WOT RPM change

[halmc]

I have an '88 Mercury Classic Fifty on a pontoon boat; because of intermittant issues with cylinders #1 and #2, I've ordered a new switchbox and expect it any day. Perhaps the switch box will cure this issue, but I'd sure like to have a bit of advice anyway:

This engine typically runs initially, only on #3 and #4, and at WOT, the tach reads about 2900. Fair enough for two cylinders. After a bit of running, #2 often comes on line. With three making power, Tach reads about 3600. Usually -- and inexplicably -- after extended idling, #1 fires up, and the tach rises to about 4300. With all four cylinders on line, I get an indicated 4300rpm, boat speed is about 17kts, about what I'd expect for a moderately loaded 20' pontoon boat. Sounds initially like an over proped boat, but to my ear the engine is turning faster than 4300, but that's what the tach says.

While I'm making the 4300 and 17kts, I have hung off the back of the boat and shorted each plug at the top of the plug, so I do know that each cylinder is making power.

Here's the kicker: a few weeks ago, before I started investigating this engine's erratic performance on the top two cylinders, at WOT, I got an indicated 5800 rpm on the tach. Unfortunately, I didn't have a GPS to meausre boat speed, but it 'feels' like the boat is as fast now at 4300 as it was then with an indicated 5800.

Perhaps the switchbox is boogered enough to either 1) cause the tach to indicate a lower rpm than the engine is actually turning? or 2) perhaps the switchbox is boogered enough to be causing one or more cylinders to be running with coniderably less than their rated power? (the latter seems distinctly unlikely)

Ideas, or opinions?

 

[j_martin]

Re: WOT RPM change

Check your prop. Somebody wanted your's and swapped with you.

At any rate, running with wide open throttle and that low rpms will do serious damage to the motor in a fairly short time. It has to spin up when running hard.

hope it helps
John

 

[Silvertip]

Re: WOT RPM change

The tach doesn't read off the switch box -- it uses a signal off the charging system. You apparently have two problems. Performance and tach. Perform a compression check and you might find one or two cylinders down on power. Have you actually looked at the plugs to see what they are telling you? Check tach wiring for loose connections. Rotate the tach calibration dial back and forth a few times and set it back where it was. Sometimes that adjustment gets touchy and just needs to be tweaked.

 

[halmc]

Re: WOT RPM change

The tach doesn't read off the switch box -- it uses a signal off the charging system. You apparently have two problems. Performance and tach. Perform a compression check and you might find one or two cylinders down on power. Have you actually looked at the plugs to see what they are telling you? Check tach wiring for loose connections. Rotate the tach calibration dial back and forth a few times and set it back where it was. Sometimes that adjustment gets touchy and just needs to be tweaked.

I think it was the latter: I have the adjuster a bit of a tweak and the tach has yet to return to its 5800 reading. Thanks! Turns out the engine did need a switchbox as it runs excellent now, although I can now (with your help) see that the switchbox had nothing to do with the tach readings I was getting.

Again, thanks.

PS, I'm on the hunt for a prop with lesser pitch.