Mariner starts fine then runs slow.......Help........

[RobAz]

I have a 3 cyl 1988 60hp Mariner. Here is the problem: Everything appears fine while running it out of the water. However when I put it on the water the boat starts and idles fine, as I accelerate everything appears ok then suddenly the motor dies. It will restart but when I put it into gear it dies. I have to rev up the motor slightly to allow it to shift into gear without it dying. Now that it is in gear it will hardly accelerate. It allows me to give it throttle but the rpm's barely come up and it only goes 4 - 5 MPH. This is at full throttle. The RPM's seem to be about double the actual speed of the boat. The motor at this point seems to be running pretty smooth. I do not notice a cylinder missing or anything like that. No excessive smoke or fuel smell. Gas tank is vented, bulb is ok. Another strange thing is that after driving for about 3 or 4 miles I consumed about 4 gallons of gas. No matter how much I shut it off then restart it still does it. I let the motor cool down for about 4 hours and it still runs slow. However the next morning the boat fires up fine and we start from the very begining again. then starts all over again from accelerating fine to hardly running. It seems like after taking it out of the water for the night Something resets itself. It has done this on 5 seperate occasions. My local mechanic is 95% sure that it is the switch box. He said that this is exactly what these Merc's and Mariners do when the switch box goes out. I am definetly not a mechanic but if the switch box goes out and I am dropping a cylinder like my mechanic is saying, wouldn't I notice the engine running rough??? and then where is all this gas going??? Any Ideas????? Please help!!!!!!<br />Thank you.

 

[12Footer]

Re: Mariner starts fine then runs slow.......Help........

Rob, I hate to agree with your mechanic, but it does have the symptoms of a bad box,or coil (if so equipped) breaking-down. Run a compression test, and try new plugs first.<br />Also, I am a bit confused. You said the RPM's are lower than normal, and in another sentence,you said they are running double..<br />If the engine runs out like a slipping clutch on a car, it has a spun hub in the prop.<br />But I suspect that if you are having problems with idle as decribed, it is almost surely dropping a cylinder. The only reason you don't notice it in neutral, is due to the engine not being under load. They'll allmost sound noramal in neutral,and only be running on 1 cyclinder.

 

[RobAz]

Re: Mariner starts fine then runs slow.......Help........

Thanks 12footer. The compression is great and I have already put new plugs in. If it the switch box, does this explain the incredible use of gas?? Is there any way to check the switch box??

 

[12Footer]

Re: Mariner starts fine then runs slow.......Help........

Yes, it would explain the high fuel comsumption, and the slow speed,high RPM thing, all caused by a slipping hub (like a rubber clutch) in the prop.<br />But the missing on one cylinder is sometyhing else. It would NOT increase the fuel consumption. But this does not mean it isn't failing. Not to confuse you, but I think (from your description), you are suffering from two totally different problems. Of course, I could be wrong,being long-distance,and all.<br />The prop can be taken to a prop shop, and the hub can be replaced for under 35buks.<br />The switchbox, well.. I'd get the shop manual,find the testing section,and test first. If bad, I would try to locate a used one. If switching them out fixed the problem,i would then buy a new one, and put the still-good used one in a waterproof baggie,and stow it in a dry place onboard.<br />And then,find out why the switchbox failed, so I don't have to buy a third one.

 

[RobAz]

Re: Mariner starts fine then runs slow.......Help........

Hey 12Footer, I just wanted to clear up what I meant about the speed and the rpm's. When the motor starts acting up, which is shortly after reaching top speed or traveling at a slower speed for a little bit longer amount of time,it idles fine, however when in gear I can only drive about 4 MHP. The Rpm's are nowhere near that of full throttle, yet they seem faster than what the boat is traveling. This is while I am at full throttle. It is not as though something is slipping but more like it just doesn't have the power to push itself. The boat doesn't even have the power to get up on the trailer. It dies once it meets the slightest amount of resistance from the trailer. Maybe this is tied into the switch box problem??? Thanks for your time.