TOPIC: 1960 merc 400 missing between 2000 and 3500 rpm

I just got my boat back on the lake after a 2 year down time caused by the impeller being torn up. I replaced it and imediately tore it up again. I have since fixed that issue. Water flows well,
The issue is it idles well, nice and smooth at 500 rpm. It picks up smooth until about 2000 rpm where it starts to miss. It sounds like it is running on three cylinders. at 3500 it smooths out and will jump up to 5200rpm with no issues.If I go from 2000 to 3500 accelerating it runs fine as well. It just will not cruise at mid speeds either idle it or go like hell .

Any work done on distributor / magneto.

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Points / coil / condenser looked at.

Not lately about 6 years ago I went through the magneto and burnished the points and checked the condenser and coil. I am thinking that perhaps the timing advance or carb pickup isn't correct and I am advancing to quickly or to slowly since it runs well at idle and at full speed.

What is 4 cycling and does anyone have a video or audio clip of it? I was reading the manual last night and found a reference to it.The manual say that if the pick up points are not correctly with the magneto advance 4 cycling may occur.

I adjusted the idle mixture some and now It runs fine in the middle but I have no high end power. It will only pull 43-4400 RPM. I am going to go back through the whole tunup process again.

check the carbs, clean passages behind the high speed jets especially

And it certainly won't hurt to go through the magneto again. The points, rotor and cap posts all tend to get a little oxidized when they sit awhile. It's easy enough to drop the mag and check it out...

I pulled the plugs tonight, checked spark using a spark tester showed about 20KV timing light showed all plugs firing, compression showed cylinder 1 #130 2 and 3 at #128 and 4 at #126. Shorting out 1 plug at a time dropped idle speed by about 100 RPM per cylinder.
The plugs were wet including outer plug threads, but inner ceramic was coffee colored. Gap was at .028.

Took her out today and found out 2 things. Atwood gas tanks don't work nearly as well as Mercury tanks, 2 that I am definitely running rich at high speeds.
I ran the Mercury tank the last 2 times out but it was empty this time so I switched to the Atwood tank(fresh gas last weekend) with it fully opened after about 10 min of running at 4k Rpm she died but just before she died the engine started running great it was like I had just gave HER full throttle for a hole shot. She had that nice crisp 2 cycle song. I restarted her and she ran about 10 more min at high cruise. I have already drilled out the vent twice on this tank I guess I still havent drilled it out enough.
Next question were the champion j6j plugs extended reach or short threads?I crossed them to Autolight 3014 preferred or 303's well the autolite 3014 were nla so they crossed to an Autolite 66 which are entended reach. The 303s are short reach. The 303's are what I was running/

short reach plugs.

thanks I will use the 303's

thanks I will use the 303's