Help: 1996 mariner/mercury 100 stumbling

[vucelick]

The motor stumbles at 3/4 throttle and above and will not pull. It runs fine below full throttle. New fuel filters and clean gas. New plugs. Still no difference.

 

[orca]

Re: Help: 1996 mariner/mercury 100 stumbling

G,Day. I suspect that you have an ignition problem. Fit a timing light and check the spark an all cylinders. then do the same under load that is on the water at high speed.

 

[vucelick]

Re: Help: 1996 mariner/mercury 100 stumbling

I checked and all the plugs are getting spark, the motor kind of "loads up" at full throttle, the second you close the throttle a bit it instantly clears up and takes off and runs smoothly and up to about 37 mph, and it will run near the engines redline, but not if you push the throttle all the way open. I have looked at the carbs with the cover off while running the boat and it appears to start loading up/stumbling at about 3/4 throttle and above.

 

[vucelick]

Re: Help: 1996 mariner/mercury 100 stumbling

any more suggestions would be appreciated....

 

[BKool68]

Re: Help: 1996 mariner/mercury 100 stumbling

What RPM's are you turning? You might be trying to over rev the motor and a rev limiter is kicking in. If that is the case you may need to just adjust your throttle stop, or go up in your prop pitch.

 

[Solittle]

Re: Help: 1996 mariner/mercury 100 stumbling

Well if you are getting spark the other likely place to look is fuel. If the engine has not been run in a while you may have some crud in the carb jets. If so its rebuild time and I would also purge all old fuel from anywhere.

 

[vucelick]

Re: Help: 1996 mariner/mercury 100 stumbling

Like I said I have drained all of the old fuel, from every where including the fuel line, tank, and changed the filters, I can turn redline without full throttle which is 5250. I saw a bit of an over-rev condition and put a 23 prop on it and it did not seem to change anything, the problem is that at no time except for the first second when coming out of the hole can I use full throttle, I can feel it make alot of power in that second, but it very quickly starts to fall off until I back the throttle down to about 3/4 throttle setting, the boat is an 18.5' flat bottom I have not cleaned out the carbs and have been running the boat for the past several months, it had sat since the previous november until I bought it in April.