1998 Mercury 40 hp bogging down on full throttle

[newbie38]

I just bought a 1998 Mercury 40 hp outboard (serial # 0G660803, model 40M?). Engine looks remarkably clean, but bogs down to the point of dying when I go from idle to full throttle. When the problem is happening, the motor will run smoothly at idle and at low throttle while in forward gear. The first time it happened I thought it might have been the spark plugs. I swapped them out and it appeared to take care of the problem. Today the problem happened again. I changed out the spark plugs again and the problem continued (the spark plugs I removed looked ok). One thing I noticed; if i went up to full throttle in stages instead of just going straight from idle to full throttle I didn't have the problem. I'm thinking maybe fuel system problem? Any advice would be greatly appreciated.

One other thing (not sure if it matters)--the motor is mounted on a Zodiac Mark II Z470 (15 1/2 feet & rated for 40 hp motor).

 

[tashasdaddy]

Re: 1998 Mercury 40 hp bogging down on full throttle

either dirty carbs, or weak fuel pump. it's definately fuel restriction, also check for air leaks.

 

[belewsonmymind]

Re: 1998 Mercury 40 hp bogging down on full throttle

Also could be the anti-siphon valve. Does the primer bulb collapse when you begin to bog? If so, replacing the valve may help.

 

[newbie38]

Re: 1998 Mercury 40 hp bogging down on full throttle

Turns out the culprit was a weak/bad fuel pump, as you all suggested. I replaced it, and the motor runs like a champ. Thanks to all for the advice!

 

[flyingdelorian]

Re: 1998 Mercury 40 hp bogging down on full throttle

I have a simular problem. The motor runs great for a minute or two then bogs down at full throttle. I can pump the bulb and the engine will pick back up and run ok until I stop pumping the bulb then it bogs back down. The bulb is never going really soft just a little less tight then when i pump it. I am thinking either a blockage in the fuel line or a weak pump. Where is the cheapest place to get a fuel pump for a 40 horse Merc? Also, I just bought the boat and don't know the year model of the motor. How can I find out what year model it is?

 

[: Casey1232008]

Re: 1998 Mercury 40 hp bogging down on full throttle

either dirty carbs, or weak fuel pump. it's definately fuel restriction, also check for air leaks.

I was hoping to find some help here. I have a 1983 mariner 115 inline 6 outboard. 3 carbs, 6 coils, 2 switchpacks (one of which has already been replaced) This engine was running strong when I got it, although it sat for a few years. I used some carb cleaner to get the fogging oil out of it. I have had this boat on the water three times since I purchased it this past winter. First time was a three and a half hour blast. Second time, it ran great for about three hours and then: I was running it wide open on the way back to the marina and began losing rpm's. A lot of rpm's! It would only give me idle speed at that point. Any more throttle and it bogs down and dies. I took it back to the lake to test everything. The boat took off great and ran for 25 min and did the same thing. Why is this happening to me?!!!):

 

[fasassvalk]

Re: 1998 Mercury 40 hp bogging down on full throttle

I was hoping to find some help here. I have a 1983 mariner 115 inline 6 outboard. 3 carbs, 6 coils, 2 switchpacks (one of which has already been replaced) This engine was running strong when I got it, although it sat for a few years. I used some carb cleaner to get the fogging oil out of it. I have had this boat on the water three times since I purchased it this past winter. First time was a three and a half hour blast. Second time, it ran great for about three hours and then: I was running it wide open on the way back to the marina and began losing rpm's. A lot of rpm's! It would only give me idle speed at that point. Any more throttle and it bogs down and dies. I took it back to the lake to test everything. The boat took off great and ran for 25 min and did the same thing. Why is this happening to me?!!!):

I am having the same issue with a 2000 125 hp runs great for a while then bogs and shuts off, will restart and idle and run great for about 100 yds at wot then does it again. It idles and runs smooth at low to mid throttle.
So far I have replaced anti siphon valve, fuel hose and primer ball, screen on pick up tube, fuel filter. Going to rebuild fuel pump when kit arrives and start over ! same deal mine sat for three years.

 

[truckinfavis]

Re: 1998 Mercury 40 hp bogging down on full throttle

I had the same problem, then i discovers water in the botom cylinder. check compression it not always a fuel problem. My motor would run fine for a bit then just die down, then hard start. but would run and idle.