140 4 Stroke Losing Revs

[JonCase]

Hello Everyone.
I have a 2010 140HP 4 Stroke. On the weekend I began driving at 4500RPM and was fine for around 5 minutes. The engine then dropped down to 3000RPM and then went straight back to 4500. It did this every 5 minutes or so.

I have checked the water separator and there was no water in there. I also checked the fuel line. It still continued to this every 5 minutes or so.

I live on a remote island where there is no mechanic. The boat has been serviced regularly (I have to send off the island).

Any ideas on what the issue could be?

Thanks in advance.

[Moonlighter]

Easy things first.

Just because there is no water doesn't mean that the filter hasn't become partially blocked with rubbish. I would replace the water separating filter element as first step.

Check Fuel and breather lines. It's not a bad idea to disconnect a couple of lines from the tank or where they attached to the water separating filter and look inside them, some lines deteriorate internally and start to clog up as a result. Some lines can crack or partially break where the hose clamp is fitted which is another good reason to undo them, take off, inspect and replace. Usually there is enough slack in the lines to cut 1/2" off if necessary to give a clean section to reattach again with the hose clamp.

Make sure the breather line is clear of obstructions - usually it's easy to remove it from the hull filling, tank the fuel cap off the tank and blow through to see if it's clear.

Also check the primer bulb for fuel or air leaks and make sure it's pointing upwards.

See how you go with that, it does sound like a fuel delivery issue.

[Solarman]

As moonlighter suggested, it sounds fuel related.

But, are any codes, or trouble lights turning on when it drops to 3k, they might just flash on, then back off as abruptly, when this occurs?

Does the motor stay at 3k long enough to try pumping the bulb?
Have you noticed if the fuel filter on the motor has air in it when this occurs? This would indicate an air leak in fuel line, like moonlighter mentioned?

Just some things to look at that might point to a solution.

Good luck.

[JonCase]

Thanks for the info fellas. I will start with the filter and inspect all the hoses. I have some spares coming in on the plane today so I will start and work my way through it all.

Thanks again.

[JonCase]

Ok, so I tried all of the suggestions above. Changed filter, inspected fuel lines and bulb. Reconnected everything.

Stars and runs fine of muffs. I will give it a water test tomorrow.

I have a 250lt tank and the boat gets used at least fortnightly. In an average trip I would use 90 litres which will get topped up on return. Someone suggested that the fuel may have lost octane as we are not running near empty and this may be causing the issue.

Any ideas on this theory?

[Moonlighter]

Stale fuel can definitely cause dramas. Though it doesn't sound all that likely in your case.

Perhaps next time you fill up, you could freshen it up with premium highest available octane fuel if it's available. Or if available add a can of octane booster from the auto store.

Best idea in hot tropical areas is to refill the tank to full after every trip. A full tank of fuel loses less volatiles than a partially full one, due to the partially full tank "breathing" in and out more due to the greater volume of vapour in the tank. That's what BP fuels tell us here in Australia. You will also lose less to evaporation with a full tank.

Hope she runs OK when you water test, let us know how you go.

[JonCase]

Water test was worse than before. I have two different mechanics telling me two different things. I will change plugs next and see if that helps.

I will also run from another tank to see if it is an onboard fuel issue.