Low RPM under load, Mercury 175 (1993)

[SunBum]

Quick question (hopefully with a simple - cheap answer!)

1993 Mercury 175 outboard.

When not in gear, the motor seems to operate fine. Revs up to 5000+ RPM with no discernible problem.

Put it in gear, and the RPMs won't go above 2000, and of course the boat has no speed.

This has happened before (both times after sitting - neglected - for long periods of time)
Once a long time ago, we had the carbs cleaned and rebuilt and all seemed well for a while.

The last time, I had the carbs cleaned and rebuilt again and the tank drained. But it didn't correct the problem.

It just seems starved for fuel so I'm going to change the little in line fuel filter (I've already changed the water separator filter).

Any other ideas?

I took a video the other day and I'll try to get it posted asap.

Thanks!

 

[SunBum]

Re: Low RPM under load, Mercury 175 (1993)

Here is a video showing the problem.

http://www.youtube.com/watch?v=Byr469Dh1IQ

Thanks again!

 

[JB]

Re: Low RPM under load, Mercury 175 (1993)

Multi-cylinder outboards can pretend to be running fine with no load and half of the cylinders not making power. When load is applied they no longer seem to be running fine.

Never believe what they do with no load, it is often a lie.

I think you have one or more cylinders dead. It will take a bit of troubleshooting to identify the culprit(s), which cylinder(s) are dead, lost ignition, lost fuel, lost compression, etc.

Good luck.

 

[SunBum]

Re: Low RPM under load, Mercury 175 (1993)

Just to add (and I know it's not a definitive test) but I checked the compression on all the cylinders and they all tested out at between 105 - 110 PSI.

 

[CharlieB]

Re: Low RPM under load, Mercury 175 (1993)

Compression test should be followed wwith a spark test on each cyl to ensure the ignition system is firing all at low speed.

Once low speed spark is confirmed a DVA test of the ignition is recommended to ensure the stator is producing adequate voltage from the high speed windings to continue firing all plugs under full load and RPM advances over 2000.

Never assume any electrical parts is bad as there is NO RETURN on electrical parts. Test correctly to prove any fault before even thinking of ordering any electrical part.