Outboard revs normal but getting bad economy / speed

[Ryannnnnnnn]

Hey, 

I have a 2021 yamaha f150 and recently noticed during my trip to Yeppoon that my fuel economy has worsened and that I wasn't getting the speed per revs as I normally would. 

Normally cruising at 40kph at 4000 rpm I'd get around 1.6 kms per litre (which I thought was pretty average to begin with?) 

During my trip I was getting like 1.3 kms per litre revving at 4000rpms which was getting me commonly around low to mid 30kph. The engine sounded healthy (no alarms or coughing) , but just lacked getting and maintaining speed, I had to work it harder to get up on the plane. 

At the start of the trip some fishing line went around my prop, which I thought I removed instantly and I also touched a bit of reef at low speed. I can see some scrapes on the skeg but not the prop. 

I got home today and took the prop off (my first time) and prop washer and couldn't see any fishing line. Only thing I noticed is the nut wasn't overly tight, it came off easily without a wrench. 

I'll probably do it again on the weekend so I can add some marine grease and a new split pin. 

 

Anything else I should check that might cause the speed / economy issue? And are there any 'gotchas' I should look out for when putting the prop off/on again (on yamaha 150)? My neighbour starting chatting midway through taking it off, so I got a bit distracted when taking note of how exactly it fitted together. I've watched a few howto YouTube videos 

[Junky]

Prop removal is easy. 

I'd say you've done it right. 

Don't tighten the crap out of the nut. Just a firm lock by holding the prop and doing it up with socket and rachet, all by hand. Do not lock the prop and tighten the nut up hard.

Make sure the nut retainer is fitted correctly, thus is paramount to not losing your propeller. 

 

 

We're you loaded up with weight?

Obviously weight will slow you down and cause more fuel usage. 

Windy condition will also cause this with cabin boats. 

We're conditions rough ie. Big swells. What I'm getting at is if you take a straight piece of rope, then zig zag it to represent swell. It will shorten that length of rope substantially. 

 

Crap fuel. 

Only use 95 or 98. That 91 is a con and ruins engines. 

 

Service the motor. 

I'm sure you've had it done, but old oil etc will cause engine problems. 

 

Worst case, sell it and get a merc... .... Joking. 

[ellicat]

Agree with everything Junky said. Only other thing is where you put your weight e.g. large full esky. The further back it is the heavier your rear end the harder the engine will need to work to get on and stay on the plane.

[turdle]

1 hour ago, Junky said:

Only use 95 or 98. That 91 is a con and ruins engines. 

Actually the outboard engines are designed and tuned to run on 91-93ron. Yamaha specifically states that if you encounter engine troubles/roughness, it may be due to the use of premium or high energy fuel, and recommends only using standard fuel. Economy also suffers when using high octane fuel. 

[Ryannnnnnnn]

Interesting OK yeah I noticed it first in some  choppy swell and also when I got over to the calm waters of the islands. I had the standard gear on board placed where it normally goes

I normally fuel up at the servo but I fueled up at the marina assuming they had quality fuel (as all the charter and boats there use it). So weird. I came back home early as I was frightened that it resulted in a breakdown and get stuck out on the water

[benno573]

All of what has been said above.

Alloy or stainless prop?

[Ryannnnnnnn]

Stainless

[ellicat]

13 hours ago, turdle said:

Actually the outboard engines are designed and tuned to run on 91-93ron. Yamaha specifically states that if you encounter engine troubles/roughness, it may be due to the use of premium or high energy fuel, and recommends only using standard fuel. Economy also suffers when using high octane fuel. 

That's interesting @turdle. I  have only run 98 in my Merc without isuue. Now I'm going to have to look up the Owner's manual. haha

[Junky]

13 hours ago, turdle said:

Actually the outboard engines are designed and tuned to run on 91-93ron. Yamaha specifically states that if you encounter engine troubles/roughness, it may be due to the use of premium or high energy fuel, and recommends only using standard fuel. Economy also suffers when using high octane fuel. 

I take manufactures "advise" with a grain of salt. 

Hey, it's worth a try but you get better economy out of higher oct. I have a 92 b2600 bravo that even runs better on 95. The fuel economy is astounding compared to 91. 

My father inlaw is a very good car and light commercial mechanic (which by the way I am happy to pm his details if anyone would like a good honest mechanic in capalaba. He's no dodo.

He even designed and financed the creation of Seadex boats Australia. His name is on every vin plate to this day) 

 

There is no way I would run any of my engines, car boat and moto x bikes on 91. 

No I do not own a Yamaha. Once you go black, you never go back. 

I did have an older yam 15hp years ago that I'd run on 95. No problems at all. 

Most 91 is just not good for engines. Ethanol free, may be okay but there's not many 91 fuels out there that do not contain up to 10% sugar. 

Run at your own peril. 

 

Also, do not service your vehicle every 15k. Manufacturers advise but your engine will hate you and will detonate in a shorter time than a motor serviced every 10k.I am only mentioning this because it's manufacturers advise. 

That's why they have "interim" services at 7.5k. That saves their bacon, however it is not a mandatory service in their eyes to maintain a warranty, so most people ignore it. Then when the motor does self destruct, 9 times out of 10 the vehicle is past its warranty expiry date. 

My father inlaw replaces a lot of blown motors...

They're not always advising on what's best for your hip pocket. 

Sorry for the rant....

 

 

@Ryannnnnnnn

How old was the fuel in the tank before you filled up?

If the boat sat for a while then perhaps the existing fuel was nearly crude again. 

 

Do you have a spin on easterner type fuel filter (looks like small car oil filter). 

Spin it off and tip out its contents into an ice cream bucket. See if there is any contaminants. 

Replace with a new one of course (fill with fuel and lube rubber gasket before you wind it on). 

Probably wouldn't hurt to remove the spark plugs and check colours, maybe even throw a new set in. 

Fill with a fresh tank of BP95, you may need to contact your bank for a loan to do this atm and go for a good blatt in your regular location. 

 

 

 

[Ed.]

There is another thing you can try, I had a 150HP XS Pro Merc on my last boat and had a range of problems for a while, wouldn't get high revs and when run at say 4300 RPM plus for a while (20 minutes or so) it would suddenly loose power for a split second and then continue as if nothing was wrong, however at a slower rev speed it would work fine all day long.

Turns out that where the fittings  such as fuel filters and fuel bulb went in to the fuel hose I had a small blockage. The fuel line I had (grey line), had a plastic liner and where the fitting pushed into the fuel line, it had sort of pushed and scrunched the plastic away from the line and it turned it into a small blockage. There was enough fuel going past it to run the motor normally but when hitting 4300 RPM it would not go past it for any length of time, after a long sustained run at higher revs the blockages were starving the engine of fuel to a point of it not being able to reach or maintain the higher revs for any length of time and also to miss a beat or two every now and then.

As this motor was overpowered for my boat I usually didn't travel normally at higher than 4100 RPM for any length of time, as it meant I was travelling way too fast for my liking and the conditions, so rarely did the problem occur. Took it to the marine mechanics a couple of times and they couldn't find any issues as they didn't run the motor at high revs or long enough in the tank, so everything came up as normal and there wasn't any error codes showing up on the scans.

It could be a long shot, but just take off the fittings and check inside the line ends to see you might have  a similar problem. Not sure if the black fuel line has a similar liner to the grey fuel line I had. It is worth a shot as it won't cost you anything to check. Hope this helps.

Cheers

Ed.

[Junky]

20 minutes ago, Ed. said:

There is another thing you can try, I had a 150HP XS Pro Merc on my last boat and had a range of problems for a while, wouldn't get high revs and when run at say 4300 RPM plus for a while (20 minutes or so) it would suddenly loose power for a split second and then continue as if nothing was wrong, however at a slower rev speed it would work fine all day long.

Turns out that where the fittings  such as fuel filters and fuel bulb went in to the fuel hose I had a small blockage. The fuel line I had (grey line), had a plastic liner and where the fitting pushed into the fuel line, it had sort of pushed and scrunched the plastic away from the line and it turned it into a small blockage. There was enough fuel going past it to run the motor normally but when hitting 4300 RPM it would not go past it for any length of time, after a long sustained run at higher revs the blockages were starving the engine of fuel to a point of it not being able to reach or maintain the higher revs for any length of time and also to miss a beat or two every now and then.

As this motor was overpowered for my boat I usually didn't travel normally at higher than 4100 RPM for any length of time, as it meant I was travelling way too fast for my liking and the conditions, so rarely did the problem occur. Took it to the marine mechanics a couple of times and they couldn't find any issues as they didn't run the motor at high revs or long enough in the tank, so everything came up as normal and there wasn't any error codes showing up on the scans.

It could be a long shot, but just take off the fittings and check inside the line ends to see you might have  a similar problem. Not sure if the black fuel line has a similar liner to the grey fuel line I had. It is worth a shot as it won't cost you anything to check. Hope this helps.

Cheers

Ed.

Wow. 

What a find. Well done.

I had a similar issue.... 

 

 

 

My engine is just the standard 150 but I could go all day with no hiccups. 

Then, on the way home, normally longer trips like 70km,  out of nowhere the engine would run rough as guts like it was out of fuel, but weird out of fuel splutter, and drop off the plane. As soon as I buttoned off to neutral it was fine. Purring like a kitten. Take off, all good. 

WTF?

Was so random it wasn't funny. 

Could go 4 or 5 trips with no problem, then the demon would appear again. 

My biggest fear was that it would happen crossing the bar. That could end on the news. 

 

After much searching for answers I came across a bloke in the US with the exact problem. 

He had narrowed it down to the gear selector cable not pushing the selector completely into gear. The engine would have a moment where it thought it was in neutral and go to neutral rev limiter. That's why it was a weird splutter. 

Simply remove the black cylindrical pawl on the selector cable and give it 2 rotations out.

Never had a problem since. 

Here I was looking for fuel problem the whole time. 

[Ryannnnnnnn]

Very interesting OK thanks guys for the tips will check it out. The fuel I had in before topping up at the marina was less than a week old (covered some kilometers that week)

[Ed.]

There is nothing like an intermittent problem to cheese you off! If something stops you can find out what it is, but when it comes and goes, but runs normally most of the time, it becomes a nightmare to track it down. Mine took a couple of years and a few trips to the mechanics without success. It was only when I took the fuel bulb off as I needed to rearrange things due to putting in a larger battery in the battery compartment that I noticed my issue.

[Ryannnnnnnn]

Could it be a spun prop maybe? It does not lose speed / power instantly like if it were slipping at a specific rpm, it just feels like I'm carrying a heaps of weight (but I'm not, weight distribution is normal and am carrying no water on the bilge) 

[Ed.]

7 hours ago, Ryannnnnnnn said:

Could it be a spun prop maybe? It does not lose speed / power instantly like if it were slipping at a specific rpm, it just feels like I'm carrying a heaps of weight (but I'm not, weight distribution is normal and am carrying no water on the bilge) 

Personally I don't think so. I had a prop hub go faulty on a 225 Evinrude about 2 boats back and if I remember correctly, when it happened the boat slowed down and the revs increased  due to the reduced load on the prop as it was spinning on the hub. It  would slip at a certain rev due to the pressure of the water on the propeller blades and so the prop would stop spinning but the shaft would keep turning inside the rubber hub. As soon as I felt the motor revving higher I would back off the throttle straight away and then the prop would grab again on the hub due to reduced pressure on the prop. It was like driving a car and slipping the gearbox into neutral but not taking the foot off the accelerator which results in the motor over revving. Only way to fix a faulty rubber hub is by replacing it, but as you are not slowing down it doesn't appear to be your issue.

There are numerous causes that can make you reduce your fuel  economy, ranging from an ignition coil/ spark plug/lead breaking down at speed, down to clogged fuel filters, injectors, etc. I had replaced all my filters before I found out what my actual problem was, on my motor there were 2 filters under the cowl and another inline fuel filter just above the fuel tank.

Forgot to mention earlier that with my motor problem, I occasionally had revs but little power, so revs were high but no actual power, so instead of doing 27 knots at about 4000RPM I would only be doing about 15 knots. That didn't make sense to me as the gear box is fixed to if I got the revs I should have being going a lot faster, maybe my prop was also slipping at the time. But in any case once I fixed the fuel line blockage issue I didn't have any more drama, so who knows!

[Ryannnnnnnn]

OK fair enough. If I have crappy fuel, is there anything additive I can add to bring it up to scratch? The tank is holding about 200l of fuel 

[Hweebe]

One potential gotcha....is that the boat speedo or gps speed you are quoting?

You mentioned you hit the skeg. A blocked outboard speed sensor in the skeg can lead to misleading speed / mileage readouts.

This has happened to me before. Simple fix with the right sized drill bit to unblock the sensor.

 

[Ryannnnnnnn]

Thanks to all who replied. Just thought I'd give an update in case it helps someone else sometime. 

Confirmed the prop had not become spun by taking the prop off and looking at the prop hub assembly . As a step before checking the fuel filters /lines I ended up putting some 'fuel doctor' in the tank (in case it was bad fuel) and was surprised to find the motor ran as normal afterwards. Whether or not it was the fix or if something in my fuel filter or fuel line resolved itself, I guess we'll never know. 

[Bobs]

On 2/17/2022 at 1:00 PM, Hweebe said:

gotcha....is that the boat speedo or gps speed you are quoting?

You mentioned you hit the skeg. A blocked outboard speed sensor in the skeg can lead to misleading speed / mileage readouts.

This has happened to me before. Simple fix with the right sized drill bit to unblock the sensor.

 

True, if you get some mud or sand in the speed sensor hole, the speed and fuel economy numbers are random. Easy to clean it out.

If I even slightly suspected bad fuel I would siphon it all out and put it in a car or something else.

[MattInOz]

On 11/02/2022 at 6:29 PM, turdle said:

Actually the outboard engines are designed and tuned to run on 91-93ron. Yamaha specifically states that if you encounter engine troubles/roughness, it may be due to the use of premium or high energy fuel, and recommends only using standard fuel. Economy also suffers when using high octane fuel. 

From my time as a mechanic working on old Minis, MGs, Rovers etc I can confirm this. Spending money on octane levels higher than your engine is designed to use is just throwing money away. The fuel companies will argue that they put in detergents etc to help "clean your engine" but realistically all that would do is take up space in the combustion chamber with something that's not producing power and actually lower your output. Of course if your engine's designed to use 95 or 98 RON you should use that, but the majority are not.

One caveat though is if you're using 91RON (or 94, depending on the brand) be very sure not to use fuel with ethanol in it. Not because it would do any damage (it won't assuming your motor's less than 30 years old) but because ethanol is hygroscopic... ie: it absorbs moisture from the air. This isn't a real problem with cars under normal use, but with boats it's inviting water contamination in the tank.