Figure This One Out
 

Twin HP 500's, carburated and one engine starts cutting out at or above 4000 RPM after about 15 min of running time. We have swapped ignition boxes, coils and wires.

We also made a harness with an ignition switch to eliminate the boat side of the electrical system. Engine still cut out after about 15 min when trying to run over 4000 RPM.

When this happens, the tach starts bouncing + and - 100 RPM or so, if you hold the throttle or increase throttle, the tach starts swinging from 3000 RPM to 8000 RPM. The engine isn't reving, just the tach. Sometimes the engine dies. If you slow to idle or shut it off, it will do the same thing even faster than 15 minutes.

He has been chasing this all summer with no luck.

Not sure, but it may help to switch one piece of the ignition swstem between the motors at a time, and see if the problem switches motors. How is the voltage of the charging system.I had a alternator start to go bad once, and the tach started bouncing all over as well.


Darrell.

Have you verified the alt. is working? Battery good? I take from your post that the wiring has been all bypassed. Tach still goes wild with temp wires? Good ground(s)? Replaced distributer? Im guessing you have removed fuel from being the problem.

Looks like two of us are thinking somewhat alike.

New battery and rebuilt alternator on that side. That was the first thing we thought of.

We already swapped High 8 Ignition modules, coils and wires. We installed a new cap and rotor on the engine that was acting up.

Have you switched dissys?




Darrell.

Almost forgot, do you have a fuel pressure guage on the motor. Is it losing pressure when the pump gets hot.


Darrell.

make sure the tach gauge is not back feeding and the ignition switch is not giving you problems.

Terry

A bad engine ground had my tach jumping all over the place. It would read correctly up to 4000 and then I think the vibration made it start to malfunction. We found it when the engine died. The ground finally fell off.

Definatley check/clean/repair grounds.

Also, a few people have run into this and the tach itself was the problem. If you are grasping at straws after checking/cleaning/repairing grounds, swap tachs and see if other motor now does this.