1996 200hp midrange misfire

[mike66]

My engine starts and idles, and will run great at 5000rpm forever. But decreasing rpms into the 4000 and below makes the engine real rough. It will be okay at idle up to around 2000. Any thoughts? Motor is new to me, so no service history.
Update: now missing at all rpm. Guess it's back to basic troubleshooting. I'll post more as I go along.

[Joe Reeves]

Other members more knowledgeable of this engine will be jumping in here (I retired 1991) but for the time being, I'll put my two cents worth in.

Do you receive any warning horn sounds, If so, which one? Does the horn beep when you turn the key to ON?

DO NOT ever race the engine beyond 1500/1800 rpm in neutral or even in gear if running on a flushette as you chance a runaway engine which will become a diesel exceeding its specified rpms! By the time you figure out how to stop it (shoving rags in the carburetors works)... connecting rods will most likely be flying out the side of its crankcase.

The idle can be set on a flushette depending on the model as follows. The idle will drop to a normal rpm due to back pressure when in the water.

Long shaft (20") - 1000 rpm
X Long Shaft (25" - 1200 rpm

Your problem.... Start with the basics:

Remove "all" spark plugs... Check the compression. What are the compression psi readings of the individual cylinders... the actual readings? Not generalities as some boaters give such as "Oh, they're great"... same holds true for the spark as statements such as that tells us nothing. The cylinders are numbered as follows, standing in back of the engine, facing the spark plugs.

2.....1
4.....3
6.....5

Now, still with all spark plugs removed.... With a spark tester that has an adjustable gap setting (available in auto part stores or build the following), set the air gap to 7/16"... The spark at cranking speed (at least 300 rpm), the spark should jump that 7/16" gap with a strong blue lightning like flame... a real SNAP! Does it? If not explain in detail.

(Spark Tester - Home Made)
(J. Reeves)

You can use a medium size philips screwdriver (#2 I believe) inserted into the spark plug boot spring connector, then hold the screwdriver shank approximately 7/16" away from the block to check the spark or build the following:

A spark tester can be made with a piece of 1x4 or 1x6, drive a few finishing nails through it, then bend the pointed ends at a right angle. You can then adjust the gap by simply twisting the nail(s). Solder a spark plug wire to one which you can connect to the spark plug boots, and a ground wire of some kind to the other to connect to the powerhead somewhere. Use small alligator clips on the other end of the wires to connect to ground and to the spark plug connector that exists inside of the rubber plug boot.

Using the above, one could easily build a spark tester whereas they could connect 2, 4, 6, or 8 cylinders all at one time. The ground nail being straight up, the others being bent, aimed at the ground nail. A typical 4 cylinder tester follows:

..........X1..........X2

.................X..(grd)

..........X3..........X4

This can obviously be modified to a 6 or 8 cylinder setup tester.

Let us know what you find.