used by carburetors to enhance cold start ability. The objective with all 3 is to increase the fuel to air ratio. While most people will call any/all of the 3 systems a "choke", only one design is a true choke. In this design a plate/butterfly to restrict the air flow into a carb. By reducing the volume of air allowed into the intake, it increases the fuel to air ratio. The second method is achieved by adding an extra passage from the float bowl to the throat of the carb. This passage is normally closed/plugged for normal operation. When activated, the passage is opened, allowing the engine to draw extra fuel directly from the float bowl, thereby increasing the fuel to air ratio. This design is actually an enrichment system rather than a choke. The third method is to use a primer. A small pump that when activated, squirts a small amount of gas directly into the throat of the carb. I have the least experience with this method and I'm unsure whether this design does anything other than that. This is another "enrichment" system. One system reduces the amount of air, while the other two designs increase the amount of fuel, but all three designs work well when functioning properly. Personally, I'd much rather have a true "choke" system just because of it's simplicity and reliability. I couldn't guess at the number of times I've seen failures because a tiny passageway becomes blocked.

If any of these systems increased the air flow in any way, it would be counter productive to increasing the fuel to air ratio, which is the only reason I mentioned that portion of Tods post.

When it comes to me and making mistakes, I can put up numbers that would rival those used when talking about the national debt. So lets go with the probability that this post is going to contribute to that number, and someone will be along shortly to correct me. But it's all good, because I'm always up to leaning something new.

No, it (at least on the F25) also allows more AIR to bypass the throttle plate along with the fuel that is pulled out of the bowl. It is obvious once the prime starter is removed and the carb is off, look at the passage. One leads to the inlet throat of the carb, the other direction goes to the engine side of the butterfly. It is kind of like it's own carburetor, inside the carburetor. This allows a fast enriched idle, which is exactly why the carb's idle mixture (if it has an adjustment) cannot be adjusted if the engine is not up to operating temperature. Mine, when cold, will start right up and idle up to about 1250 RPM. That is without touching the throttle. After a little while, it will settle down to a 810-820 RPM idle speed once operating temp has been reached. This is also a case where the thermostat needs to be working as well, as if the engine block doesn't warm up but the prime starter does, and leans the mixture, the engine will not idle very well. It will run, just will be a little lean.

This was the second best setup next to EFI, well, as long as the carb isn't plugged and all components are working as designed. Some other motors use prime start as well. Specifically the F40's and F50's. The F40 4 cylinder and the F50 both used two prime starters and a mess of hoses to connect the "choke" carbs and the "non-choke" carbs together. Also, they were weird in that they used staggered jetting, so if you guys are pulling one of them apart, watch the jet sizes and compare to the service manuals. This goes for all multi carb motors, some are all the same jet sizes but it is wise to check to be 100% sure. I did an F50 once and didn't know any better, and couldn't read the sizes, so I assumed they were all the same. Put it back together and synchronized the carbs but the owner said it was never right. 4 or 5 times I had it to the water and couldn't figure it out. Finally I had the carbs back off-for like the 10th time-and used a magnifier to look at the jets, and sure enough, 3 different size pilot jets and 4 different sized main jets. Put them in the correct locations, resynchronized, and that motor ran absolutely beautiful. The loudest thing you'd hear with it idling in neutral or in gear, was the pee water hitting the lake-and nothing more.

Some Yamaha 2 strokes used prime start too. Biggest complaint was the same as it was with the 4 stroke. No start when cold. Took forever to finally get it kick off, once it was running it ran perfect. Pushing the key or working the choke on the remote control does nothing. The 2 stroke prime starters also had a manual over-ride with a red lever on it that you could turn and manually enrich the mixture. But it was blubbering fat WAY richer than with prime start alone. To make matters worse, it was not marked which was was "on" or "off". One way it was smoky rough idle the other way it was correct. That is how you know.

A lot of the 25 and under Mercs from the 80's also used prime start as well, but of those that I've worked on or seen apart, I've not seen anything that suggested those did anything other than the extra fuel. I've seen carbs where the bowl vent runs back up to the inlet side of the throat (carbs on my motorcycle do this) but sounds like you've got something different. What you're describing sounds interesting, design wise. Hope to see one for myself some day. Thanks for the info.

Follow the passages. One is drilled diagnoally to intersect the "inlet" which is in the throat. The other, goes from the prime start area down to the back side of the butterfly. As air passes over the area in the prime start cup, it draws fuel from the bowl in through the tube. The tube has holes in it for emulsion. Just like a mini carburetor-inside the carburetor. It works awesome when the bowl isn't plugged solid. That is a manual start F25 carburetor, vintage 2000 IIRC. It was brand new when I bought it, but broken, so it never got used.

Wow, totally different than anything I've ever worked on. That 3rd pic looks more like the inner workings of the valve body in an automatic transmission.

Thanks for your help Todd. I'll be rebuilding the carb in a couple weeks and check it out.