Yamaha 115 not revving right…

[Stevie135]

Hi All

I have an Avon 5.6 adventure (yes I know not the best boat :-) ) and she has a 2007 4stroke Yam 115 on her. High hours at around 1200 but just had a full and expensive service at my Yam dealer for peace of mind as the boat is new to me.

Tried it out on a local water sports river near me but would rev much beyond 4200 before sort of chocking or just refusing to go further. The dealer suggested prop issue as it had a large stainless screw on it so fitted the original ali prop… seemed better at first got up to 5200 and 34kn but wouldn’t go beyond (I know that’s near max so was ok with it) but after that initial climb started doing the same again and not really getting past 4.2 to 4.5 and struggling if I tried harder. The dealer have told me to try replacing the fuel lines as it sounds like fuel starvation. Sort of feels right and I have a way of testing that, but wanted to know if there’s any other potential causes. I know the fuel pump was replaced during service - could it be this?

I’m perplexed but want to get this right before I trust my family in the rib

Many thanks

Steve

[spartacus]

More surprised by your dealer’s attitude. You have just had it serviced?

You can get fuel line issues. Another contributor could be the fuel filter (assuming the filter was changed) and the priming bulb. If it’s old fuel, decant and use for your mower. Start with that and report back.

Does the boat have internal tank or portable fuel tanks. At the engine end, get yourself a clear jar and press the connector with a screwdriver and pump at least a litre of fuel off. Just ruling out water.

[Stevie135]

Thanks for the reply. To be fair the Yam dealers have been good - after initial trial they took it back in and went over again but nothing in engine management and it ran up fine from their perspective.

It’s fresh 95 fuel in an internal tank. They suspect collapse of fuel hose… the issue seems to get worse as the engine heats up tho. Strange for sure.

[neil.mccrirrick]

I’d be considering a coil breaking down under load.

[Stevie135]

Coil? Could you expand a little more please? How would I diagnose/prove? Many thanks

[Pipster100]

Fuel line could be an issue but I’d bet money on the injectors, there’s very fine pre filters and the slightest blockage of the injectors will result in the engine bogging down under load. It would rev freely with no load. You need to check your dealer has the equipment to balance the injectors or they may send them away. Make sure they also replace all the filters including the VST. I could be completely wrong but seen this before on Yamahas that have supposedly had a full service, and to be fair to dealers I’m not sure the VST filter is a service item every time.

[Pipster100]

I should also have said the slightest blockage of the injectors too will result in the symptoms you describe, these can be acoustically cleaned.

[neil.mccrirrick]

Coil can break down under load and provide a weak spark. It can also be intermittent so good some day which would explain you having full power and then not

This https://www.bornagainboating.com/bad-coil-outboard/ page explains in detail and how to test. Good luck