Mercury 50 hp outboard, runs at half speed

[whmstang99]

My Mercury runs at half speed for about 20 minutes, then it takes of like a bat out of hell and will run fine for the rest of the day, next day same thing.
Any ideas

 

[racerone]

Three cylinder or 4 cylinder ?---serial # helps too.

 

[whmstang99]

Sorry for delay, 4 cyl, serial is 6224280, beleive it is a 1982 50 hp Merc, and thanks, hope you have some advise. I think I am also fowling my plugs, took hem out today, and they were black, put new ones in and it ran nice and then started poping and coughung and would not holdd and idel.I can tell when it won't run at full speed when it doesn't hold an idle, because when it does warm up and take of at full speed, it goes back to holding an idle

 

[racerone]

Many folks want to throw money and parts at a problem motor.---But take these steps and answer questions.--Did this motor sit for a length of time with fuel left in the carburetors ?-----Did this problem start suddenly ?---Does the primer bulb go hard when you operate it to start a cold motor.----Have you done a compression test ?----Test the spark as it must jump a gap of 3/8" on all 4 cylinders.---Answering with " I have good spark " will not do.

 

[hotrod53]

You may also only be running on 1 carb. These 4 cylinder Mercs will run.. although not well, on 1 carb. the fact that it eventually runs "like a bat out of hell" has me leaning towards electrical though. You could also have a bad coil and your dragging a cylinder until the coil decides to fire.

IMO.. check spark first, then check/clean carbs if you have spark just to insure they're both working.

 

[whmstang99]

I all, been off for a while. Thanks for all the feedback. So I rebuilt the carbs, and they came out great. I was real happy, started right up, held a smooth idle no skipping. Got her on the water, started right up, ran smooth at idle, opened it up and half speed. Ran it for 1/2 hour she never took off. Runs smooth holds the idle but half speed, so I am thinking electrical . How do I test a stat, a coil, and a rectifier. Will a regular volt ohm meter do it? I did a compression test and I an running at 110 in my top three cylinders but at 90 in my forth, some folks told be that this shouldn't be causing the problem like I have. I didn't change the main gas line and ball coming in any help would be great

 

[Redwing4900]

Does the motor sound like its running right? Maybe its a prop that's been spun? I had that similar problem replaced the prop and it ran great. Prop was pricey though at 130 bucks at west marine. Just throwing out a simple solution that worked for me. How's it going?