Outboard carburetor - question on Pilot Jets

[sy_gilana]

Hi.


Some comments above are misleading.

The pilot jet is effectively disabled at anything above idle. This is achieved by a change in differential pressure across the jet compared to the reservoir. Specifically there are a series of small holes in the throat that straddle the butterfly. Look at them and move the butterfly.


The idle circuit senses vacuum on the downstream side of the butterfly when closed. This vacuum draws fuel from the side of the main jet, but the vacuum is partially collapsed by the position of the idle mixture adjuster. (sometimes called "air screw") and completely collapsed when the butterfly moves past those ports.


The #48 jet (0.48 mm) should be changed about every two to three years, as erosion will take it out of spec. Symptoms will include "sniffing" at idle and a higher idle descending. Opening the "air screw" will lean the mixture back, but its a temp solution.


The #110 Jet nearly never needs replacing. The emulsion tube does. (I have never found the correct alignment of the emulsion tube, if you know please advise)

Your problem is elsewhere, and I suspect its the balance passage of the main jet to the vent on the top of the carb under the stainless steel plate. You must shoot some canned magic through every passage.

If you have an ultrasonic jewelry cleaner, this is the time to use it.

[Ranger2727]

Soak and soak and soak with PB blaster tapping each time may take several days of tapping. Make sure you loosen the shift linkage, if not the shift linkage will keep the unit from separating

[Marqus]

@ Jenna1: Thanks, relevant experience report, though I found no water in the bowl during any of my stall events.

@ Gilana: Thanks for expert level detail, esp how the activation and deactivation of the Idle jet is linked to the position of the throttle butterfly. I am assuming here that Idle Jet and Pilot Jet are synonymous; pls correct me if wrong.

Earlier in the thread Sailmonkey mentioned the pressure venting of the float bowl, and here you have nailed the channel that achieves that via an opening visible under the stainless plate for this carb model.
Though as I mentioned above, in my tests I could achieve normal WOT the moment I cleared my Pilot jet, probably suggesting that my vent channel was ok.

[Marqus]

Thanks for the replies so far.

As per my opening post, I'm not seeking help to "fix" a carb (I fixed it weeks ago). Instead I started this thread to try to understand how carburetor jets work and interact.

After having a couple of motor breakdowns at inconvenient times earlier this year, I found that cleaning the small Pilot jet fixed the problem on each occasion. The bowl, inlet needle valve and larger Main jet looked fine.

So I thought, if the Pilot jet gets clogged, but the Main jet remains clear, the motor cannot idle but it should run WOT.
If I could do that, it would be a great Get-Me-Home tactic when stranded on the water.

But experimenting with that seems that the motor will not run WOT (for more than four seconds). And I am trying to understand precisely why that is so. That's all this thread is about. Not fixing, just pinpointing why.

[tkeithlu]

I certainly learned from gilana's detailed response. It brings to mind the wisdom that "carburetor" is french for "You're going to regret messing with this."

[AKA-None]

This thread is really simplified as there are multiple methods used by carb manufacturers to achieve the same results

Eg a simple carb for a lawnmower is far different than a rodchester or a Webber

Some use and some don’t use the idle jet during wot

To know how your carb works requires knowing what your carb is

[Thumbs Up]

There are variables on an outboard that can mimic carb problems. Weak spark and low compression come to mind. A weak spark will run it fine at no load. You can make a simple spark gap tester. You should see a bright blue spark jumping a 1/2 inch gap from both wires simultaneously. You could be dumping too much gas (bad gasket or leaky needle valve), too little gas (bad fuel pump diaphram or filter). You could have combustables in the crankcase (sometimes a problem for 2 strokes). Could you have lost a part? (Ball check or rubber plug that covers jet?) Have you tried pumping the fuel line bulb at wide open throttle to see if it runs better or worse? Also pinch off or disconnect the fuel line and see if it runs better or worse. Have you tried fresh fuel? You have indicated that it is already fixed. Did you buy a new carb (ethanol in fuel eats carbs)? Compressed air is the only correct way to blow out tiny carb passages (not wd40, though you can turn the can upside down in a pinch). If "cleaning the pilot jet" kept solving the problem, wouldn't that indicate that dirt was getting in (no filter or rotten fuel line). Couldn't this dirt have been affecting the float valve and caused flooding? If there was something clogging the pilot jet you could see it if you hold it up to the light. Maybe just the teardown and cleaning the bowl is what was "fixing" it. If you had little black specks, replace the fuel line, the rubber is corroding. "Runs good at WOT" for a few seconds sounds like flooding followed by fuel starvation. A bad gasket or bent (overtightened) flange could cause this. (Pumping the bulb floods it, then fuel pump fails to suck). I am curious to know just what the fix was (new carb, new carb and new fuel line or ???)

[Thumbs Up]

In my above post I noticed that CF has taken the opportunity to link to a walmart ad every time I typed "fuel line". How lame is that?
Upon revisiting the OP's jet photos, I see little black specks.

[Shaneesprit]

It’s a carburettor if a jet is blocked your screwed, that’s a good reason why fuel injection is just better - I don’t have an outboard because all of the lifting and maintenance if you add it up it’s quicker to Row with oars.

[AndyEss]

Originally Posted by Shaneesprit

It’s a carburettor if a jet is blocked your screwed, that’s a good reason why fuel injection is just better - I don’t have an outboard because all of the lifting and maintenance if you add it up it’s quicker to Row with oars.

Did you ever try to row an inflatable any distance?

[sy_gilana]

An update to this for your benefit. Never use a wire or tapering needle to clean jets. For instance, the idle jet #48 on a Yamaha 9.9/13.5/15 2 strokes will change size by reaming out with a wire. The idle mixture will have to be compensated. This will result in a rough running motor.
Use Ultrasound, or chemical (Not Acid) or monofilament nylon.

On a related troubleshooting consult.






Motor, Yamaha 15 Enduro 2 stroke.
Symptom, starts easily, runs perfectly for approx 4 minutes at idle, then loses power. Runs perfectly full power planing for about 2 minutes, then loses power. After owner swapped out the pulser coils, exciter coils, HT coils, No change. Consulted with me.



I asked about spark plugs, he said had just changed to new plugs before trouble started, AHA!. I said put in the old plugs to test. Motor ran perfectly. Took brand new plugs to test, ran for 4 minutes. Broke open the ceramic part, found the insulating ceramic near body cracked. Surmise that as the spark plug body increased temperature, the crack in the ceramic opened, and spark leaked to ground. Perfect while cold.

Just something to bear in mind while troubleshooting.