RPM's drop when put in gear

Hi and welcome to YF.
How old is your boat/engine and when was she tuned up last?

Even a sick motor will idle OK, but when put under a load (i.e., when you put it in gear), maintenance issues manifest themselves.

Pull a sparkplug...black/dirty? Could be a clogged air filter, weak electrical (distributor/rotor/coils/points/timing), or old fuel/carburetor adjustment/clogged injectors.

And, yes, it might be twenty feet of dockline wrapped around the prop.

Good luck.

My next door neighbor just went through a similar issue and while investigating the problem discovered that the linkage, when shifting into either forward or reverse, momentarilly accuates an ignition kill switch, designed to reduce the loading on the transmission when moving from Neutral into gear.

The cure in his case was to replace the sticky shift cable to the outdrive and have everything adjusted by a qualified mechanic. Hope this helps.

Good point, some Merc's have a micro-switch on the shift to retard the ignition (try Sierra Parts, cheaper) its a small thing often overlooked.

Oh yer, good old fishing line around the shaft as well

Fish

very good and point about the shifting! Also had a similar problem with the 7.4L 454 whereby RPM would drop/hesitate on plane or docking for no mechanical reason from the engine...turned out to be a faulty battery terminal.

Stardust's gas engine

Based on a couple of things I guess we are looking at a carburetor engine here. Being an inboard you could turn the propshafts from inside by hand to see if there is excessive drag on the propshaft. If that is OK, while underway I would place both engines at about 12-1400 RPM and advance only one at a time to full throttle. If the engines are sound and running gear proper, they would reach close to the same RPM. If your problem engine is much lower, we may be looking at things as simple as fuel starvation, or problems with burned valves/compression loss or anywhere in between.
Loren mentioned looking at the spark plugs. This is very accurate indeed. They should be rather dry and a golden brown like color on the porcelain. Wet, dark, deposits (and salt taste on the tongue for raw water cooled boaters in saltwater) are not good signs. That reminds me, leaking water from inside exhaust manifolds often sounds like the carb is flooding at low RPM but clears when you race up the engine RPM.
Sorry for the long post. Hard to diagnose an engine over the internet.

Rick

Don't forget that leaking manifolds and risers kill more gasoline inboards then any other cause. Check the exhaust ports in the cylinder heads to see if water has been leaking into them from the manifolds and risers. The manifolds and risers are usually changed every 3-5 years in saltwater. I've rarely found a burnt exhaust valve in gas inboards. Usually it's a hole in the piston/bent piston rod from water intrusion from the exhaust manifolds, then next would be a melted piston if the gasoline is bad and/or the ignition timing was set overzealously.

Hi,

There is a member here called Kiwipushrod who has previously posted some pretty sound info about Gas V8's and the like.

He hasn't posted here for a while but you could always try sending him a PM and see what he thinks.

Having been down this road before can i "strongly assume" your going to pull the other head while your at it?