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Hi,

I have 1988 Yamaha 70. Starts up great and runs fine. However after putting the throttle wide open - after about 15-20 seconds the engine begins to back down even with the throttle open. If I back off on the throttle for few seconds I can push the throttle back open and the engine responds but will begin to back down again after another few seconds. At mid/slow speeds it runs fine and it idles fine. I thought it may be a timing issue. Any help would be much appreciated.

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I ams sure it is not a timing issue. Sounds like a fuel delivery problem. Specifically, I suspect a carburetor problem like a clogged jet.

It may also be a simple as fuel flow restriction. When the motor begins to loose power, is the fuel primer bulb soft? Does squeezing the fuel primer bulb bring the motor back to life?

Start with the simple and obvious stuff first. Most problems are something simple.

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i agree with Tom, sounds like a fuel flow restriction to me. Try a simple test by loosening the gas cap and see if there is a difference.

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From the description, I am guessing your fuel system can keep the carb bowls full at mid-throttle and below. When you first go to full throttle, you're using the fuel out of the bowls, and once it's used up, you lose power. You back off to mid-throttle, the bowls fill up again, and you can bring it back to full - for a few seconds anyway.

So I think you have a problem delivering fuel to the carbs - either a flow restriction or possibly a bad fuel pump. Unlikely to be a plugged jet - if that were the case, you wouldn't be able to make it to full speed in the first place.

To check the fuel pump, have a passenger pump the fuel ball while at full throttle. If you stay at full power, that's it.

Tim

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Thanks for the replies all.

The fuel line is a little old and likely needs to be replaced - something I meant do. A buddy has had the problem and says that while the lines may be ok, the bulbs tend to go.

Going to start with that and pick up a new line today.

Thanks again!

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I agree with the above contributors...fuel supply is most suspect. Way back I had a Slickcraft SF235 with a very similar problem...back off at high rpm...actually had to shut down to get engine operating again only to have it repeat the scenario. After a few of us brainstormed over some brewskis we realized there was sediment in the fuel tank that drew to the feed filter and clogged it at high rpm. When the engine shut down, the debris dropped from filter head and the engine restarted and ran for a while as though nothing was wrong...until the sediment cycle repeated itself. Drained and vacuumed the tank and all was well. I know you have an exposed tank; however, if the fuel line/bulb replacement doesn't work...check your tank for sediment, especially if it's metal.

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Caution my friend!
On a 2 smoke, the fuel is the life's blood of your engine.
Low fuel = low lube, no fuel = blow engine.
Don't run it until you go through the fuel system.
Or you are running the risk of killing a cylinder.

FYI "Drained and vacuumed the tank"?
Too exciting for me. vacuum plus fuel = flame thrower.
That could ruin your whole day and cost you some hair, boat, or a house!

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Karlow...I'm convinced God doesn't want me yet...for sure I've given Him ample opportunity over the years :o)...Actually we let the tank dry out sufficiently and used a looooong grounded nozzle...it was amazing how much sediment we got out. The operation was performed by a marine mechanic as a wonderful alternative of ripping the tank out of the deck.

Quick question re fuel flow since its mentioned above. I see a lot of people installing additional in-line filters on fuel lines...can those filters possibly increase the gas/oil ratio to the engine and result in damage?

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I changed the line but have not run it yet. I did notice that the old fuel line was on backwards - likely the source of the issue? Replaced trim/tilt so when reinstalling line is when this oversight occurred - surprised mechanic did not catch that. There were absolutely no issues before this point. Will see how it behaves soon enough.

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If the fuel line (and more to the point, the primer bulb) was reversed, then that is your problem. Primer bulbs have check valves in them that allow fuel to flow in one direction only.

I suggest replacing the primer bulb because it should not have allowed the motor to run at all in the reversed position.

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Yeah, that'll do it:-). The simplest of things... The irony is that if your primer bulb was in decent shape, your motor wouldn't have run at all and you would have found the problem very easily!

If it's an old line along with an old bulb, just replace the whole assembly - it's not worth fooling with.

Tim

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Second that, and go with OEM as the primer bulbs are much better quality than the aftermarket ones. It's well worth it.

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Ran the boat with the new line. There is some crud in one of the tanks that appears to be the problem. The uncontaminated tank had everything running perfect. Thanks for the input - looks like the problem has been isolated.

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Be thankful you didn't have this problem with built in tanks like I mentioned above...much to be said for simple boats :o) Glad it's solved.