525 SC Popping
 

Help me please!
The other weekend I had my twin 525 SC'd Scarab out and it ran great. Next morning, I got in and rode it down to the ramp. First idle zone, it acted like it was loading up on fuel, so I'm thinking, stuck float. I didn't have anything to remedy that at that moment, so I hobbled to the ramp and loaded her. I got to working on it this weekend and it still ran like shirt.
I check the floats thru the bowl screw and the float levels were fine. It is popping thru the exhaust, both sides and generally sounds bad.
So, I figure ignition.
I swapped distributor caps, no change.
I swapped plug wires, no change.
I swapped carbs, no change.
I ran a compression check and all check good, 115-120 psi.

Any ideas where to head from here?
Should I change distributors?
Would the ignition module cause the erratic miss-fire?

Thanks,

Scott

So, I figure ignition.
I swapped distributor caps, no change.
I swapped plug wires, no change.

Did you check your plugs?

Last two boats that I was on that started poping badly through the exhaust both rounded a cam lobe.

I've seen the rev limiters go bad and have the exact same symptoms as you've described. It's the little black box between the module and coil. # 43 shown below.

http://www.mercruiserparts.com/Show_...2D+0F348963%29

You can eliminate it with the little wire harness from an early Thunderbolt IV without a rev limiter, say a 1989 454 Magnum.

Good luck,

Kurt

I looked at the plugs and other than appearing pretty sooty, they seemed fine.
I plan on changing them out next weekend as my next step.

I'll look into by-passing the rev-limiter.
I am sure (I hope) that it has something to do with ignition.
Stupid sparks!

Fuel/lean condition? Maybe you sucked up sediment from the bottom of the tank sitting over the off season and clogged a fuel filter? Do you have fuel pressure guages?

I thought about junk in the fuel.
It was some pretty old gas.
I am only running the fuel/water separator, which has not been changed this season, yet..
But it still puzzels me since I am running both engines from the same tank.
Could water in the separator do this? Ithought it would just not run, on water..

I had a similar situation a few years ago with an 2000 HP500 Carb engine. It ended up being the power valve was ripped and was dumping fuel and causing it to pop and skip. Hard to diagnose, I first did all of the stuff you talked about but still didn't work. Eventuallly I swapped carbs and the problem was solved, So i took the carb apart and the powervalve needed to be replaced. cheap fix but a long time getting there.

I thought about the power valve, but when I swapped carbs, it made no change.

I was also going to investigate the coil.
A friend suggested that a cracked housing might make it behave in this fashion.
Anyone ever hear of this?
I thought when a coil went bad, it was bad, dead, usless.

Coils can sometimes be intermintent. Sometimes coils have a harder time producing a steady output at lower RPMs. Spin the motor harder and the coil will sometimes work without interuption.