bad impeller or bad thermostat?

[fastback4]

I have a 2005 Yamaha 90tlrc 2 stroke on a 19 foot G3 bass boat. We were cruising around cave run lake for about 15 minutes at mid-range throttle when the engine suddenly started running slightly rough with no full power but still not missing or backfiring. There was still a water stream coming from the engine and there were no warning lights or buzzers sounding. We headed back to the dock and put the boat back on the trailer. Today I hooked up the water muffs started the motor and it idled decent but not great. I pulled the three plugs and they looked okay but a little more Brown than I like to see. Next thing I did was drain the 3 carbs and all three had good fuel no debris. I restarted the motor let it run for a few minutes and took temperature readings at the water stream the thermostat and the three heads with my digital thermal coupler. The thermostat and the 3 Heads all had temperatures between 132 and 135 degrees. The water stream on the other hand had a temperature of 85 degrees but when I turned the engine off the reading of the excess runoff at the water stream was 125 degrees. I did shoot compressed air through the water stream tube and I did get a better output after that but the same results in temperature. So I'm going to guess it needs a thermostat but I never get these things right the first time.

[99yam40]

need to test your alarm system to make sure it works when it should and indicates properly for overheat and low oil.

if the alarm system functions as it should look at fuel

[Jason2tpa]

The water stream on the other hand had a temperature of 85 degrees but when I turned the engine off the reading of the excess runoff at the water stream was 125 degrees.

Water exiting tell tale should always be cooler than block temps when engine is warmed.

Agree with both replies ^^^^^. This isn't a cooling issue, it sounds like a fuel issue. Carb cleaning time.

[rejesterd]

I would not assume you have a cooling issue. The behavior you described sounds carb/fuel-related. Try some carb cleaner spray first and then run it at full throttle for a few minutes. But you might need to take the carbs apart and clean them.

[rejesterd]

The MAIN JET, part #21:

2005 90TLRD Yamaha Outboard CARBURETOR Diagram and Parts

is not accessible W/O pulling the float bowl(s).



.

Ah, so it'd likely be a waste of carb cleaner spray.

[fastback4]

Fuel pump and fuel filter looked good with no water in either. It's time to disassemble and clean 3 carburetors I suppose paying attention to jets (main, idle, nozzle. Rats

[fastback4]

Thanks TownsendsFJR1300 and all for your advise and concern. I have a screwdriver set specific for brass orifices, and 2 cans of sea foam....LOL

[fastback4]

On the middle carb item 11, pilot screw, there is a cap item 42 covering this screw. Do I need to expose the screw pull it out and clean the orifice?

[99yam40]

I would not assume you have a cooling issue. The behavior you described sounds carb/fuel-related. Try some carb cleaner spray first and then run it at full throttle for a few minutes. But you might need to take the carbs apart and clean them.

running a sick engine at full throttle for a few minuets to see if that helped could destroy a motor if running lean on one or more cylinders.

Did you check your alarm system to make sure it works?
that should be done from time to time just to be sure they work

[99yam40]

We were cruising for about 15 minutes at mid-range throttle when the engine suddenly started running slightly rough with no full power but still not missing or backfiring.

Today I hooked up the water muffs started the motor and it idled decent but not great. I pulled the three plugs and they looked okay but a little more Brown than I like to see.

If it is carb related it all needs to be cleaned, remove the caps/plugs and Pilot screws.
what you wrote deals with more that the main jets

[fastback4]

Per your advice I only removed and cleaned things that were easily accessible and not hidden by plugs. The carbs were easier to disassemble than I thought they would be and I did have a stand by carb kit just in case but I was able to split the halves without ruining the gaskets. There was obvious dirt of some sort in each of the carburetors After cleaning and wiping down all the parts I reassembled in the same order in which they came apart. I did much earlier invest in a really good mechanics manual for this 90 horsepower motor. Setting the float Bowl height seems to be my biggest fears but I believe I have this one down. I do not have the means to run it in a test tank with a test prop, so for now it runs pretty well in neutral at 900 RPMs. Tomorrow we'll take it to the river and open it up thanks again to all. I forgot to add that I did test the engine hot and low oil sending units and they tested good

[fastback4]

with five gallon of fresh fuel we headed to the River today. The engine started on one bulb squeeze and two revolutions of the engine and idled nicely. Moved away from the no wake zone, nailed it and held on. Once again we are good to go.