89 merc 100 hp drops 1 or more cylinders just before plane.

Hey guys, this is the first time I have posted on here, but I need some help. The fish are starting to bite, and my boat is broke. 3 years ago I bought an 89 Mercury 100 hp that needed a rebuild. I have always worked on engines, so the rebuild was no big deal, however, I'm not very good with the electrical side of them. Last year, I broke a magnet in the flywheel. When it quit, the first thing I did was rebuild the fuel pump, because it kind of acted like a fuel issue. Before I found the broken magnet, I had rebuilt all four carbs and replaced the fuel pump. Then I saw the magnet was loose and broken. I replaced the flywheel and it ran great for about 3 trips to the river. Now I have a new problem. When I hit the throttle to jump up on plane, it jumps out of the hole great, but then I seem to lose at least one cylinder just before I hit plane, and that's all it has. I have tried pumping the bulb, and choking it as that was suggested to me by a boat mechanic in order to see if it was a fuel issue or not. Nothing changed when I did that. When I am running it on the hose at my house, it runs perfect, when I leave it on the trailer at the ramp and rev it, it runs perfect, when I take it out and the pump (it has a jet pump on it) gets a full load on it, that's when it poops out. Also my fish finder shows voltage on the screen, and I have noticed now that when I start the boat it shows about 13.5 vdc but when I bring the rpm's up, it will climb quickly to around 16 vdc and then the fish finder shuts off saying "high voltage". It has the original (I think) black stator in it, and I have been told by a couple different mechanics that these were notorious for failing. I am ready to start ordering parts, but I don't want to just throw money at it. Also, I'm worried that I may have more than one problem. Any help would be greatly appreciated. Man I need to go fishin'

Re: 89 merc 100 hp drops 1 or more cylinders just before plane.

If the charging system is making 16 V, the stator is working perfectly. How old is your battery?

How clean are ALL battery cable connections?

As a lead/acid battery ages the maximum charging voltage rises, many autos would fail as they regulate voltage very tightly. Outboards will allow the charging rate to 'float' with the aging battery.

Pull the battery out, take it in somewhere that has a real adjustable load battery tester for a proper load test to determine your battery 'health'.

As for dropping a cyl, start with a simple spark test using an adjustable gap tester set to 7/16 inch.

Now disconnect the throttle cable, disconnect the plug wires, remove the spark plugs, pull the lanyard, use a DVA meter and test each trigger while cranking. While testing each trigger move the throttle from idle to WOT and back while watching the meter for any weakening, indicating a broken trigger wire failing connection as the wires are moved with timing advance.

Reving in Neutral doesn't prove much, a lightened load does not cause full ignition load, nor full fuel load, and just can't zero in on a problem as quickly as loaded system tests.

Re: 89 merc 100 hp drops 1 or more cylinders just before plane.

Disconnect the yellow stator wires from rect/reg and retest. If problem goes away the rect/reg is bad causing stator to overheat. 16 volts is way too much voltage since you have a regulator..

Re: 89 merc 100 hp drops 1 or more cylinders just before plane.

Well, I got desperate and took it to a very reputable boat shop in the area, and they did find that my battery was bad which was causing the high voltage issue, which was surprising, because the battery was only a few months old. They had to drop the jet pump off of it and install a regular lower unit so that they could put it on the dyno to load the engine. Today the mechanic got that done and they were able to test the stator, triggers, and according to him, the whole electrical system, and they said it tested perfect. One thing that bothers me is that they said it did not have a rectifier/regulator. They said it had a 9 volt stator on it. He tested it and said it ran perfect. I picked it up and took it straight to the river and, surprise, it wouldn't get out of it's own way. The overcharging issue was gone, but when I hit it, it falls on it's face. It's definitely missing. So, now I'm back to thinking it's a fuel issue. A couple years ago when I was fighting the broken magnet, and I thought it was a fuel issue, as I stated in my last post, I rebuilt the fuel pump, twice actually, and then I found an electric low pressure fuel pump that I put on it. After I got the flywheel replaced, it ran better than it ever had before. It was just snappier out of the hole. I'm beginning to think it isn't getting enough fuel. Tomorrow, the original fuel pump is going back on. I feel like I need to get it back to original before I go any farther.

Re: 89 merc 100 hp drops 1 or more cylinders just before plane.

When was low on power during acceleration did you try squeezing the primer bulb to assist the fuel pump to keep the carbs full?

Did you try momentarily 'bumping' the enrichener/choke button to give a touch more fuel?

If one of these makes a difference it tells you a direction to follow and find/cure the problem.