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71 Johnson 100hp V4 spark loss after warm up

7

70wellcraft

Member
My 71 Johnson 100HP V4 will run great for the 1st 15 minutes or so after launch but then like a switch, it will only run on like 2 cylinders. Then I let it set in the water for 20 mins and its running good as new for another 10 mins or so. Its very repeatable. I have tried swapping coils and MSD spark units (I converted it from the stock box 3 yrs ago) but still same problem. I suspect the following;

- The motor has 2 sets of points. Maybe one set is heating up causing those cylinders to not spark.
- There is a thermo switch assembly that is for some reason jumpered to the Amplifier on the terminal block. This switch controls choke soleniod. This is per the stock schematic. could this switch cause this issue?
- There is a Temperature switch that turns the overheat horn on when engine gets hot. Could this cause the problem?

Thanks for any input!
 
Are the points properly adjusted ?------------Are the distributor cap and rotor absolutely clean.----Have the wee boots on the distributor cap been replaced.---------Get rid if the thermo switch connections as it has nothing to do with ignition.
 
SOS - I have tried the above and still same problem. I took 200 - 400 grit paper and cleaned all carbon from inside distributor cap and rotor metal areas that are encapsulated in the plastic and wiped with contact cleaner. I also checked the points and very slightly adjusted on set. Disconnected temperature switch. The motor runs like a champ for 10 - 15 mins until it warms up. Then the same old thing, it looses power drastically and acts like its running on 2 or 3 cylinders or barely firing. Its like a switch. Which leads me to the question about any limp modes that may be in the engines circuitry when it gets hot? To me, it doesn't seem any hotter than the normal at temperature should be. The fly wheel is very warm to the touch and the Hot Horn has not gone off. Could the Temperature Switch that connects to the Hot Horn cause any of this? I can't tell from schematic. It did go off on me several years ago when the water impeller wasn't pumping enough. I'm miffed on this one and its tried my patience. About time to bring it in to a shop.
Any help or tips is appreciated.
 
Have you run with a timing light to see if you can pinpoint one or more cylinders failing on spark ?-----------Are the head bolts properly torqued ?----No fancy circuitry on that motor for a " limp mode " at all.---Flyweel gets warm because it is part of the generating system , very normal !--------
 
Well, I have just replaced the impeller to make sure it was ok and it was fine. I also checked the hot horn and it works also, so if the motor was actually getting too hot it would have gone off.

The next thing I would like to know about is the stator. IF the stator has a heat related wire break, would it cause the conditions I have? does the stator have to be working 100% to get full charge to the ignition system or could the motor run totally fine off the battery? I have found other blogs with Mercury outboards that have had bad stators after warm up that work fine until the engine warms up. Then they let set for an hour to cool and then it runs fine again... Sounds a whole lot like my problem.
Comments?
 
UPdate ---- Last night I checked out the Rectifier assembly and stator while not running. The stator checked out ok but the rectifier assembly did not. 1 of the 4 diodes was open with both polarity checks, should be open one way and short the other. So I am going to get a new rectifier and then test the stator output voltage with the defective rectifier and after I put in the new rectifier. There should be a big difference. Maybe this explains why the motor runs good for 10 - 15 mins and then spark starts to get really weak? Maybe it drains the battery voltage enough to not have good spark?
 
I am using a standard 12 volt marine deep cycle battery. It didnt seem to have any problem cranking after the motor would loose power and stall after the 15 minutes or so of running. However I did notice one time that the positive cable at the battery was getting very hot and nearly smoking after cranking for a relatively short period after one of these events. The cable was clean and tight so it wasnt that. The battery may have just been over taxed already from supplying the engine ignition juice instead of the stator? I like the ice chest idea. Where can I get a unit like that that runs on a DC accessory plug?
 
Well we took the boat out yesterday and what do ya know, it works! The inverter was the problem. Ran all afternoon with no power loss or stalls. The only thing it still does that I would like to figure out is after running hard for a while, it doesnt like to idle. Any ideas on that? I have rebuilt the carbs and fuel pump in the last 2 years as well.
 
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