1972 50hp johnson running rough at idle/ tune up details

Hi just bought a 1972 johnson 50hp Model 50es72c . When i first got it on the water it took forever to start finally got it going and it seemed to be idleling away nicely. I then put the cowling cover back on after switching it off started it again and it seem to be running a little rougher. I then set off to see how she faired on the water, she seem to run ok at medium to top speed but seemed to be lacking so trolling or idleling. After about 20 mins of running around to engine started hunting (speeding up then slowing down) and eventually stopped. I had to get it back to the dock by opening the warm up lever on the controls to full to get her to start then laboured back to the dock. I have just got into boating the last couple of years but am fairly mechanical. I bought a couple of carby kits online and I am waiting for them to arrive. Does anyone have any ideas regarding this problem if the carby kits don't work? Or any ideas on how to tune up to motor? Any help would be greatly appreciated.

Re: 1972 50hp johnson running rough at idle/ tune up details

Since you just bought the engine, there are a couple of things you should check.
Start with a compression test, should be somewhere around 125 maybe higher, but should be within 10% of each other.
Do a spark test, spark needs to jump a 7/16" air gap.
Make sure there is no water in the lower unit, change the oil and install gaskets on the fill/vent screws if you don't know when it was changed last.
You might want to think about replacing the water pump impeller if you don't know when it was replaced last.
Make sure when you overhaul the carbs that you remove the core plugs on top of the carbs and clean all the passages in there, make sure all passages are clean.
If you are going to maintain the engine yourself, you might think about getting the factory manual, will take you through all procedures.

Re: 1972 50hp johnson running rough at idle/ tune up details

Thnks for your reply. Put it on the water again yesterday had the top cover(cowling) off and the motor ran great but as soon as I put the cover back on it started to run rough and stalled. Seems like its not getting enough air when the cover is on. I looked in the cover and took some old bits of foam off that was hanging loose slighty but this didn't make much difference. We took the boat out for a 20 minutes or so without the cover on and it ran fine. Any suggestions?

I have carby kits, a workshop manual and I seemed to cure the hunting(speeding up and down by using another fuel tank with fresh fuel)

Re: 1972 50hp johnson running rough at idle/ tune up details

dam1981 said:

Thnks for your reply. Put it on the water again yesterday had the top cover(cowling) off and the motor ran great but as soon as I put the cover back on it started to run rough and stalled. Seems like its not getting enough air when the cover is on. I looked in the cover and took some old bits of foam off that was hanging loose slighty but this didn't make much difference. We took the boat out for a 20 minutes or so without the cover on and it ran fine. Any suggestions?

I have carby kits, a workshop manual and I seemed to cure the hunting(speeding up and down by using another fuel tank with fresh fuel)

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Still having problems with my engine. I have been told it could be the gasket seal between the powerhead and lower unit is shot or corroded . Has anybody had this problem before? Any answers Would be greatly appreciated.

Re: 1972 50hp johnson running rough at idle/ tune up details

I've never had it happen to me, but I have heard of this problem happening when there is an exhaust leak into the motor housing space, usually from the exhaust cover gasket or as you say some lower unit seal. There are several threads about this kind of problem, so search around and see what you find.

Rgds