HELP!! Merc 25hp losing spark

[will941s]

Alright merc experts, I need your help BAD! I have a merc 25, 2 stroke tiller, 1991 year model. Bought it as a dead motor, pretty much everything was bad. Powerhead has all new gaskets, 125 PSI compression now. I rebuilt the carb, new stator, new trigger, new switchbox, new starter, t stat and waterpump. Here's no issue.

1st time ran it, it took off like a scolded dog. About 5 min into it it shut off. Started right back up, hit about 75% throttle and sounded like it was missing. Been that way sense. From idle to 75% it's great, past at it misses.

Got a new gas tank and fuel lines. Re checked carb and float, rechecked sync on the motor. Ran it again today with some new coils, same issue. Changed switch boxes, same issues. Here's what I've found. Idle is set at 2* like it should, but when you throttle up, round about 19 or 20 * timing, #1 plug stops firing. It's supposed to set at 25 * timing WOT but can't get there....right before it hits WOT it stops firing #1 coil. #2 will fire all the way through. Any ideas on what's going on here? Have y'all seen this before? Thanks in advance

 

[will941s]

Bump

 

[will941s]

Anyone? Looking for ideas on this, I would be hesitant to say the stator is crapping out, but maybe it's possible when it gains more voltage with the higher RPM's. Or could the trigger be phasing out on the #1 side?

 

[will941s]

Update: Went through the sync procedures again, it'll hit 25* all day at WOT without the carb synced in the system yet. Once the carb linkage is hooked up and added to the equation (able to gain RPM's) then the #1 plug loses fire toward WOT. Swapped coils, swapped spark plugs, put another coil on #1....no change. #2 never loses fire. Any idea on what could be causing this? Tried two different switch boxes too

 

[CharlieB]

Look for movement in the kill circuit wire. If tiller model inside the handle at throttle twist may be making just the slightest ground.

An easy test, disconnect the kill circuit at the ign module and test run. If it runs perfect you can be pretty sure you are on the right track.

 

[will941s]

CharlieB, thanks a lot for the reply. I did disconnect the kill circuit from the system with no change. Im sure this motor doesn't have a rev limiter built in, it's beyond me. Perplexing of why it'll hit WOT timing without the carb opening, but losing spark 5* shy when the carb is in play. It almost HAS to do with the increase in RPM's what what would do it?

 

[will941s]

Another update: Auxilery stator, that charges the battery, found the zip tie that holds the wire to it is melted and the the leads coming out look melted also. It's still producing voltage, but being that those wires leading down to the rectifier follow right on top of the trigger wires, could that be affecting the trigger operation at higher RPM's?

 

[will941s]

Solved....! My theory on the whole issue was it was trigger issue, trigger "phasing out" at higher RPM's. Wrong! Installed a new trigger...same problems. So....bit the bullet and replaced my brand new Sierra brand stator with a CDI brand. Problem solved! So the yellow and black wires are the "low" windings, black and white are the "high speed" windings. When it was getting up in RPM's it was crapping out causing the switch box to crap out. Thanks Sierra for producing a part that performed flawlessly for about 5 min before breaking.

 

[CharlieB]

I have ALWAYS preferred CDI parts. Met one of the owners many many years ago at a seminar. Great Guy!

 

[will941s]

Agreed, I should have went with them from the get go. The only thing to be cautious with regarding CDI stator on a merc 25, is the output wires exit to the right, then have to be immediately be looped back to the left under the windings to go to the switch box. They use a head shrink shield on the first 1" of wire and thats where you have to bend it. Feels like your going to break the wires loose but has to be done or the flywheel will cut the outputs. Otherwise, great products