91 merc 40 hp cutting out at wot

I have 91 merc 40 hp 2 stroke 4 cylinder,was rebuilt due to over heating...stock pistons and rings with a good honning,new switch box,fuel pump and water pump kit & triggar coil,cleaned carbs and new plugs...5 tanks of mixed fuel with oil injection hooked up for break in...I have disconnected the oil injection and am mixing my fuel 50;1...compession is 125/120/125/120...engine is cutting out and wot...don't notice the cut out at low speed...scan tool put on to measure the rpm and it is all over the map.The engine is cutting out sharply and intermitey..Spark test is strong jumping 1/2 inch,plugs are clean. I don't know what to change at this point. stator or voltage regulator is suggested but no one knows for sure or how to test for that matter..this money pit may find itself at the bottom of the lake ...anyone run into this problem ? or have any suggestions/
thanks

Re: 91 merc 40 hp cutting out at wot

If runnung of at low speed and cut out at WOT . sounds like it may have fuel issues, not enough at WOT

Re: 91 merc 40 hp cutting out at wot

I just rebuilt the fuel pump and have not tryed it out...I will be on the water this weekend...thanks for responding

Re: 91 merc 40 hp cutting out at wot

What kind of scan tool are you talking about you attached????? Rapid shutting on/off could be a bad ground, coil/s not grounded,loose wire on switchbox or a bad rectifier

Re: 91 merc 40 hp cutting out at wot

mac tools em170...good thinking i will check that out...when you refer to rectifier ..is that the same as voltage regulator ?

Re: 91 merc 40 hp cutting out at wot

It is also possible to have a broken, yet abutted wire, on one of the triggers. Disconnect the throttle cable, remove spark plugs and ground all wires or pull the lanyard to kill the ignition switchbox. Move the throttle lever throughout its range while cranking/testing each trigger to see if there is any variation as the wires are moved.

You will need to conduct running DVA tests of the ignition system, triggers, stator, switchbox output to coils, to isolate any possible component problem. Test each item at speed. Then again test each coil output at speed, also check each spark plug wire. It wouldn't hurt to Ohm's test each spark plug just to make sure they all read the same.

Once you verified the ignition components good/bad start digging into the fuel system.

Re: 91 merc 40 hp cutting out at wot

check switch box connections...found loose wire that goes to coil..tightened all connections and engine running great. tach is reading erratic..new tach...not a deep cycle battery..measures at 12:40 volts. When engine is running it reads 12:30..does this mean that the voltage regulator is finished?

Re: 91 merc 40 hp cutting out at wot

Yepper on VR finished. Should be getting up to 14.5 V while running if discharged battery isn't pulling the voltage down....regardless, running vs not running definitely has to be a higher voltage to be functioning.

Mark

Re: 91 merc 40 hp cutting out at wot

thanks i"ll look into replacing it