Ok, not sure it's the Ignition, but nothing else makes sense.

I returned my engine to stock in preparation for selling the boat. Rebuilt 454 with a Qjet carb and TB IV ignition. It started and ran PERFECT....For about 2-3 hours. All of a sudden as I'm idling back through the marina at 800rpm, it went from prrrrrr, to prkkprkkprkk. Basically almost like it's running on 6 cylinders.

No mechanical noise at all, and great oil pressure, and it wasn't gradual like a fouled spark plug. It was like someone flicked a switch. It went from running perfect to crap instantaneously. Coolant was at 140, oil pressure at 50psi, no noise, smoke, or anything else.

I messed around checking for lose wires, but didn't find anything. I checked the timing and it's still dead on.

Before I cancel my weekend plans, does anyone have a clue what might be up? If the TB IV goes, does it have some sort of "fail safe" mode or something?

Any suggestions would be greatly appreciated

no failsafe on a TBIV, but they are fairly bulletproof.

I hate to say it, but your description leaves a lot to the imagination.

Is it only running poorly at idle? Will the boat plane and run at cruise or is it running too badly to try? After cooling and retrying is there any change - worse, better, same?

Will it idle smoothly? Can you make it backfire?
Have you dumped your water separator into a glass jar to check for water in the fuel?

There are too many variables to be able to take much of a stab from your description. Give us more info.

MC

how are the plugs?
sounds like something may have run lean...to detonation?
.02

Sounds like water in the fuel to me also. I've had it happen and it just kind of spudders for second and then stalls. It happens when the fuel/water sep filter gets full of water. Dump the filter into a clear glass and wait for about 30 mins and look for separation.

I was wrong about it not being internal to the engine. The lack of any sort of rattle or noise, coupled with the way it went so quickly made me think it must have been ignition or at least electrical.

Turned out to be a pushrod sawn in half. Whoever put the engine together didn't check alignment of the guide plates and one of the rockers was riding partialy off the valve. The side load forced the pushrod into the guide plate and sawed it in half. Luckily I don't think anything was damaged. Put everything back together and will try firing tomorrow.

thanks

Next guess was going to be broken valvespring or bent pushrod. I don't think I would have guessed "sawn in half" pushrod though..

Somebody needs to go back thru the motor checking for other such careless shenanigans..


MC

Hopefully the oil filter caught most of the metal.