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Mercury 35HP - runs rough at lower speeds

R

rstier

Member
Hi,
My 1988 35hp Mercury outboard is running crappy at lower speeds. For the most part, it starts and idles ok, with an occasional stall when switching from neutral to forward. A lower speeds, it runs pretty rough, often sputtering/hesitating. At higher speeds/rpms, it runs pretty smooth. I wanted to get some opinions on where to start. Does this sound like a fuel delivery problem or possibly the timing? The plugs are new and the gas is fresh. It's been this way for a few years and doesn't seem to get worse or better. Any advice would be much appreciated!
Thanks,
Ron
 
Could be a few things but I would start by tearing the carb down and giving it a good cleaning and throw a kit in as well.

Other issues will cause what you note (bad crank seals, chipped/broken reed, bad base gasket etc etc etc) and being an 88 model particularly the crank seals could be on their way out (20-25 years seems to be their lifespan before getting worn).

From your symptoms I would "suspect" that it's fuel and not ignition - so either not getting enough fuel (when under load) until you really open it up OR sucking too much air through a bad seal/gasket.
 
Thanks, this is helpful! I'll go through the list and see what I can find. The crank seals/base gasket don't sound like fun. What's the best way to check those? I guess with the base gasket, I could spray Gumout or Starter Fluid at it to see if it gets sucked in and the RPMs increase.
Thanks!

galamb said:
Could be a few things but I would start by tearing the carb down and giving it a good cleaning and throw a kit in as well.

Other issues will cause what you note (bad crank seals, chipped/broken reed, bad base gasket etc etc etc) and being an 88 model particularly the crank seals could be on their way out (20-25 years seems to be their lifespan before getting worn).

From your symptoms I would "suspect" that it's fuel and not ignition - so either not getting enough fuel (when under load) until you really open it up OR sucking too much air through a bad seal/gasket.
Click to expand...
 
Often with a base gasket you experience a "sneeze" from the motor (but not always).

The upper or lower crank seals are kinda like a little metal cup or doughnut with a rubber top holding a small circular spring in place (think of a drum, like rock band drum, with hole in the middle - the spring keeps the rubber snugged up to the crankshaft). Anyhow, they do eventually wear and the rubber tears away. When the piston comes down forcing the air/fuel/oil into the cylinder, if the seal is compromised, some of it gets "blown out" from the bad seal.

The gas evaporates and leaves the oil behind. So the tell-tale signs of this can be a gooey mess in the lower cowl (bottom seal) or the appearance that the powerhead has been sprayed with WD-40 (or the like), particularly immediately under the flywheel.

The upper seal is somewhat less work to replace, the lower seal involves pulling the powerhead itself - doable at home, but a fair bit more work.

Other gaskets that could be the issue include the pair of cover gaskets - one at the intake cover (which is on the right side of the powerhead, back near the cylinders if you are behind the motor) and the other on the exhaust cover (left side - same perspective). These would be "less common" but if other things don't pan out something to investigate.

Likewise the reeds on this are "internal" to the engine block and quite the chore to get at.

But I would look at the carb first. I (believe), if you have the year correct that it should be one of the older Tillotson sidebowl carbs. While I am not aware of an entire kit for them still being available from Merc (been out of production for a long time now), a number of the parts, particularly anything you would typically need, is still available and from your description of what it's doing I would suspect some "crud in the carb".
 
Great explanation, thanks for taking the time to help!

galamb said:
Often with a base gasket you experience a "sneeze" from the motor (but not always).

The upper or lower crank seals are kinda like a little metal cup or doughnut with a rubber top holding a small circular spring in place (think of a drum, like rock band drum, with hole in the middle - the spring keeps the rubber snugged up to the crankshaft). Anyhow, they do eventually wear and the rubber tears away. When the piston comes down forcing the air/fuel/oil into the cylinder, if the seal is compromised, some of it gets "blown out" from the bad seal.

The gas evaporates and leaves the oil behind. So the tell-tale signs of this can be a gooey mess in the lower cowl (bottom seal) or the appearance that the powerhead has been sprayed with WD-40 (or the like), particularly immediately under the flywheel.

The upper seal is somewhat less work to replace, the lower seal involves pulling the powerhead itself - doable at home, but a fair bit more work.

Other gaskets that could be the issue include the pair of cover gaskets - one at the intake cover (which is on the right side of the powerhead, back near the cylinders if you are behind the motor) and the other on the exhaust cover (left side - same perspective). These would be "less common" but if other things don't pan out something to investigate.

Likewise the reeds on this are "internal" to the engine block and quite the chore to get at.

But I would look at the carb first. I (believe), if you have the year correct that it should be one of the older Tillotson sidebowl carbs. While I am not aware of an entire kit for them still being available from Merc (been out of production for a long time now), a number of the parts, particularly anything you would typically need, is still available and from your description of what it's doing I would suspect some "crud in the carb".
Click to expand...
 
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