Help diagnosing outboard motor problem

[robdew]

Over the weekend my 70 HP Mercury Force outboard started acting up.

Here are the symptoms:

• At full throttle I only get about 1000 RPM instead of 4000-5000
  at random RPMs the overheat buzzer in the throttle box stars buzzing

I have plenty of coolant water flowing.
My first though was we forgot to mix oil into gas on the last tank full, but we're 90% sure that's not the case. If these symptoms describe EXACTLY what would happen if the oil mix were way to low, we'll assume we're wrong and undermixed or forgot to mix.
Any other ideas?
At a slow crawl, just above idle, everything seems to work correctly.
I am planning to take this into the shop but I want to do any troubleshooting I can before turning it over to them.

(I am aware of the Force motor reputations. No need to revisit that)

[teecro]

First thing to check is cylinder compression...

[Bamaman]

I don't know what year motor you have and if it's oil injected.

1000 rpm's is an indication that your motor may be electronically thrown into a fail safe mode. My old 85 Yamaha will do that for a number of reasons. If the oil injection's not working right, it'll go into fail safe.

Unless you have a service manual with a troubleshooting section, it may require a professional's opinion.

[Grumman]

only running on one cylinder I bet, check compression, and spark on each one. FORCE engines love to blow piston rings

[BoatCop]

Alarm buzzing and low RPMs is a sign of fuel starvation. The alarm is going off because, as it's a fuel/oil mixture, lack of fuel/oil can cause major wear or engine failure. The alarm is not just for overheat.

Check filters, fuel lines and connections for restriction or leak. As any leak is usually on the suction side, it may not be visible. Use a can of starting fluid to spray on any connections or suspected leaks (with engine running at idle). If the engine RPM increases, you've found your leak.