Low RPMs at Full Throttle - 8HP MERC 4-stroke

[TheInterestHunter]

I'm only getting 3,000 rpms at full throttle on my 2005 8hp 4-stroke Mercury Outboard. I should be getting 5,000 to 6,000.

I put a tiny tach on there and adjusted the idle to the recommended 900 RPM's when in simply put in gear (Forward). No problems there.

When in full throttle, the manual says it should be at 5,000-6,000 rpms. I'm only getting about 3,000 and a little lower. What's going on?

I wanted to do the tachometer reading because I figured that the engine should put both me and my friend on a plane in my 10' Zodiac dinghy, and it wasn't.

HISTORY: I've had some dirty carb and fuel line issues which has given me frustrating performance over the last while. I dug in further and ripped open my old dirty fuel filter. I found a lot of weird crystallized amber-colored translucent pieces in the fuel filter, as well as a lot more upstream of the filter. Weird? (Any explanations on that one?). I've cleared out those things, bought a new fuel tank, hose, and fuel filter... and the engine's running pretty awesome, other than the low rpms. Now I'm trying to solve that.

Any troubleshooting ideas? I'm hoping a cylinder isn't bad. I'm going to pull a spark plug wire today while it's running to see what happens.

Thanks for your help in advance.

 

[TheInterestHunter]

Re: Low RPMs at Full Throttle - 8HP MERC 4-stroke

I pulled a spark plug wire while it was running. The engine started dying. My other hand holding the motor started getting little shocks though. It concerned me so I hit the kill switch. I'm guessing this isn't a recommended method?

 

[TheInterestHunter]

Re: Low RPMs at Full Throttle - 8HP MERC 4-stroke

A friend suggested the prop could be the case. The only history I do know about the engine is that the seller used it to duck hunt. My friend said that I should check out the prop and that the duck-hunter probably put that prop on there not for the purpose of speed. He thinks by getting a different prop I'll get higher rpms and more speed.

 

[TheInterestHunter]

Re: Low RPMs at Full Throttle - 8HP MERC 4-stroke

LINKAGE: I also did an exterior look-over of the carb after bringing the boat in from the test-drive and tach-reading of 3000 rpms. I noticed the linkage for the throttle wasn't perfectly positioned. In full throttle postion, the linkage still had about 4 millimeters to go before it was truly fully open. So the tiller handle was in full throttle position... but the linkage still had about 4 millimeters to go. Perhaps this will give me more rpms too. I adjusted the linkage and am waiting for a calm day to go out and test it.

 

[TheInterestHunter]

Re: Low RPMs at Full Throttle - 8HP MERC 4-stroke

Being unaware that the Tiny Tach tachometer had different settings, there's a chance that the settings are wrong for my motor and it's reading half the rpm's that the engine is really getting. I'll report back (once the weather warms up in a week).