70 HP Mercury 700 2 Stroke runs rough with cowl installed

[pdeal]

I have a 70hp (3 cyl, 2 stroke) Mercury 700, serial number 4830257. You guys helped me a bunch last year getting it running. Last year I got the timing set pretty well. Also last year I rebuilt the fuel pump and carbs.

We've had it out a few times and it sometimes runs good and sometimes runs rough. Also on the water sometimes it's hard to restart. I have been messing with it today and I noticed it seems with the cowl off it runs nice and smooth and starts really easily. When I put the cowl on it still idles ok but starts to miss when I rev it some with the warm up lever. For this testing I'm Running it in a barrel of water only in neutral.

I read some on this that it can be an exhaust leak? I can't really see where this could be. I suppose it could be rebreathing some of the exhaust fumes since it's just sitting there not moving but I did have similar issues on the lake. Or could it be a slight mixture change when the cowl is installed? When I'm reving it in the barrel, trying not to go too fast, I'm pretty sure the throttle isn't even to the cam to open the carbs up yet.

Anybody got an idea where to look?

 

[pdeal]

I think maybe I figured out the issue. There is a little hole in the bottom of the block just below the exhaust cover (cover on the opposite side of the engine from the ignition stuff). This hole has been dripping oil. We figured it was some sort of drain or something. We didn't think it was a mounting bolt hole because it's smaller than the other bolts in this same flange. In looking for an exhaust leak with the engine running, cowl off, I put my finger under this hole and got burned from the hot gasses coming out of it. It was pretty caked up but I tried to put a 1/4-20 bolt in the hole and it went in and tightened up. Anyway, once I put this in, put the cowl on, and ran it some it does seem to run better.

The engine could also use a new set of spark plug wires so I will get some of them on the way. They might also be contributing to rough running I suppose.

And, by the way, it appears that this engine is a 1978.

 

[Texasmark]

Glad you figured it out. Interesting problem but "exhaust gas recirculation" does reduce combustible volume in the cylinders. Question is, what and why the hole.

 

[pdeal]

Well my best guess is that it's a bolt hole with the bolt missing. This is an old engine that's been monkeyed with a lot over the years so this is very possible. I saw the hole earlier and I thought it was a weep hole or something. But when I discovered it was shooting out exhaust gasses and given the issues I was having it seemed clear that it ought to be plugged. I've since had the boat out and it does run much better. I did discover that I have a float that's not sealing along the way so I now have new float kits on order.