72 Johnson 125hp starts fine runs good then dies after 10-15 minutes

[LOSTBOATER72]

HELP!!

I have completely rebuilt and during the process replaced 1 piston on my 72 Johnson 125HP. It ran great for the first year or so after the rebuild but last year when taking it out it started fine, ran great for 10-20 minutes then at top speed just died as though the fuel was cut off to it???? After taking off the hood, waiting 15-20 minutes i would again start and then die shortly thereafter, say 5 minutes or so. The last time I took it out it was quite embarrassing as I had to be towed in by a mastercraft. Now I know the lower unit is working ok as I have the fine mist coming out the back of the engine. Not sure what else it could be.

The only modifications I have done is I have bypassed the shift switch in the trottle as I had replaced 3 times at $100-$150 each time so got tired of it. I have put in a trottle switch which as worked great so far but other than that no modifications have been done. I did have to replace the powerpack but this was well before this problem started. Any ideas?

HELP!!!

Live in warm-hot California and the boat is never near snow or freezing weather.

Thanks one and all,
LostBoater72

 

[Vic.S]

Re: 72 Johnson 125hp starts fine runs good then dies after 10-15 minutes

Agree it sounds like lack of fuel. First thought being that the fuel tank is not venting. Try taking the cap off. If it then carries on running it is the tank venting or lack of.

Could be some other fuel pick up issue, air leak into fuel line or defective pump but I don't think it would run for that long at high speed if it were.

Could also be the ignition failing when it gets warm ... could check sparks when it stops but before it will restart.

Definitely check the tank venting first.

If it dies then its probably fuel. If it just drops dead in its tracks then ignition but you probably wont notice the difference at high speed. See what happens running slowly.

 

[emdsapmgr]

Re: 72 Johnson 125hp starts fine runs good then dies after 10-15 minutes

I had this same problem on a 72 100 hp. It turned out to be ignition related. The sensor under the flywheel was bad. It ran great when it was cold, but when it reached normal operating temps, the weak sensor just quit. No spark. If any component in the ignition system is weak, the normal heat of the engine can cause it to malfunction. A factory service manual is helpful to diagnose ignition problems-you might want to double check the stator output too, as these ignition components are not inexpensive (as you found out with the power pack) and you don't want to buy a part you don't need.