Questions on a 1990 Evinrude 100hp

Stanley.Wilson

Recruit
Joined
Jun 24, 2012
Messages
4
Ok guys, I have two questions. First questioin is, I just bought a boat from a man and the engine ran great when I test ran it and the first few time once I got it home but now it wants to miss when it is 1200 or less rpm. It runs smooth at the higher rpms but when you get it in that lower rage it misses. It actually misses pretty hard at idle and it will kill it. What is happening here?
Second question is, I had a 1978 evinrude 115hp before this boat and I had studied up on switching it to a 140hp. That motor sounded pretty easy just swapping carbs and the exhaust houseing. What would i have to do to this motor to make it a 115hp or 140hp?
 

emdsapmgr

Supreme Mariner
Joined
Dec 9, 2005
Messages
11,551
Re: Questions on a 1990 Evinrude 100hp

If the engine does not run well at dead idle, but seems to miss, that could be a "lean sneeze." That's probably a carb problem, caused by a lean condition inside the carb. Residual dried fuel tends to restrict fuel flow in those small, dedicated idle passages. That manifests itself in what seems to be is a miss. Once you crack the throttle just a little, other jets take over and it probably runs just fine at other rpm's. Only way to correct that is to disassemble the carbs and clean these passages with some type of carb cleaner- like a Gumout product. Converting a 115 to a 140 is not just a bolt-on proposition. The 140 heads are special high compression ones-different and NLA, so they are hard to find in salvage yards. The 140 block is likely ported differently from all the others. A marine machinist may be able to make this change once the block is torn down. In addition, you'll have to add the exhaust filler blocks in the exhaust chest-not sure if your block has the pre-drilled holes for the filler blocks or not. Once those blocks are installed, you will need to have a machinist mill (deck) the outer face of the exhaust chest to be sure it is completely flat-to mate to the special 140 exhaust manifolds. Yes, you will need the larger carbs. Your 115 should already have the special intake filler blocks.
 
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