Clean the carb
- You could likely battle various symptoms for a long time, but I think it best to remove and clean the carb so that you know it is OK. A carb that hasn't been cleaned in years will have a layer of varnish deposits on all fuel surfaces. In cold climates there may be significant varnish/sludge buildup. This varnish will slowly diminish performance over the years by reducing the size of the carbs jets, nozzles and passageways. The Atomic 4 carb is an easy style to haul apart for cleaning. Most shops will be glad do this for a fee, but after you've done it once yourself you'll forever be outraged at what they charge. There are many things that can affect a carburetor's performance, but usually it is a combination of several minor problems. I always start with a complete disassembly and cleaning so that I know I am starting with a good carb.
- There are two types of spray carb cleaner. One is for spraying into a warm running engine and the other is for cleaning disassembled carb parts. The first type has very limited benefits. The second type is great for breaking down crud and varnish.
- When you sprayed into the dirty intake screen (backfire suppressor) it may have sent a lot of crud into the carb. There is a small passage in front of the choke butterfly that leads to the float bowl. It is used to equalize the air pressure inside the float bowl with the air pressure at the carb intake. This keeps a vacuum from forming in the float bowl. By spraying carb cleaner into the intake it is possible to force dirt through the passageway and into the float bowl, but highly unlikely that it could cause the float to stick open.
- Spraying into a dirty intake screen could cause crud to pool in the bottom of the carb. In the bottom of the carb body there is a scavenge tube that sucks any overflow of gas up and into the intake manifold. If your engine had been slightly overflowing gas for some time it would have been sucked up and burned in the combustion chambers. This is one possible cause for the soot as it would be quite rich. Dirt may now be plugging the scavenge tube and not allowing it to suck up the extra fuel which could sit in the bottom of the carb body. When running it will try to draw it in with the air flow but after shutting off the engine any remaining fuel could leak out.
- The fuel/air mixture is delivered by jets & nozzles. Fuel flows through different jets and nozzles depending on engine speed and loads. However, if one jet becomes blocked the vacuum will draw fuel via another jet. If the small idle jet is blocked the engine will draw the extra fuel it needs via the larger main jet. This will soot things up as it will be a rich mixture. Drawing idle speed fuel through the main jet could also cause slight amounts of fuel to pool in the carb body.
- Fuel can leak past the float valve and overflow the float bowl. Usually this is caused by a bit of dirt holding the valve open. This valve seat and valve may have a coating of varnish deposited over the years by aging gas. If a bit of this varnish flakes off it will leave a small gap that gas can seep past. Once disassembled, the float valve the seat is best cleaned with a small pointed wooden pin. Spin it between thumb and forefinger while pushing it into the float valve seat. This will burnish the seat smooth and wear away the fine layer of varnish.
- Dirt can enter the float valve from any point in the fuel delivery system. Your Racor water separating fuel filter is an excellent choice between the tank and pump. Also install a small polishing filter between the pump and carb. My polishing filter is a small metal disposable in-line automotive type. When reinstalling a cleaned carb always replace the polishing filter and the short fuel hose between it and the carb.
- The static adjustment of the idle mixture is one full turn out from being seated. After the engine has warmed up slow it down to idle and slowly turn the mixyture screw counter-clockwise until the revs start to drop. Then turn it back in until the revs peak.
- Improper ignition timing can also cause the engine to soot up and lose power. A distributor overhaul is relatively straight forward but at a minimum you should check that the timing is correctly set. Warm the engine up and, with good dock lines, place it in gear at high revs. Loosen the distributor and rotate it slightly back and forth looking for the highest revs possible.
Good luck with your Atomic 4. Hopefully after a few tweaks she'll give you another six trouble free years.
Dana, S/V Kiskadee