RPM Reduction, Yamaha 90TJRY

[Dusty-Pants]

I have a 2000 Yamaha 90TJRY. I have had an intermittent problem for a couple years where the engine speed will drop form 5200 to 4000. It can run great for a couple weeks then one day it will decide to cut out. I can run indefinitely at reduced RPM. Seems like disconnecting the fuel line then running engine dry and repriming helps restore the WOT.

When not misbehaving the engine runs great.

After returning from Lake Erie a couple days ago where it decided to go wonky 3 out of 4 days, I have decided to try to figure out the problem.

This year have replaced all fuel line and components from the tank to the engine and added a fuel/water separator. I have vacuum/pressure tested the fuel pump and tests OK. I have tested the plug wires, and coils and seem to test OK. I have checked the wires in the engine compartment for shorts, and all look like new.

My thought at this point is dirt in a carb or all carbs since in hindsight the problems seems to occur mostly when running is rough water but not always.

Any thoughts would be appreciated.

[rodbolt17]

I would replace the pump and clean the carbs.

if it was RPM reduction it would drop the RPM to 2500 or so and miss like crazy.

don't know how the exhaust tubes on jets hold up in freshwater but a hole in the tube near the socket it mounts in can drive a tech nuts.

[99yam40]

I have a 2000 Yamaha 90TJRY. I have had an intermittent problem for a couple years where the engine speed will drop form 5200 to 4000. It can run great for a couple weeks then one day it will decide to cut out. I can run indefinitely at reduced RPM. Seems like disconnecting the fuel line then running engine dry and repriming helps restore the WOT.

When not misbehaving the engine runs great.

After returning from Lake Erie a couple days ago where it decided to go wonky 3 out of 4 days, I have decided to try to figure out the problem.

This year have replaced all fuel line and components from the tank to the engine and added a fuel/water separator. I have vacuum/pressure tested the fuel pump and tests OK. I have tested the plug wires, and coils and seem to test OK. I have checked the wires in the engine compartment for shorts, and all look like new.

My thought at this point is dirt in a carb or all carbs since in hindsight the problems seems to occur mostly when running is rough water but not always.

Any thoughts would be appreciated.

Hard to believe you have run this motor with a problem like that for years without causing major problems with the motor.

If there is trash plugging fuel passages then some cylinders would be running lean and that is never good for long periods of time.

Too much fuel to a cylinder or not enough will cause a cylinder not to fire properly but too lean will cause motor damage quicker usually.
Fuel pump leaking into crank case on #3 will cause problems
Someone playing with main jets in carbs can be a problem that is hard to pin point also, ask me how I know

Carbs are simple to open up and clean.
Make sure to follow the service manuals steps for link and sync after reinstalling the cleaned carbs.

Is this motor something you have had since new or was it a used one you picked up?

[Dusty-Pants]

99yam40,

I purchased used in spring 2012 so this is my third season with the rig.

When it acts up I normally fiddle with the fuel quick disconnect so I do not run long peroids of time. 10-15 seconds, just long enough to see if WOT is there. The Erie trip I decided to run for awhile to see if it would self-clease so to speak. No luck with that one.

Would you suggest a full carb rebuild (new floats, jets, etc) or just drop bowls and clean?

What do you mean by "Fuel pump leaking into crank case on #3 will cause problems"? Is there a way to check for this?

More info...when I tested fuel pump with vacuum on input side, the vacuum only held for 2-3 seconds. The pressure test on output side held for about 30seconds. According to the manual these tests fail if pressure or vacuum cannot be acheived. My question is... must it be able to hold these values for any peroid of time?

rodbolt17,

What is the scoup with exhaust tubes? Is this something that can be checked without dropping the jet? I replaced the water pump last year but did not look at the tubes.

I really appreciate your responses

[99yam40]

unbolt fuel pump but leave the hoses attached.
Pump primer bulb and look for fuel coming out of pulse port to see if pump leaks

[Dusty-Pants]

Update

I have removed the carbs and cleaned and relinked and synched. One carb had some blackish thick oil consistency "stuff" in the float valve area. Since I am not a carb expert I am just hoping this may have been the culprit.

While I was throwing money at the problem I decided to replace the fuel pump, even though it tested OK.


After two outings (about 50-60 minutes of running) the engine has performed flawlessly. However, it has done this in the past so I am still uncertain if the problem is resolved. I suppose time will tell.