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1996 115 low speed misfire

P

pullendrag1

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"When running this egine, it s

"When running this egine, it seems to have a low speed misfire until around 1200 rpm.After that is runs alot smoother. The carbs have recently been cleaned out, adjusted 1 1/2 turns out.I omhed out the coils, tested good, plugs are new along with all filters . Any suggestions ?"
 
"Hi,u could try a compression

"Hi,u could try a compression check first.that will at least tell you if theyr even,or down one cylinder"
 
"Check all wiring,especially e

"Check all wiring,especially earths.thats my two cents worth.youll need Graham for the rest"
 
"If you are sure it's a mi

"If you are sure it's a misfire and not a "cough or sneeze" being caused by a lean carb condition, see if you can isolate what cylinder is mis-firing (may only be one) - at 1200 rpms I think you will find that cylinder 3 and 4 are not firing at all.

You may be developing what I call a "phantom" ignition problem. If it's not an individual coil just starting to give up it is almost always a bad trigger (that always tests perfect when you put the meter on it).

Although, because of the rpm you note it at there is a slight possiblility that it is a low speed bobbin on the stator starting to go or if this is one of Merc's 2+2 models (and believe that's what you have), that rpm is a little low, but close to the point where the second two cylinders start firing, and that "cross over" can be a little rough."
 
This is a Yamaha 2+2 motor

This is a Yamaha 2+2 motor...you can be sure of that if only the top two carburetors have idle mixture screws and the bottom carbs do not. It is meant to run on just the top two cylinders untill approximately 1800 RPM when the bottom two cylinders finally get enough fuel to fire. Ignition is constant on all cylinders...the bottom cylinders receive only enough fuel for lubrication but not enough to fire until the throttle is opened and the motor hits 1800 RPM. You said the carbs had been rebuilt....there is a procedure called synchronization where the carb linkage is adjusted so the carbs are "synchronized" to open at the correct point. You would need a manual to do this. When adjusted properly this motor should idle reasonably well and the transition from two to four cylinders should also be smooth.
 
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