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I’m working on a well-used, “darkside” (Mercury) for a neighbor that hasn’t run for many years and I have a couple of carb questions. The serial number is 5757744, and I can’t find that on any lookup sites. I have no idea what the year is. When I go online to marineengine, I found the exploded views by putting in SN 5404657 and up which I found as a possible beginning of a range in my old (2004) Sierra catalog. Exploded view below. After careful disassembly and cleaning, I learned how it works (much different than most other carbs); and cleaned it out good. Two questions:
1. Where should the float level be at? and how is it measured? It was so far off it was not closing the inlet valve.
2. The manual choke assembly, (parts 23 through 28) which is a spring-loaded brass piston that moves to block or allow a gas passage in the diagram below, is sticking in the choke OFF position and difficult to break loose. I think part 23 is hanging up on part 25, or else the piston on the end of part 23 is hanging up in the upper carb housing. I have put light oil on these parts but that didn’t help much. The piston and housing appear very smooth, as does the portions of part 23 that I can see. To remove part 23 from part 25, I think I have to take off part 27 to get at part 26 which I think holds 23 captive in 25. Can I pry off 27 and then disassemble? I’m hesitant to put too much force on part 27 because I don’t want to break it. Hopefully we have a Mercury expert here that has run across this problem before and can put me on the right track! Thank you!
Dave
I did some searching online for the carb and learned that Mikuni is a big time carb maker, making carbs for a wide range of ATVs, motorcycles, and a bunch of small engines, like Tecumseh. I’ve found about 100 of their carbs for sale online, and some are like the one on this 3.6 Mercury, but none with that goofy piston choke. Unless I get more tips by tomorrow, I’m going to carefully pry part #27 off #25 and see if I can resurface parts so they slide without binding up.
What is the gas/oil mix for these motors? The current owner has never run it and doesn’t know. If I don’t hear otherwise, I’ll tell him 24 to 1 to be on the safe side.
DaveI finally found my service manual. (90-43183)
Setting the float level is indeed odd: The trick is, you hold the carb upside down and hold the float so that it just touches the float pin. You don’t let it collapse the float pin. While holding just touching, the height from the bowl flange to the bottom of the round float should be 7/8″ to 31/32″.
Although the manual shows the choke assembly disassembled, it does not have specific wording on how to do it. #27 is a “rubber cap” and under that, is #26, the retaining clip. Prying off the rubber cap seems a safe bet.
I have no info on fuel ratio but I bet it was 50:1. I’d be real happy and confident with 24:1.
There weren’t many of them around in my area. Those like yours are technically not “Gnats,” which were named so only the first year and were TWO cylinder, 4 hp. They pretty much stunk (The Mercury tech rep called them “f—ing Guh-Nants.” but they are cool, in that the real Gnats were the last Fond Du Lac produced, American portable motors. When they started with the Mikuni carbs and everything else, it started getting vague about their origin. Be very diligent about the gear oil. They did tend to lunch gears if they lost any of the small amount of gear oil they had. I would check it after every use; but that’s just me.
Long live American manufacturing!
The way they want you to adjust it is strange but that’s exactly what the manual said to do.
The power head appears to be based on the old school, Mercury single cylinder power head that goes back to the 1960s 3.9 hp. When right, those actually didn’t run too bad for a one lunger. You may be surprised….
Funny thing about the two-cylinder “Gnat” type engines: They had an integral fuel tank, like yours. They would run out of gas with half a tank of fuel left, because the bottom of the gas tank was lower than the carb inlet. Can’t make this stuff up. If you bought one that had the optional fuel pump and remote tank connector, then that wouldn’t happen. I don’t know what the tank/carb level relationship is on yours; but be advised that it might be possible to run out of gas unless the tank is quite full…..
Long live American manufacturing!
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