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1999 Yamaha 150hp 2-stroke model C150TXRX loses RPM's at WOT on plane then drops off of plane

R

rjw4259

Contributing Member
This model engine is Yamaha C150TXRX 2-stroke. Guy's I have been fighting this problem for four boating seasons now, last year in the shop for over 2 months at Dubberts Marine in port Clinton. They replaced the anti-siphon valve fitting, water/fuel separator, rebuilt the carburetors, both fuel pumps, new fuel hose+primer bulb+engine fuel connector from water fuel separator to engine. Over $2500 spent on this problem and I still have it! This happens after 5 to 10 minutes WOT on plane gradually loses engine rpm and comes off plane. There is a flat primer bulb when this happens. I have checked for tank vent clogged by opening the fuel fill cap when it happens so bees nest in fuel vent should be ruled out for cause. I myself have checked the fuel pickup line, it is a solid plastic tube with no corrosion or breaks in it. I sure ain't taking it back to Dubberts and waste another whole season waiting for them to even look at it again, after over $2500 of repair bills from them its still got the same problem. Guys I am at my wits end on it, I am wide open for suggestions of what it could be that we haven't already replaced or checked?
 
Flat primer bulb is telling you that there is a restriction between the bulb and the fuel pick up probe inlet.

Prove it to yourself. Get a milk jug. Clean it out. Fill with gasoline. Run a hose from the milk jug to the motor. See if the motor now runs well. If so, back to the boat.

If it were my boat I would remove the anti-siphon valve. Replace the fuel hose if necessary. Pull the fuel pickup tube if necessary.
 
Agree with Boscoe....its got to be the pickup but more likely the anti-siphon valve. Before going through effort of accessing the tank, hook up a portable fuel tank to motor and run at WOT (or a milk jug lol). Assuming the result is no fuel starvation, remove anti-siphon valve as Boscoe noted.

If you also informed the service shop that the primer was going flat then I kinda feel like they took advantage of the situation....as a flat primer bulb implies a restriction – not a pump failure, sour fuel/water separator, gummed up carbs, and/or deteriorating fuel lines. Just my opinion of course.
 
Guys, some additional information on my fuel starvation problem. Attached is a photo of the fuel bowl showing the level of the fuel at 5000rpm's versus at idle, the first picture is at 5000rpm's, the second is at idle. As the picture shows there is a fuel filter in there atop a spring and a little orange ring at the bottom of the bowl. You can see the level of the fuel fluctuating as the engine speed is increased. My question is it normal for the fuel bowl to look like this as the engine is speeded up, I can also see what looks like big air bubbles occasionally? Thought this observation may be important. Thanks guys!
rjw4259

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Yeah I think Dubberts really missed the diagnoses of the problem, the service rep did not replace the water/fuel separator until I bought it back to him with the same problem. They did all the carbs and fuel pumps first then replaced the fuel line and the water separator when I bought it back with the same problem. To their credit the engine ran fine when they lake tested it for about a half hour, got the boat back from them in late august 2020 and it ran fine for the rest of the season. The first time I took it out this year it ran great for about 10 minutes at WOT then back to the same old thing that plagued this engine for the last 3 seasons. The first time it happened I connected it to a spare tank I had on hand and it acted the same way with the portable tank as with the main tank. I will do that again just for grins but don't expect it to have any different results. That should at least rule out the fuel tank if it fails the same. Just frustrated that this engine just keeps running like caca no matter what you do to it! If the things that were done to it last year appeared to fix it I wouldn't expect it to all of a sudden start to exhibit the same problem, at least should get at least one season without engine problems! Thanks for your comments and suggestions, I'll Be back in touch with you and Boscoe once I find out more about it!
 
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Boscoe, Wouldn't a 5 gallon fuel tank do as well as a milk jug, just run the fuel line to that. Fuel tanks are expensive but I have several 5 gallon fuel cans I could use?
 
The flat primer bulb is the key indicator here. Without getting in to depth with how a fuel pump works, just keep in mind the fuel pump drawls in fuel from the fuel source and pushes the fuel to the carbs/injectors. Think of the fuel pump as a siphon hand pump...pulling up on the pump handle pulls fluid in and pushing down on the handle pushes fluid out. If the fluid tank you were siphoning fluid from was vented and became empty, you could continue to pull/push the handle without resistance even though no more fluid would be siphoning out. This mimics an outboard running out of fuel....an empty vented fuel tank and the primer bulb would not be collapsed. But if you were to pinch the siphon tubing on the pull-side, thereby creating a resistance to drawing in fluid, you could only continue to pull up on the siphon handle until the tubing collapsed.....and this mimics what's happening with your rig....your fuel pump continues to drawl in fuel until the primer bulb collapses. What you've got is a restriction between the fuel tank and primer bulb. The restriction would not come from the fuel filter since its up stream from the primer bulb.

Were all the fuel lines replaced?....from tank to motor? Outboard fuel lines can delaminate...meaning the inner tubing fractures into layers which can cause an obstruction of fuel flow. You would not expect this with new fuel lines....but if the fuel line between your fuel tank and primer bulb is old then delamination is a potential cause of the restriction.
 
rjw4259 said:
Guys, some additional information on my fuel starvation problem. Attached is a photo of the fuel bowl showing the level of the fuel at 5000rpm's versus at idle, the first picture is at 5000rpm's, the second is at idle. As the picture shows there is a fuel filter in there atop a spring and a little orange ring at the bottom of the bowl. You can see the level of the fuel fluctuating as the engine speed is increased. My question is it normal for the fuel bowl to look like this as the engine is speeded up, I can also see what looks like big air bubbles occasionally? Thought this observation may be important. Thanks guys!
rjw4259

View attachment 27088View attachment 27089
Click to expand...

That is normal.

The flattened primer ball is the clue.
 
yeah we are completely replacing the fuel delivery system, fuel pickup, anti-siphon valve, all hoses, water/fuel separator. Everything downstream of the primer bulb will be replaced. We fueled the engine from a separate tank and it ran much better, still getting some fluctuations in rpms could be from bad or fouled plugs(changing those today). Thanks for your help, have a super day.
 
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