Losing power

R

rockyrr

Member
Joined
May 5, 2007
RO Number
26166
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29
We have a 21' walkaround with a Yamaha 150 4-stroke. After 3 or 4 minutes running it on the water it loses power and we can't get it back up to full speed when at full throttle. We had it looked at twice by a mechanic and he could find no problem. He did clean some filters, but that didn't solve the problem. The mechanic thinks we have a tank with "bad" gas. Recommended that we just run it to use up the gas or have the gas pumped out and replaced. Engine starts fine and runs fine for about 3 or 4 minutes, then loses power (can't get boat up to full speed). Any thoughts or feedback will be appreciated.

Thanks
 
I would get a small portable approved gas can and run the outboard on that to confirm your mechanics thoughts. If this proves to be true get rid of the bad gas now don't run it on bad gas and cause potential damage to your motor trying to burn off bad gas.
Bill
 
Is the vent to the gas tank clogged?

How much gas do you have and how old is it?
 
How soon can you run it again at full power? A few minutes, hours?

I'd start with fuel, either the vent, filters (both primary in the boAt and the secondary on the engine), fuel hose collapsing, ati shoehorn valve etc,

If it takes the engine to cool down before it can develop full power then I d look at an ignition problem, like a coil acting up when hot.
 
What's an ati shoehorn valve? Is that something specific to outboards?
 
quote:
Originally posted by cwms

Is the vent to the gas tank clogged?

How much gas do you have and how old is it?

Click to expand...


Gas is relatively fresh...a mixture of new gas and gas that is 3 or 4 months old (we use Startron and another gas stabilizer).

You raise an interesting point about the gas tank vent. We do have a lot of trouble adding gas. Could a clogged vent affect starving the engine from getting gas?

Thanks
 
quote:
Originally posted by getakey

could it be anti-siphon valve?

Click to expand...


What is the anti-siphon valve, where is it, and how does one check it? Thanks.
 
quote:
Originally posted by PascalG

How soon can you run it again at full power? A few minutes, hours? Maybe a few hours.

I'd start with fuel, either the vent, filters (both primary in the boAt and the secondary on the engine), fuel hose collapsing, ati shoehorn valve etc, What is this?

If it takes the engine to cool down before it can develop full power then I d look at an ignition problem, like a coil acting up when hot.

Click to expand...


Thanks
 
quote:
Originally posted by PascalG

How soon can you run it again at full power? A few minutes, hours?

I'd start with fuel, either the vent, filters (both primary in the boAt and the secondary on the engine), fuel hose collapsing, ati shoehorn valve etc,

If it takes the engine to cool down before it can develop full power then I d look at an ignition problem, like a coil acting up when hot.

Click to expand...


How does one go about "unclogging" the vent?

Thanks
 
quote:
Originally posted by rockyrr

quote:
Originally posted by cwms

Is the vent to the gas tank clogged?

How much gas do you have and how old is it?

Click to expand...


You raise an interesting point about the gas tank vent. We do have a lot of trouble adding gas. Could a clogged vent affect starving the engine from getting gas?

Thanks

Click to expand...


A blocked vent will do both...starve an engine and make it difficult to fill the tank.
 
to test the vent run with the cap loose.

Bad gas can cause pre ignition overheating.
Overheating from any source could cause the problem.

Personally I would start by dumping the gas. If it is on a trailer the job is easy. Just siphon through the fuel line into cans on the ground.
 
Venting causing problems seems to be mentioned often....Hmmm.

Thanks, folks
 
Bad gas doesn't make sense. First of all, it isn't very old and secondly, seems if bad gas was the problem, the engine would run bad all the time, not just after a couple of minutes.
Like Pdecat says, just open the fill cap and run the boat to see how it goes. Don't just loosen it, take it off but keep an eye on it to make sure gas isn't splashing out of it while running.

My money is on the vent first, bad gas last and some kind of engine issue in-between.
 
Will give the gas vent theory a try (i.e., removing/loosening cap).

If that's the problem, then how do you unclog the vent?

Thanks
 
quote:
Originally posted by jtybt15

What's an ati shoehorn valve? Is that something specific to outboards?

Click to expand...


Lol... Anti syphon ... Damn spailsheik :)
 
permanently mounted fuel tank have a ball vale in the fuel pick up on top of the tank to prevent fuel from siphoning out if the line is cut. With time, the ball can get sticky.

Check the vent fitting, some can get corroded, clogged with salt, corrosion, etc..

Years ago, I almost crash in a single engine plane when the a large paint flake got loose in the cap and blocked the vent hole. Engine lost power just after take off... Some air was able to get in as the engine wasn't pulling a vacuum in the tank and recovered... Cycle happened 3 times until I returned for emergency landing.

So yes, blocked vent can ruin your day :)
 
quote:
Originally posted by rockyrr

Will give the gas vent theory a try (i.e., removing/loosening cap).

If that's the problem, then how do you unclog the vent?

Thanks

Click to expand...


Buy enough vacuum line to reach the whole length of the vent line and run it through. Make sure the vacuum line you get is flexible, but stiff enough to push through the pipe. An old long dipstick would work too, but I'd be afraid of it breaking off in the loop.
 
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