Your engine is a 2+2 as is my 2002 yearmodel of the same engine just purchased last summer. You run on 2 cylinders at low RPMs and once your engine gets up enough RPMs, there is enough fuel being fed to the other two cylinders for them to participate in your propulsion....book says that's somewhere around 1800 RPM. Your problem as was mine on my shake-down cruise of my new used boat, is that you have too much prop pitch for the available HP.
So, looking at available vs HP posted on the engine Nacille, the question is what happened to it? I assume that at one time things were working for you and this is a recent event.....so we will leave the prop pitch alone. If not, come back with that information.
After lots of work trying this and that, having had no idea as to what the PO had done or not done for/to the engine, I came up with the following efforts and have a great running boat:
1. I found that the lower 2 plugs liked to foul and with half the HP, you cannot make the hole shot. With new plugs I could make the shot but couldn't achieve top speed/RPMs, or anywhere near it until the plugs self healed and fully came online due to combustion temperatures burning off the contamination.
First time out, with minimum idling to get from the launching ramp out past the No Wake marker, I could punch it (fast transition of the throttle to ¾ or higher) and with the expected delay for the lower 2 to catch and come online, I had a clean hole shot and could run on up to the 5250 RPM rev limiter allowable max rpm....with trimming out appropriately as the boat picked up speed.
In retrospect, I found that the engine had a rough idle (about 700 RPM, in gear boat moving) if level or powerhead tilted back....allowing unburned fuel to pool at the top of the cylinders where the spark plugs are located. However, an easy solution to that was to tilt the engine slightly such that the power head was slightly tilted up in the rear and the idle was smooth as silk.
On subsequent outings, having applied the above tilting, where half the operating time was at trolling speeds, the bottom two plugs would foul out somewhat and when I would firewall the throttle, working the trim and coming up to speed, you could feel the engine intermittently gain RPMs as it was cleaning up the plugs, finally hitting the rev limiter RPM.
After some work I decided that I could minimize the fouling by changing the spark plug type and heat range. Since I don't run for long periods at WOT, I don't need to worry about the OEM recommended plug temperature rating being exceeded to the point where pre-ignition or melting of the electrodes would occur...if in fact that would happen.
I dropped down 2 heat range numbers (8 to 6 but on NGK plugs, lower numbers are increased heat range, unlike some other mfgrs. which are hotter with higher numbers) and went with an Iridium plug; Iridium for 2 reasons: Having a fine point, all the charge is confined to a small tip area making for the hottest/largest volume of energy dissipated in one spot, not diluted by a large contact area.
Second, with the Iridium, looking at the insulator of a conventional plug vs Iridium (identical construction otherwise) there was more insulator material between the tip electrode and the shell of the plug making for a longer path for the charge to bleed off for plugs that were loading up with unburned fuel-carbon, thus decreasing spark energy leakage.
The plugs I chose were as follows:
https://www.ebay.com/itm/Set-of-Fou...e=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649
The plug is exactly the same in all respects (including manufacturer and are genuine, not counterfit per NGK "how to tell the difference guidelines") except for heat range and tip type. I am having excellent results.
2. Other things I did/do, and in retrospect, probably not necessarily a necessity, were/are:
A. Use Pennzoil Premium Plus, semi-synthetic TC-W3 oil....almost no smoking, Walmart.
B. Not knowing the engine's history, installed repair kits on the accelerator pump (squirts a slug of fuel into 3 and 4 when it's time to bring them online), fuel pump, primer bulb-fuel line, new main fuel filter (which upon tearing apart had a goodly amount of accumulated crud---blockage), new small 3-4 cylinder fuel filter (clean internally).
3. What I didn't do was to remove and mess with the carburetors as nothing indicated that they needed my attention, nor link and sync nor anything else. But had what I did not worked, that would have been the next place I was headed.
You may or may not get lift from this "dissertation" of sorts. Depends on your engine and it's history. Good luck.
