1967 Evinrude 18hp Fastwin low speed sputtering and missing

I am currently working on a rebuilt Evinrude 18 hp Fastwin and its a 1967 model. It runs like a dream at high speed but spurts and misses terribly at low speed idle in gear and in neutral. I have preset the low speed needle to 3/4 turn per the service manual. It still misses badly no matter what needle setting I use within a reasonable range. I checked the packing gaskets around the needle and they look good so I don't think air is bleeding in, and I assume that the timing cam and roller should be just between the two marks on the cam at idle? Is this correct? It smooths out nicely at medium to high speed but medium speed is too much rpm to shift into gear. If I get the rpm low enough to safely shift it sputters and misses and dies most of the time if I'm not quick enough to increase the throttle. Any advice or thoughts would be much appreciated! Thank you in advance!

The needles might be worn out. When was the last time a carb kit was put on the motor? Just trying to rule this out.

Sometimes you can unplug one plug wire to see if the opposite cylinder will run the engine. Both cylinders should do an equally good job of running by themselves. If the engine runs on the top cylinder but won't run on the bottom one, then you have narrowed the problem down.

Popping or sneezing is from a lean condition. A bad crank seal is most often the culprit. Air leaking into the crankcase at idle(high vacuum)disrupts the fuel air mix for that cylinder. This causes the lean pop or sneeze. You have noted that the problem goes away at medium and full throttle.

The leak(s) don't affect fuel air mix at higher speeds because there is no vacuum in the crankcase...high air flow through the engine. The leaks are tiny compared to the amount of air flowing at higher speeds. The leaks are larger in comparison to the small amount of air going through the engine at idle.

Other air leaks can be from the fuel pump, crankcase parting line, transfer port gaskets, etc.

Another thing that can happen sometimes is the fuel pump will leak fuel into the cylinder it's running off of. That will gag that cylinder with too much fuel. If you can suck air/fuel from the vacuum port on the back of the pump, it's leaking fuel into the crankcase.

You may be on to something. I did as you said and the bottom cylinder is the missing one. Runs ok on the top cylinder only. Where is the crank seal located that you mentioned? What would be the first thing that you would check as the most likely culprit?

Great help here so far guys! Thanks so much I do appreciate all the help I can get!

Another issue is when the fuel tank is nearly empty it seems that the motor will loose significant power. I have about 8 ft of fuel line between the tank and engine. The motor is freshly rebuilt with new rings, carb kit etc. it has about 4 hrs run time. It just has nev run very well at idle.

Were the ignition components changed at rebuild?
Ensure that the points are set correctly, test the coils and condensers.
If you have determined that one cylinder is not firing then the cheapest thing to do would be to start swapping ignition components.
Swap the plugs and see if the opposite cylinder fires. If no pop .... swap the coils , then the condensers, then the points.

I had the same thing just happen with a 53 15hp - top fired, bottom would not fire. I did the swap 'o' rama up to the point of the points. I figured the points were good as they had good continuity when closed. After swapping everything except the points the lower was still not firing. So, I ended up getting an old set of points I had hanging around and surfacing the contacts. I installed the points and cranked the motor. It was then very apparent that both cylinders were firing.

flyingVranch,

ImVho, the Fast-Twin is one of the cheapest/toughest/simplest/longest-lived outboards ever built.
(My "new to me" FDE-12, 1958 Johnson, that I got from an estate sale, is my choice to power my 16ft Lone Star 1958 deep/wide V-bottom "tin" open fisherman. = I expect that it will last longer than I will.)

IF it was my OB, I would "individually test" each & every component in the following list, to assure that each one is fully functional:
(Because a component is "new" or was replaced during the "re-build" does NOT mean that it is OK. = Not long ago, I bought "new coils" for a 1957 Johnson Golden Javelin and one of those new coils was BAD.)
1. points,
2. sparkplugs,
3. sparkplug wires,
4. condensers,
5. coils,
6. fuel pump
and
7. Assure that the carb(s) are properly adjusted & functioning properly.

IF all those components "check out" as GOOD, you MAY have a problem in one or more of the electrical "grounds".

just my opinion, satx

Is there a good spark in both cylinders?

A quick update. I found out that it was mostly too lean on the idle speed needle. I turned it out 1 1/2 turns and got it fine tuned and it runs nearly perfect now. I do think the crank seals are leaking as it dribbles a little oil mixture from under the flywheel. It only misses very seldom now and is running very nicely and good enough for me. Thanks for all of the great help and replies!