1994 Mercury 75 ignition issues

[bacchus99]

I wasn't getting fire to cylinder 1 and sometimes 2. Did a DVA check and 0 volts to #1 and #2. #3 had correct voltage. Ran thru all DVA checks and symptoms pointed back to switchbox. Installed a new Sierra box. DVA checks out now and we have good voltage to all 3 cylinders and spark jumps a 7/16 gap on all cylinders! Yaay it's fixed right? Well not so fast. Put it in the water and she runs pretty good except seems to be idling slightly rough, lacking power to get up on a plane fast(altho it will plane) and at WOT i noticed just slight surging kinda(like its running fine then just kinda backs off then up). Almost like its losing power not really RPMS if that makes sense. Compression is 128psi every cylinder. 99.9% sure fuel system is clean clean. Went thru gas tank, new lines and bulb, filters, rebuilt fuel pump and cleaned carbs twice with fine wire thru holes and compressed air and carb cleaner...really feel like fuel system is good to go.

Any ideas on the symptoms its doing now? Trigger ohms test ok. It has the red stator with the little adapter module that is connected between the stator and switch box. The white/green and green/white stator wires test ok ohms. The yellows test slightly high on ohms at .5 instead of the recommended 0.165 - 0.181. This motor has a separate voltage reg and rectifier block. Tach does jump around some.

Man i need some help lol. Thanks in advance.

 

[bacchus99]

I'll add that coil dva voltage is erratic when idling on the muffs...dva meter just bounces around all crazy on all 3 cylinders...not sure if that is a clue. Voltage is perfect when spark plug wires are pulled off plugs and just measuring dva voltage at cranking speeds.

 

[bacchus99]

I'll add i don't think this motor has a rev limiter. I'll have to remove the bottom cowl to be 100%...that would actually make the most sense but i don't see one and in the merc documentation it looks like the non oil injected version has it and the oil injection version does not. Mine is oil injected.

I talked with merc customer service and he said the ignition system seems happy given the dva readings at coil are good at cranking speeds. Not sure why it seems to runs kinda weak and bleed power/surge at WOT. We verified the rectifier terminal block and voltage reg have nothing to do with the ignition system only the charging system...so if either one of those were bad it shouldn't affect ignition anyway. Maybe the sierra box isn't that great? Gets pretty good reviews on the web. I do have some new sierra coils i can throw in there just to try.

I'm at a loss to what to try next besides bring it in. I have put countless hours into this motor and want to fix it myself! Any help/ideas of something to look at or try would be greatly appreciated! Thanks!

 

[bacchus99]

Gonna throw a new rectifier in there and see what happens. I know that really isn't quite ignition related but i've read it has solved some issues somehow. Cheap enough try...plus it should fix the tach so that's a bonus.