2 Stroke Carb Jet Cleaning

[Jason2tpa]

1998 S115TLRW

I experienced power loss as rpms approach 3300. Sounds like it's flooding if i throttle up. Because the carbs are due for a good cleaning im fairly sure the jets are gummed up. As a quick fix, what are your thoughts if i were to dump the 4 bowls, spray compressed air using the straw directly into the main jets, then some B12 carb cleaner into jets.

If the issue is the main jets I would think this works.

Townsend. Boscoe, Rodbolt, 99yam...like to hear what you studs have to say. Big thanks as always.

[Jason2tpa]

Update:

Yep, it did the job. But time to pull the carbs for a complete clean job.

[zenoahphobic]

It would not be flooding (as you suspect) if the jets are gumming up. The needle and seat maybe, the muck preventing complete close off.

Having said that I know a racing person who insists jets wear out, through the abrasion of the flow of lots of fuel. Maybe microscopic I say.

[TownsendsFJR1300]

+1 ^^^ re flooding. Clogged jets won't make it flood. A leaky needle / seat will.

Couple ways I check them if in doubt. I use a Mity vac automotive brake bleeder tool. Hook it up to each carb fuel inlet, flip the carb upside down, float/needle closed. You should be able to pull a vacuum W/O it leaking ANY AIR. Do this to all inlets.

Or, pull the bowls, float/needle installed. set the carb in normal operating position. I hook up a small auxiliary gas tank (old mower tank) and turn the valve on while holding the float gently closed (like the fuel would). It should stay dry until you release it.

Kinda rare (saw once two months ago on a generator), but sometimes the float fails/leaks, injests fuel and NEVER closes/sinks. Probably NOT your issue un-less you have mucho fuel coming out constantly.

And very important, check your float levels too.

If its apparently running pretty good now and your cleaning them for preventive maintenance, you can still check for any leakage. I would definitely pull all the jets and un-less you can see light thru them, IT DON'T COUNT.

[Jason2tpa]

Perhaps "flooding" is wrong word to describe the sound I was hearing. It must be because after cleaning out the main jet like I described the issue was solved. My bad. I'm not sure what word you use to describe what I was hearing. As I went for more throttle it wasnt there. But like I mentioned, the carbs need a total clean and kit installed. I can't access the pilot jets without removing the carb assembly, and only had a couple hours to try to fix the problem. Looks like I was fortunate by taking a stab at the main jets. Appreciate the input guys as always.

[zenoahphobic]

A fault condition doesn't necessary have a sound. I guessed you were just putting something like flooding that you may have experienced to emphasise increasing throttle did not make the engine sound (feel?) any better.
Opening the throttle makes more throatier deeper sound even when the engine is sick.

[TownsendsFJR1300]

And what I mentioned about the jets is that sometimes, they have smaller horizontal holes, besides the main larger hole. Those easily clog up.

Running a dose of Yamaha's RingFREE at 2-3x's the recommended amount should help, as long as some fuel can flow...

[Jason2tpa]

Throatier, deeper...exactly!! Let's establish an adjective for this sound lol

A fault condition doesn't necessary have a sound. I guessed you were just putting something like flooding that you may have experienced to emphasise increasing throttle did not make the engine sound (feel?) any better.
Opening the throttle makes more throatier deeper sound even when the engine is sick.

[Jason2tpa]

Good to know, thanks bud.

And what I mentioned about the jets is that sometimes, they have smaller horizontal holes, besides the main larger hole. Those easily clog up.

Running a dose of Yamaha's RingFREE at 2-3x's the recommended amount should help, as long as some fuel can flow...

[Jason2tpa]

Can you tell me what the "slow air bleed jet" does? Im curious. It's number 14 here: 1998 S115TLRW Yamaha Outboard CARBURETOR Diagram and Parts

And what I mentioned about the jets is that sometimes, they have smaller horizontal holes, besides the main larger hole. Those easily clog up.

Running a dose of Yamaha's RingFREE at 2-3x's the recommended amount should help, as long as some fuel can flow...

[TownsendsFJR1300]

That schematic shows two "slow speed jets" (#9 as well) besides the high speed jet.

I don't know why Yamaha would have another slow speed jet (must be a reason), I would simply make sure all are clean.

Someone else may chime in re the 2nd jet. I've never seen two slow speed jets in one carb...

[boscoe99]

Can you tell me what the "slow air bleed jet" does? Im curious. It's number 14 here: 1998 S115TLRW Yamaha Outboard CARBURETOR Diagram and Parts

The air bleed jet allows for a bit of air to enter into the system to facilitate the easier flow of gasoline through a very tiny tube. Might seem assbackwards but more gasoline can flow through with air mixed with it than will gasoline without air being mixed with it.

Here is the theory from a document I have. Might be a bit more than you care to know about how stuff works.

The basic principle of the air bleed can be explained by simple diagrams, as shown in Figure 2-13. In each case, the same degree of suction is applied to a vertical tube placed in the container of liquid. As shown in A, the suction applied on the upper end of the tube is sufficient to lift the liquid a distance of about 1 inch above the surface. If a small hole is made in the side of the tube above the surface of the liquid, as in B, and suction is applied, bubbles of air enter the tube and the liquid is drawn up in a continuous series of small slugs or drops. Thus, air “bleeds” into the tube and partially reduces the forces tending to retard the flow of liquid through the tube. However, the large opening at the bottom of the tube effectively prevents any great amount of suction from being exerted on the air bleed hole or vent. Similarly, an air bleed hole that is too large in proportion to the size of the tube would reduce the suction available to lift the liquid. If the system is modified by placing a metering orifice in the bottom of the tube and air is taken in below the fuel level by means of an air bleed tube, a finely divided mixture of air and liquid is formed in the tube, as shown in C. In a carburetor, a small air bleed is bled into the fuel nozzle slightly below the fuel level. The open end of the air bleed is in the space behind the venturi wall where the air is relatively motionless and at approximately atmospheric pressure. The low pressure at the tip of the nozzle not only draws fuel from the float chamber but also draws air from behind the venturi. Air bled into the main metering fuel system decreases the fuel density and destroys surface tension. This results in vaporization and control of fuel discharge, especially at lower engine speeds. The throttle, or butterfly valve, is located in the carburetor barrel near one end of the venturi. It provides a means of con*****ing engine speed or power output by regulating the airflow to the engine. This valve is a disk that can rotate on an axis, so that it can be turned to open or close the carburetor air passage.

[Jason2tpa]

Oh boy, why did I even ask. ***, Boscoe, you knocked it out of the park with that diagram...advanced flow dynamics. Appreciate the response. I'll come away with now knowing that the jet slowly bleeds air and i should keep them clean. Haha.

Townsend...can you top that diagram!?!

[boscoe99]

Not to let townsend get the upper hand on me, here is some text from an older Yamaha service manual. Back in the day when they described what in the hell is going on.

I like to think of the term "pilot" as meaning idle mixture.

[Jason2tpa]

Townsend will have to step up big time to top that diagram. Nice one, thanks. This one helps me to better understand what exactly is occurring with the mix at low speed (idle) and high speed.

Would it be an accurate assumption that if the engine is experiencing sluggish pick up when throttling up from idle, these pilot air jets may be gummed up? I know this is a very general question because low end slug could be lack of compression, spark, fuel, etc....but let's just assume all other necessary systems are properly functioning. If the engine is sluggish at the low end, but runs good once the main jets open (high end), these pilot air jets need to be addressed?