Mercury M800 one cylinder not working

cpdinc1800 · #1 August 26th, 2008, 04:51 AM

Hello All,

I have a 1978 M800 80HP 4 cylinder 2 stroke outboard that is running on three cylinders. The motor starts fine and idles for as long as I want it to. I removed the spark plug wires one at a time when the motor was running and the #1 cylinder makes no difference in the RPM, while the # 2, 3 or 4 will stall the motor if I remove them. There seems to be plenty of spark on the #1 cylinder, I grounded it to the block and it seems bright blue, not yellow. The plug will come out wet after running the motor, and the top carb looks like it was cleaned by the previous owner. I took it apart just to be sure all the passages were clear of debris and I found nothing, not even a little sludge. If I put the boat in the water, I can make 1 or 2 short full speed runs on plane, but I can tell it does not have the power it should. After those short runs, the motor dies and I have to start it again and it will idle as long as I want it to, but I cannot give it any gas in gear, only in neutral. It will rev in neutral but die in gear if I try to go faster than an idle. I swapped the coils and wires for the #1 and #2 cylinders and it made no difference. I even swapped the wires running from the switch pack to be sure the wiring was o.k.

On a side note/question, the motor is mounted to a 1996 KeyLargo 166CC center console. The cavitation plate is not even with the keel because the motor will not mount any lower on the transom. It is at least 3 inches higher than the keel. The transom has a stepped shape to it's profile, where the keel is about 6 inches in from the transom, almost like a jacking plate. Should I mount the cavitation plate even with the bottom of the transom or the bottom of the keel? It has no tilt trim control and I have been trying to change the tilt using the manual tilt pin, but as the motor barely runs my experiments have been useless. Turning the boat in anything but a WIDE turn causes the prop to lose grip, the motor bogs, and then I'm idling back to the dock.

Any insight would be appreciated. I have been working on this boat all summer, it was a damaged hull when I got it and I have rebuilt most of it and would like to use it before the season is over.

achris · #2 August 26th, 2008, 05:35 AM

Do a compression test... I'm betting you'll find low compression on the top cylinder... With the cav plate 3"

above the bottom of the boat you will not be getting enough water.... I suspect the top cylinder has overheated and lost compression, hence it's not contributing any to the engine...

Chris...

cpdinc1800 · #3 August 26th, 2008, 01:45 PM

Sorry, I should have mentioned that I did a compression test-@150 in all four cylinders. The only two times I actually ran it in the water at any speed, I checked for the "pee" stream and it was working. The so called high speed runs were shorter than 2 minutes. that's all it would last before going to the limp home idle speed.

Chris1956 · #4 August 26th, 2008, 04:43 PM

That motor does not have a "limp Home" mode built in to it. If it is idling only, something is failing. You might try pumping the primer bulb while running. if it gets better, rebuild the fuel pump.

BTW- I am not sure I understood your description of the place of the A-V plate. There are hulls that cannot corner at high speed w/o ventilation, however they pump water fine. I recommend a visual on the motor while on plane. look over the transom. The A-V plate should be within an inch of the water. Any more than this and the motor will suck air instead of cooling water. You can plug the top water intake hole and enlarge the bottom intake hole, with a round file. That may help.

cpdinc1800 · #5 September 3rd, 2008, 03:15 PM

I took the boat out at 5am Wednesday, after working on it over the weekend.

I took off the top carb to check it, since it did not seem to have any fuel being sucked into the venturi. The float adjusting tab was keeping the float more than the 11/16 it said in the manual, although it may have gotten bent when I removed the gasket, I can't be sure. I reset it, put it on, primed the bulb, and fuel came out every hole. I took it apart again, removed the other carb, and adjusted the float to the second carbs float height (@14/16?). Could the float needle stick from sitting too long? Can the motor run on one carb, and would the bottom carb be enough to feed the #2 cylinder, but not the #1?

In regards to it's driveability, it will plane, but not as fast as a I thought a 16ft center console boat with an 80hp motor would. Not nearly as fast or quick as my 19ft Bayliner cuddy cabin with 110hp. At full speed (30?) if I turn back and forth in a weaving motion, the back of the boat feel like it is going to slide out. If I make a sharp turn the boat will seem to bog, then rev, and then bog as it looses traction in the water. The cavitation plate looked like it was riding level with the water, with the vent/intake above the plate out of the water. I can't post pictures of the height of the plate in relation to the stepped transom (camera only goes down to 1024x768). The plate is 3-4" above the bottom of the boat, and 3-4" below the second step of the transom. The motor is a 20" and it's resting on the transom as low as it can be mounted.