2012 2.5 hp 4 stroke gains speed while trolling

[Rick.]

Hello to All. My son bought this motor new in 2012. Has had little use but always has run very well until today. This was the first time being used this season. I would set the throttle for a troll speed of 2.0 mph and it would start at that but then start to gain speed and rpm all on its own. Continued to perform this way all afternoon. Vent was open and wind was not a factor. Any ideas what may be causing this? Thank you. Rick.

 

[Texasmark]

Not familiar with your engine. What vent was open? how do you control the throttle? Having had little use, it could be that it was "congested" from sitting up and as you ran fresh (one would assume) fuel through it and lubrication warmed up and thinned, internal resistances were reduced and it improved it's efficiency. Not uncommon. Happens to me on a 115 hp.

 

[Rick.]

Thanks Texasmark. Vent on the internal gas tank was open and new fuel was used. It has a tiller arm with a twist grip for throttle. The throttle has a little friction tensioner to hold the throttle where you want it to stay. I understand what your saying but it kept doing this throughout the afternoon and I did run full throttle a couple of times for ten minutes hoping that might smarten it up. If this was a two stroke I'd be looking at the carb but I've no experience with this tiny four stroke. Rick.

 

[dingbat]

The first thing to look at is the throttle linkage. Make sure something has not come loose

 

[Rick.]

Thanks dingbat. Had a look at the throttle linkage and it appears to be just fine. Two cable system to the carb and a separate pull knob for the choke plate. The cable system functions perfectly and is impressive compared to older OMC small outboards I'm more familiar with. There is no free play at all in the Mercury cable system. Hope you gentlemen have some other ideas. Rick.

 

[QBhoy]

No chance of it being a partly sheared shear pin ? Sometimes like this they catch and other time’s they don’t.

 

[Rick.]

Checked and the pin is good. I pulled the spark plug and it looked a little suspect (not obviously fouled). I had a new one so I gapped it and put it in. Should have checked that first. Hope that does it, I'll report back when the wind stops blowing. Much appreciated. Rick.

 

[Texasmark]

No chance of it being a partly sheared shear pin ? Sometimes like this they catch and other time’s they don’t.

Side comment, I am out of suggestions to solve the posted question: QBhoy, on your comment about a "shear" pin, you, as we did back in the day before the rubber slip hub, called the "DRIVE" pin a "SHEAR" pin also because when you needed them the most....for driving the propeller, they would be sheared, letting the prop free wheel....like when fishing the windward side of Rip Rap as an example, the prop would hit something and it would shear. Not fun.

Recalling that one thing, and the OMC gear case bulge out the front put me on to Mercurys because they had a rubber slip hub that reconnects when you back the throttle to idle and back to WOT and get the heck out of Dodge (Dodge Kansas....outlaw town of the old west...Matt Dillon US Marshall jurisdiction). There was no pin....just a dozen or so tabs to bend so that you could get the prop off the splined shaft when you wanted to do something prop related.

Dad bought a 1955 or 56 Scott Atwater 10 hp and believe it or not, on the tiller arm was a rubber grommet with 2 cotter pins and 2 SHEAR pins....right there where you could get to them.

 

[QBhoy]

Side comment, I am out of suggestions to solve the posted question: QBhoy, on your comment about a "shear" pin, you, as we did back in the day before the rubber slip hub, called the "DRIVE" pin a "SHEAR" pin also because when you needed them the most....for driving the propeller, they would be sheared, letting the prop free wheel....like when fishing the windward side of Rip Rap as an example, the prop would hit something and it would shear. Not fun.

Recalling that one thing, and the OMC gear case bulge out the front put me on to Mercurys because they had a rubber slip hub that reconnects when you back the throttle to idle and back to WOT and get the heck out of Dodge (Dodge Kansas....outlaw town of the old west...Matt Dillon US Marshall jurisdiction). There was no pin....just a dozen or so tabs to bend so that you could get the prop off the splined shaft when you wanted to do something prop related.

Dad bought a 1955 or 56 Scott Atwater 10 hp and believe it or not, on the tiller arm was a rubber grommet with 2 cotter pins and 2 SHEAR pins....right there where you could get to them.

Haha. Excellent and very true.

I remember getting my first outboard when I was about 6 years old (about 35/36 years ago)...it was a wee mercury 2.2 direct drive. My dad learned quickly to carry a few spare shear pins that summer .

 

[Faztbullet]

It is either a air leak on intake runner, venting on tank as gravity feed or carb needle sticking. Only a air leak or low fuel supply will make it run lean and increase RPM.

 

[Rick.]

Thanks Faztbullet. Will have a look for those items. Intake runner? I'm assuming that should be intake rubber. I does speed up like an engine does just before it runs out of fuel except it's not running out of fuel. I know the tank vent was open but next time I will remove the cap completely. Rick.