115 Yamaha TLRP 2 stroke rough running. Needs tune up!

[judojim]

I have a Yamaha 115 hp 2 stroke model 0115TLRP, (serial 6E5-L-320813), which starts up and idles, but it runs really rough with "sneezing/coughing" and smoke. This engine has 4 cylinders.
Thinking it has something to do with fuel I removed and cleaned the carbs and blew out the jets, then installed new gaskets (carbs to engine), changed the fuel pump, removed the 6 month old gas and put in new gasoline (50:1 mix) with a small amount of Sea Foam. Also, installed new filters all around.
Then, I fidgeted with the 5 adjustment screws on the carbs…(4 for each of the cylinders’ fuel jets, and the one idle screw), but it still runs really rough.
Question: does anyone have a link to a repair manual for this outboard engine, and can anyone help with some advice as to what to look for next?

 

[ClassicAQ]

Here is a link that will direct you the service manual for a 1991 115TRLP:
https://www.vansoutboardparts.com/s...991-yamaha-115p-130p-150p-175p-200p-225p/file

To my knowledge, each carb has 2 air adjustment screws (aka mix screws, aka pilot screws)...1 screw for each of the 4 cylinders. These screws meter the amount of air administered into the air/fuel circuit. There is also a idle adjustment screw. However, this screw does not effect the mix. It functions to increase or decrease idle throttle.

Sneeze/cough is attributed to a lean mix. Assuming the smoke is white, this is caused by the seafoam, which is normal. If you look in the service manual at chapter 2 page 5, you'll see the spec setting for the pilot screws are 5/8 +/- 1/4 turns out. This spec is more or less a starting point for fine tuning carbs. Remove upper cowling, silencer, and then put the engine on muffs. Allow engine to warm up to normal operating temp (10-15min depending on your climate). Cut engine off. Then use the following procedure to tune carbs and idle speed:

* Turn in each of the 4 pilot screws until they seat (do this gently, as you don't want to break the needle). Take note of how many turns each screws takes to seat.

* Turn out each of the 4 screws 5/8 turns.

* Back out the idle adjustment screw so it's not touching the lever (throttle plates closed).

* Now turn in this screw so it's touching the lever, then turn in a tad (you want throttle plates to be slightly open)

* Start engine and observe the rpms and listen to how the engine idles.

* If engine struggles to remain running turn each of the 4 screws a 1/4 turn IN (less air => richer mix). If this causes rpms to increase and engine to idle better, then continue to turn screws IN 1/4 until engine is happiest. Then skip to last step. If the engine stalls, continue below.

* While engine is off, turn OUT each screw the 1/4 turn you just turned in (now your back to 5/8), then turn each screw OUT another 1/4. Restart engine. Rpms should be slightly higher and engine should idle better. Continue to turn each screw 1/4 OUT until the engine is happiest.

* Final step is to fine tune idle rpms now that you've fine tuned the mix. Check your rpm gauge before turning idle adjustment screw. You want 750+/- 50. Turning IN screw will increase rpms, OUT decreases. A very very small turn equates to a large increase/decrease of rpms.

 

[ClassicAQ]

Side note: Tuning carbs is more or less a learned experience verses following directions directly from a manual. You might find that the 4 pilot screws are turned out to a different setting in order for the engine to be happiest. This is ok. And it's ok to turn these screws in/out during carb tuning until you found the perfect setting, as you won't hurt anything as longer as the final setting allows for optimal performance. When the screws aren't set correctly is when over time you'll find decreasing engine performance...which is what you're experiencing now.

 

[judojim]

wow, this is the best advice I've ever seen given to anyone, on any forum. You are a blessing indeed. Thank you, thank you, thank you. I sincerely thank you.
I have done what you advise and I thank you again for the link to the information about this engine.
The engine is now running pretty good with the new air/fuel screw adjustments. I do realize that carb tuning is more of an art than science. However, I also checked the timing with a timing lite and found the timing to be off a couple of degrees. My next step is to follow a friends advice and put the boat in the water, and check the carbs and timing there where I may find that it runs a bit differently than while on the hard. Better I hope. I'm also wondering if it may be worth my while to buy a set of gauges to attempt to "sync the carbs." I've seen a few YouTubes that explain syncing, but I'm not sure if it is worth the effort. I don't see anything about syncing in the service manual, so it may not even be possible for this engine.
Thank you again.
You are the best!
Jim

 

[ClassicAQ]

however, I also checked the timing with a timing lite and found the timing to be off a couple of degrees. My next step is to follow a friends advice and put the boat in the water, and check the carbs and timing there where I may find that it runs a bit differently than while on the hard. Better I hope. I'm also wondering if it may be worth my while to buy a set of gauges to attempt to "sync the carbs." I've seen a few YouTubes that explain syncing, but I'm not sure if it is worth the effort. I don't see anything about syncing in the service manual, so it may not even be possible for this engine.

My pleasure, glad to hear it's running better. Now that you have a service manual, follow the instructions for setting the timing. It makes no difference if boat is in or out of water for this adjustment. However, as I mentioned in my previous, the engine should be at normal operating temp prior to syncing carbs.

Re: carb sync gauge
The intake of newer model engines are designed to allow for the hose hook ups that connect to the gauge. But your engine does not. I'm sure a master mechanic could figure out how to do it, but the old school method detailed in the manual does the job.

 

[judojim]

Thanks again, I will follow your advice and use the service manual you sent me to adjust timing.
However, I think I found the intake vacuum hoses. (see photo). They were all painted blue (like the engine) and hard to see, so if I have trouble trying to measure the timing linkages as described in the manual, then I may try to find some gauges and use these vacuum tubes for further refinement.
Thanks a lot.
Your the best.
Jim

 

[ClassicAQ]

However, I think I found the intake vacuum hoses.

There are no vacuum ports on the intake of 2 strokes to attach a carb tuner. I know exactly what the arrow in your picture is pointing at...I thought the same thing with my '98 2 stroke. But that nozzle is not removable. Intakes designed to allow for a carb tuner have a hole on each manifold. The hole is plugged with a threaded screw. Prior to carb tuning, you unscrew the plugs and screw in the tuner's vacuum tube.

Prior to their discontinuation, it's quite possible 2 strokes of the early 2000's have these ports. But definitely not a 1991 engine, such as yours.