Warmed up High Rpm issues

[drewwitlin]

Hi, I have a 1995 Mercruiser 4.3L lx IV thunderbolt ignition. Motor operates great cold at full throttle, The first 20mins at WOT is fine. 4800rpm. 20 mins later the rpms bounce all over the place over 4000 rpms. anything over 4000 and its loss of power, the rpms get very eratic, the rpms surge higher and the boat jerks in loss of speed, under 4000 its better, but you can kinda tell its happening just not on a severe scale. I replaced carb, had a spare, checked spark plugs, replaced water seperator, replaced coil ignition, checked distributor cap & rotor. Still fails, tried the gas tank vent, fails. New spark plug wires in May. I am stumped, and have a electric fuel pump on order, but 220 dollars, what to do?

 

[tarheelebbtide]

Re: Warmed up High Rpm issues

You're running a 13 year old motor at wide open throttle for 20 minutes plus?

Seems to me that not many are going to hold up to that kind of intensive use.

 

[Don S]

Re: Warmed up High Rpm issues

Run this test.
http://forums.iboats.com/showthread.php?t=167561

 

[drewwitlin]

Re: Warmed up High Rpm issues

update

got worse today, now i have a clunking terrible starting noise, boat wont start, turns over, no spark, has gas. used the thunderbolt IV electrical troubleshooting chart, first item, check voltage at positive side of coil. Run position, No Voltage. Proceeded to chase the reason for no voltage. Took the positive leads off the coil, no voltage, checked resistance from start switch to coil side of wire, good. Start switch deamed good. Jumped the kill switch, no change. ran a lead from battery to + side coil, boat runs. These 2 purple wires that the coil uses, one i learned is for the tach, the other i traced to the wire harness. Where is the voltage supplied from, why is their no voltage.