1985 Evinrude 40 hp tiller starts fine, runs about 30-45 seconds then misses on one

[Leefy]

Just bought this boat and motor. 1985 Evinrude 40 hp tiller. Starts great and idles fine. Take off out of the hole and it is fine but after about 100 yards you can feel a jerk and then a slow down and can tell it is missing on one cylinder. Replaced the spark plugs but is the exact same way with new plugs. Boat has been stored for the past 9 years but the owner claims he has started it every year. I have fresh gas mixed 50-1 as per this forum recommendation even though it has a tag on the engine stating 100-1. I added about 4 ozs of sea foam gas additive but have not run it enough to see if that has anything to do with it. The owner claimed is was running fine when he quit using it. He is into flying airplanes now and I don't believe he was jerking me around.

Once it starts to miss you can tell is is running on only one cylinder even at idle. Won't come out of the hole etc.

Any ideas?

Leefy

[Daviet]

Have you checked the spark when it acts up, you can use a timing light to check it real quick when it acts up. What about pumping the primer bulb when it acts up, does it help?

[Leefy]

Haven't tried anything yet but will give both a try. Thanks

[Daviet]

Let us know what you find.

[Leefy]

One thing I discovered is the fuel tank has pressure in it. Maybe it should but don't know. When I unscrew cap it really blows out. Don't see any vent. Is metal tank I assume is original.

[Leefy]

Took it into a dealer and they determined the carbs were gummed up. Cleaned them and now it runs great top end. They said it didn't idle very well so changed the jets from .50 to .35 and thought it was better. I took it to the lake tonight and it doesn't idle for beans. It dies if I try to idle too slow. Don't get to the end of the throttle and it dies but is really going slow at that point. They said to run some sea foam and it might help. Any thoughts on what else to do. I did screw the linkage stop on the side of the motor assuming it is to control low idle speed and will see if that helps but I don't have muffs so have to wait for the lake. The screw didn't even make contact with anything so I turned it in three turns and now when you back off the throttle it at least comes against the screw.