Dobb

Guys,
I have a 454 330HP with the thunderbolt ignition. For the last month or so I was getting a subtle exhaust pop during deceleration and some itermittent throttle reponse. I decided to do a tune-up and replaced the plugs (NGK's), new 8mm wires, cap, and rotor (rotor was a PITA). I traveled up to Grand haven from Holland and all was good, running great. On the way home it was just not running right...The subtle exhaust pop was constant and I lost a lot of power (could only turn about 3800 rpm's at WOT...then the power would come back. Today I decided to change out the Coil and give it a test run. It was running great for about 30 minutes then the subtle exhaust popping took over and my power was gone.

It appears that this seems to happen when the motor is warmed up, and is also a bit intermittent. There are some pick-ups in the distributer that look a bit rusty which someone had mentioned could be the problem...but why would the motor warmed up matter? Sorry for the long post but do you guys have any ideas. These are my first issues with this boat and its killin' me.

Thanks,
Dobb

John B

I would check voltage output at the plugs. Timing. Moisture or oil in the distributor. Low voltage to the coil, ballast resistor-if it has one.

This sounds more like a fuel problem... plugged filter, anti siphon valve plugged, vapor lock, weak fuel pump, low float level.

You may want to do a compression check, it's easy and will normally save time if you have a more serious problem.

Good Luck, my engine is a Sterling getting fixed during the best summer I can remember.

JB

Dobb

JB,
Thanks for the response. I think the one thing I am going to try is the anti siphon valve coming from the tank. I described the problem to a friend and he said when his warmed up it did the same thing.

Thanks,
Dobb

FOUNTAINATLAST

I ran across an older Sleek that had the exact same symptoms wouldn't get over 3000 RPM after about 15 minutes of running perfect. Had a plugged fuel filter/screen in the carb (quadrajedt carb). When the boat sits the dirt would settle on the bottom of the screen and after running a few passes the dirt plugged it up again, hopefully this will help out!!!???

Krumbsnatcher

I can see the motor laying down at a certain rpm then going lean due to a fuel starvation issue. However, since it is constant i would guess one of your cylinders ran a bit too hot and now your exhaust valve is slightly tuliped and allowing at higher rpm's some of the fuel intake to be absorbed in the exhaust chamber. Then when the exhaust exits it is igniting the left over fuel.

just my .02 probably wrong. My second guess would be a weak valve spring.

Dobb

Thanks Guys,
Because it runs for 20-30 minutes with zero issues I don't believe it is motor related and sure hope I am right. The more I think about it, the issue occurs when I am low on fuel. I plan to check the fuel-oil separator and and some dry fuel and regular fuel and see what happens. I am getting a new gear lube reservoir put on so I don't have the boat back until tomorrow.

Thanks again,

mr_velocity

Had the same problem with one of my motors at the end of last year, would run great but after a hard run it would pop at idle. Went through the whole ignition thing, no change. Winterized the boat and decided to worry about it next season. With 240 hours on the boat it was time for springs, when i pulled the valve cover off found 1 broken exhaust spring on the side that was popping. It was the inner spring so I didn't see it until I pulled the spring off. Pull the valve cover and take a look, I can't believe I didn't eat the valve.

GOODT

From The Sound Of It (punn) I Would Say Its An Exaust Valve Or A Bad Guide (warms Up And Sticks)

Joe
American Performance Marine
410 573 0057

u150

Mine was doing the same thing- turned out to be the distributor pick up coil. I bet that is your problem. If you try to clean the rust off they will die completely. Merc. updated the design so the new ones are much better.
They only run about $50.

Dobb

Guys,
Just added some dry fuel and regular fuel and it seemed a bit better. I always run the tank low which may be part of my problem. Is the fuel/water separator the only "fuel filter"? I think when I say popping you guys are thinking a loud "pop". It is really more of the engine "missing", not a loud pop. As I cruise along it can be all good and then suddenly "miss" a bit. Also, it is worse at WOT...It is just a steady "miss". Thanks for all the help...

Dobb