Three years after I bought my '92 Merc 40 4cyl 2stroke, it refused to idle but ran OK at cruise speed.
Local dealer mechanic diagnosed the problem as sticky needle valves in the carb bowls. He took off both carbs, cleaned them, re-installed. Result: boogered every fitting on the carbs (pliers + butter knife); stuck one of the floats; no tie wraps on fuel lines; stripped 2 of the 4 starter motor bolts; engine still did not idle. Solution: advance the timing 10 degrees and set the idle speed up to 1700 rpms.
I finally figured out the problem myself: both relief exhaust ports (those little holes just below the power head) were clogged with carbon. Solution: bought 2 carb rebuild kits to undo what the mechanic did; fixed stuck float; added fuel line tie wraps; 2 longer bolts for the starter; reset the timing back to spec; reset the idle back to spec; dropped the lower unit and scraped out a huge volume of carbon stalagtites (2 cups of carbon chunks); cleaned similar carbon buildup in the exhaust manifold. After that, re-adjusted idle mixture screws under load.
My bad anyway - since the motor was on a pontoon, I cruised at 2000 rpms; not enough to keep it clean; 3000 rpms and no issues.
Local dealer mechanic diagnosed the problem as sticky needle valves in the carb bowls. He took off both carbs, cleaned them, re-installed. Result: boogered every fitting on the carbs (pliers + butter knife); stuck one of the floats; no tie wraps on fuel lines; stripped 2 of the 4 starter motor bolts; engine still did not idle. Solution: advance the timing 10 degrees and set the idle speed up to 1700 rpms.
I finally figured out the problem myself: both relief exhaust ports (those little holes just below the power head) were clogged with carbon. Solution: bought 2 carb rebuild kits to undo what the mechanic did; fixed stuck float; added fuel line tie wraps; 2 longer bolts for the starter; reset the timing back to spec; reset the idle back to spec; dropped the lower unit and scraped out a huge volume of carbon stalagtites (2 cups of carbon chunks); cleaned similar carbon buildup in the exhaust manifold. After that, re-adjusted idle mixture screws under load.
My bad anyway - since the motor was on a pontoon, I cruised at 2000 rpms; not enough to keep it clean; 3000 rpms and no issues.
