Yamaha 150 TXRC 2005: Engine won't go past 8 mph, tach erratic

[Psalm 99]

Thought I'd fixed the problem (dirty carbs). Launched boat and just got up to 2500 rpm and about 18 mph when engine speed dropped immediately down to 1600 rpm and boat slowed to about 8 mph. Didn't touch throttle. Engine jumped a few times to higher speed and, for a few seconds, went back to cruising speed before dropping immediately down to 1600 rpm. Engine not responding to throttle above this speed although will respond below this rpm. Tach bouncing between 1600 - 2500 rpm without touching throttle, but mostly just settled in at 1600 rpm. When tach bounces around between 1600 - 2500 rpm, there is no change in engine rpms nor boat speed (stays at 8 mph). Coming into dock, engine responds well to throttle when actual rpms are 1600 or below. However, close to idle (750 rpm), engine just about stalls down to about 400 rpm. I've verified that all three temperature sensors are fine, oil level sending unit right at spec, new crank position sensor, all ground wires are clean, and both thermostats work as per spec. New fuel lines, primer bulb, fuel filters and carbs just cleaned. Can't do a full check on the CDI unit as Yamaha does not sell the 'winky-blink' tester anymore. Fuel is new as is fuel/water separator. About ready to bite the bullet and take it to the local mechanic but any recommendations would be appreciated.

[walleye1]

Do your carbs agan in a ultrasonic cleaner and flow test them after. they can be hard to clean.

[TownsendsFJR1300]

I'd start with draining the carbs. Catch what comes out and see if crap in the fuel pump, rest of the system, re-contaminated the carbs..

When you cleaned the carbs, did you remove ALL the jets and physically see light thru them or just a spritz of carb cleaner?


Some history as well please... Has the engine sat with un-treated fuel for sometime, used often? Problem JUST arose? Engine new to you?

[boscoe99]

The winky blinky part number has been changed. Superceded by YB-06795. Since boats.net does not sell it here is a link to a place that does.

http://www.simyamaha.com/Yamaha_Diag...p/35-06795.htm

Does not sound to me like a CDI issue. Sounds like a fuel issue. Have you tried misting some gasoline into the air intake port when the motor is bogging down/acting up to see what happens?

[Psalm 99]

Pretty sure carbs are clean. I found the carb floats were slowly disintegrating (soaked floats in fuel for a few days and found black 'dust' plus chunks up to 1/16" dia. in bottom of cup used to soak floats). Just finished replacing all 6 floats with Yamaha parts (set float height per manual). Replaced needles also. While carbs were off, cleaned all accessible ports. I have a sonic cleaner and have cleaned the carbs a number of times. I bought this boat used about 4 years ago and this problem continues to surface. Last year, engine appeared to finally be good and it ran well for about 100 hours. Then problem happened again. I'll be cruising at about 3200 rpm and engine will just immediately crash to 1600 rpm with no throttle response above this setting. If I leave the throttle set high, engine may suddenly jump back up to throttle setting only to crash back down after anywhere between a few seconds to several minutes. The last few times, I started to notice the tach would bounce around (1600 - 2500) but mostly just stayed at 1600 no matter how high I pushed the throttle.

[boscoe99]

Looks like it is time for the marine doctor.

[pstephens46]

You had the black specs in bowl or bowls. Cleaned and replaced floats. Ran good for a bit. Right? Going off of last thread. Now running poorly again. Odds are there is more funk that has choked the carbs. Left over from floats? Or something else. That is the mystery. Ask Townsend. Sometimes carbs have to be cleaned multiple times. PIA.

Guy here locally had a Whaler. I think that was the make. Had a fiberglass fuel tank. He left ethanol in the tank for an extended length of time. Had issues for months after replacing the tank. Having said that, my guess is your problem is not nearly as severe.

[Psalm 99]

Sure appreciate the quick responses. Maybe a little history: Bought boat/engine on e-bay about 4 years ago. Engine had 325 hours on it. Shipped to Hawaii from Florida after sale. This problem has been showing up since I got the boat. When it runs, its a great engine; but periodically, it just snaps down to 1600 rpm where it runs smoothly but won't respond to throttle past this point although it does respond well to throttle under 1600 rpm. Giving false readings at the tack is something fairly new. Really appreciate the link for a CDI tester as I think I've pretty much tried everything else I can think of. I foolishly ran the engine when the boat first arrived before draining all the old fuel from the tank and filling with non-ethanol fuel plus a recommended dose of Yamahas Ring-Free. Since that time, I've spent a lot more time working on the engine than fishing. Because the tach is jumping all over, I'm leaning toward an electrical issue (maybe make that 'wild-guessing'). The tach gets an electrical signal from the engine and its erratic behavior may indicate a bad electrical signal being sent from the CDI unit to the ignition coil(s) (really wild guess?).

[99yam40]

need to make sure there is not something going on with fuel supply , spark, and make sure gauges will indicate a fault in alarm system maybe

Monitoring fuel pressure, peak voltages into CDI, and out of it to coils, and make sure gauges will indicate if low oil or over heat will show

you do know that if low oil or over heat is indicated the RPM will drop to around or below 2K

[TownsendsFJR1300]

To add, the RingFree does help to keep the fuel system clean but it is NOT a fuel stabilizer...

If that fuel is mixed with a bunch of old fuel, you now have a bunch of bad gas and the issues that go with it.

It sounds like your main jet(s) are clogging up with something. Perhaps bouncing around is sucking up water from the tank.

Draining the carbs (at least should give you an idea if there is water in the tank).

Check your spark plugs for one or more not burning basically the same as the others...

I've used (what some call snake oil), K100. If there's water in the system, it mixes(and keeps it mixed) the water with the fuel(no separation)
and the engine eventually burns it off. Been using it for about 11 years, NEVER had to drain my separator for ANY water (21 year old hull and tank).

Plus one on spritzing the intake with PRE-MIX fuel when it starts acting up. If it takes off, you KNOW you have a fueling issue..

[Psalm 99]

Replaced the two fuel pumps about a year ago (for the second time in 3 years). I've never verified that the alarm systems are working correctly although I have verified in accordance with the Mechanics Manual that the oil level sensor and all three engine temperature sensors are per spec. Also, I replaced the Crank Position Sensor a few months ago. Testing the peak voltages into the CDI unit is a good suggestion (I think a special connection is required to do this). From the manual, it seems that, if there are good sparks from each coil, no further testing of the peak voltages into the CDI unit and out of it to the coils is necessary. I have a spark plug tester which I'll use to test the coils. All plugs a pretty new and I replace them each year. Manual also suggests looking into the spark plug caps. I suspect I have a bad CDI unit but, at close to $1000, I'd like to eliminate any other possibles.

[panasonic]

Try spaying some fuel in the intake as the others suggested. Need to narrow it down. If it speeds back up with the fuel then you have a fuel problem. If not, on to the electrics.

[99yam40]

testing Peak voltages while the problem shows up will tell you if one or more of the signals is dropping

[Psalm 99]

Good advice both about the peak voltages and starting spray application. Any thoughts about why the tach reading becomes so erroneous during the time the engine true rpms have crashed? I'm thinking the location where the tach data is sent from may be tied to the part(s) causing the rpm crash.