Yama 150 rpm loss

WWWes

Recruit
Joined
Jun 15, 2009
Messages
5
Hey
I have a 2000 model OX 66 (SX150TXRY) and have a problem with rpm loss. This occurs after about 10 minutes when the motor is good and warm. The rpm loss is sudden and occurs without warning. It does not completly shut down but instead seems to drop back to 3 cylinders. Just as suddenly as it drops off, it starts up again. The suddenness of the problem makes me think it is electrical. I got a switch and wiring harness from a local shop and bypassed mine but I still have the problem. One thing that I think may be significant is that whether I use my ignition switch or the replacement, I am not getting awarning buzzer at start-up. Once it starts this mess it will continue until after it cools off.
I have replaced the fuel/water separater and the inline filter because at first I thought it was a fuel issue. I took it to a local marina and they ran it on the water twice but it ran okay for them so $250 and 20 gal of fuel later they have no clue what is wrong. It is expensive to test it will not miss a beat at idle or low rpms so it has to be run on open water.
Any advice would be much apprecited.
 

rodbolt

Supreme Mariner
Joined
Sep 1, 2003
Messages
20,066
Re: Yama 150 rpm loss

claen the VST filter and monitor fuel system vacum at the lowest lift pump intake.
if you still have the inline check valve between the engine filter and the lift pump remove it, unsrcew it and remove the guts and reinstall the body.
loss of fuel rail pressure can and will result in violent shutdown.
you get NO buzzer at key on power with a single engine yamaha. to test the buzzer on your motor simply pull the Emergecy lanyard and crank it. should get a buzz while cranking.
 

WWWes

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Joined
Jun 15, 2009
Messages
5
Re: Yama 150 rpm loss

THANKS Rodbolt! The VST filter was almost entirely clogged. Got that clean and she is good as new.
 

WWWes

Recruit
Joined
Jun 15, 2009
Messages
5
Re: Yama 150 rpm loss

Hey Rodbolt
I posted a rpm loss problem a few weeks ago and you nailed it. My VST filter was completely clogged. Problem is, it keeps happening. Last week my mechanic convinced my to waste my fuel (80 gal) and clean the tank. I had reservations because it didn't make sense that trash was getting past a 10 micron Racor filter but I gave the go ahead. Once again the motor ran great for about 50 miles and then suddenly shut completely down. When I tried cranking it sounded like it was starved for fuel. After several tries I could get it to crank but it would not run higher than 1200 rpm. I have not cleaned the VST filter yet but I suspect it will be clogged. I did remove part of the fuel line between the inline filter and the VST tank and visually inspect. It appeared to be clean. Any ideas as to where this contamination could be coming from. Also, what are the symtoms of a failing fuel pump? I'm sure you are aware how expensive a pump is and I will replace it if that will solve my problem, but I need to be sure. Once again, I appreciate your input.
 

WWWes

Recruit
Joined
Jun 15, 2009
Messages
5
Re: Yama 150 rpm loss

I am still struggling with this problem. After 5 trips to the same marina I decided that I needed a new set of eyes on my problem. What my new mechanic found was astounding.
1. My VST tank gasket had been improperly installed and was in 3 pieces.
2. There was water or some other separated contaminate in my VST tank after I had paid to have my fuel tank cleaned.
3. Incredibly, my previous mechanic thought it would be a good idea to punch holes in my VST filter. I guess he was tired of cleaning it every Monday. I'm sure I probably have injectors to clean now.
And yes these are Yamaha certified mechanics!

Obviously I need to clean my tank. I have decided to do it myself. I plan to tilt my tank so the contaminate goes to one corner then pump to lowest 10 gal into a waste container. The other 70 gal I will filter into a storage tank then clean my fuel tank. My question is what is the best method for cleaning my tank. The tank is built in such that it is removable only by cutting the deck open. I will do this if I need to replace the tank, but only as a last resort. The tank is accessable only via a 2-3" inspection port on the top. Any suggestions would be appreciated. Also, the picture I have attached is fuel from my VST tank which shows the contaminate at the bottom. If this is water, how does it keep getting past 10 micron f/w filter?
 

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