1983 Merc 50hp is chugging after a minute of running.

[gsh]

I put the boat away last fall and it was running great. This year it started up just fine, but after about 30 seconds it slows down and the engine struggles.

I can slow down to an idle and go 5-6 mph all day long, but anything over that eventually leads to the motor slowing and struggling.

I tried giving it some chose and it ran worse. I checked the filters and the lines, they're all good. I checked the bulb and it was firm. The fuel pump was rebuilt in 2021.

I just can't seem to run it very long without it acting like it's flooding.

 

[jimmbo]

There are 2 carbs on that engine, each feeding 2 Cylinders. If one isn't feeding gas, the engine will slow down and won't be running smooth, but that would actually be a very Minor Concern. The Oil needed to Lube Engine is mixed with the Gas, so no Fuel means no Oil...
What do the Plugs look like?
While you have them out, do a Compression Test.
Is there a Strong Spark on each Plug Wire. By Strong, I mean, can it jump a 3/8" Gap?
When you winterized it last Fall, when it was running great, did you put any Fuel Stabilizer in the Tank during the last running of it?

 

[gsh]

There are 2 carbs on that engine, each feeding 2 Cylinders. If one isn't feeding gas, the engine will slow down and won't be running smooth, but that would actually be a very Minor Concern. The Oil needed to Lube Engine is mixed with the Gas, so no Fuel means no Oil...
What do the Plugs look like?
While you have them out, do a Compression Test.
Is there a Strong Spark on each Plug Wire. By Strong, I mean, can it jump a 3/8" Gap?
When you winterized it last Fall, when it was running great, did you put any Fuel Stabilizer in the Tank during the last running of it?

I forgot to stabilize the fuel before putting it away, but I just got done rebuilding the carbs. It looks like they had never been touched since 1983 with all the original gaskets. It wasn't too bad, but there was a some black buildup in the idle adjustment screw.

During the rather complicated process of removing the twin carbs, I discovered that one of the screws holding my fuel pump together was really loose.

I just got it back together and it runs OK on the muffs in my driveway, if a little rough. My next step is to check the plugs. It runs rough enough that setting the idle is difficult because it's hard to tell when the engine starts to bog down because it's too rich.

 

[racerone]

They all run OK on a hose in the driveway.-----Nobody can see or touch your motor.-----These motors are easy to diagnose.----If you answer the questions in post #2 you can save yourself a lot of time / $$$ / aggravation.-----Was water pump impeller replaced about every 4 or 5 years?---

 

[jimmbo]

If it runs a Little Rough, on the Muffs, it will run a whole lot rougher once it is in the water and there is some Back Pressure on the exhaust

 

[airshot]

Did you do a proper sync on the carbs ??

 

[gsh]

They all run OK on a hose in the driveway.-----Nobody can see or touch your motor.-----These motors are easy to diagnose.----If you answer the questions in post #2 you can save yourself a lot of time / $$$ / aggravation.-----Was water pump impeller replaced about every 4 or 5 years?---

Impeller was replaced 2 years ago and it spits water just fine.

The plugs were a little oily, but seemed OK. I'm thinking it was just me adjusting the carbs and they ran a little rich for a few minutes before I shut the engine off.
I have a spark tester, but can't seem to find it in the mess of my garage.
Compression checked out fine 2 years ago when I had it worked on by a mechanic, but I haven't checked it since.

 

[gsh]

Did you do a proper sync on the carbs ??

According to the manual, I did.

I plan on taking it to a the local shop and have him check out the sync job I did just in case. He said he could do it in a few minutes for next to no cost.

I wanted to take it out to the lake today, but the wind is 20 mph with 25 mph gusts. not a good day for being out.

 

[gsh]

Well, I got it to the lake today so I could do a proper test under load.
I backed away from the dock and ran it for a few minutes barely over idle to let it warm up.

I then slowly increased power until I was at WOT. Everything ran well and the engine sounded smooth. During the initial acceleration it coughed a couple times, but that was it.

I then ran it around the little lake 3 times for a total of roughly 4 miles and that took me about 9.5 minutes. Never a cough or sputter. I then shut it down and let it sit for a minute. It restarted easily and easily got back to full power.

I also noticed that the idle was a LOT smoother.

I think it was a combination of actual resistance on the engine and the ability to open it up that cleared the rough idle problem up.

However, it definitely seems that the original main issue with the chugging was fouled carbs.