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Carburetor or Ignition Problem on S/C 454

Old 05-27-2007, 10:55 PM
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Default Carburetor or Ignition Problem on S/C 454

Need some help trying to resolve this problem. Engine loading up pumping black smoke at idle and under load feels to be missing. Black smoke both exhaust banks. Engine now wants to die at idle. Engine Mark IV 454 with B&M supercharger and 800 CFM carb off a 500 HP carb that is manifold referenced. Ignition mercuruiser with V-6 module timing set at 28 degrees before top dead center.

Had it out first time this seasons she was running great starting and idling at 650 RPM with no throttle. All great till pulling kids on tube. Swung into a corner tight hit the throttle and all of a sudden she choked out when I pulled back the throttle. When I started it up ran rough and would not idle and was was missing. Started getting black smoke and rich gas smell. Started limping back to the dock with black smoke then she cleaned up and started running smooth. Decide to keep boating after that since power runs and starts around the dock were smooth.Seemed there were no problems. Ran another 30 minutes great. Hit a speed run on the way back in. Launched it off a good wake came down very very hard throttled back and engine started running poorly worse than before. Limped all the way back in after that and never cleared up. or smoothed out Have black soot around exhaust tubes pumped black smoke like a diesel. Will only run with extra throttle at idle or start by hitting throttle a lot.

Need the experts here to help me in troubleshooting this. Where should I start? I was going to look at ignition first see if all cylinders are firing. But wonder if a bad carb float could do this. If the carb was dry most of winter would anything dry up and cause this? What is the quickest way to see if getting spark at all cylinders. Thinking I could use the timing light to see strobe flash on all plug wires.

If Carb issue is problem where do I look first. Any chance fedding it some carb cleaner would help. Since she screwed up both times coming off throttle fast I'm wondering if something is sticking maybe varnished up. Missing cylinders and roughness is puzzling though. Seems like running on six or seven cylinders.

Considering putting in new plugs tomorrow least would let me know which cylinders may be culprit. Any one have issues with mercruiser ignition doing something like this? Intermittent missing then strong.

The experts here have always came through in the past and I have been able to nail it down quickly and get back to boating. Thanks in advance.
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Old 05-27-2007, 11:33 PM
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sounds like your running rich and fouled some plugs.
good luck finding the problem.
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Old 05-28-2007, 05:57 PM
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Holley carbs have a power valve that fails and leaks more gas than the engine can burn. Sometimes all it takes is a little carb backfire to rupture it.

Pull the forward float bowl and metering block. The power valve threads into the metering block. The chamber being wet with fuel is a dead giveaway the power valve is bad.

Put in a new set of spark plugs if this is the problem, as they are probably fouled pretty well.

Good luck and keep us posted.
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Old 05-28-2007, 07:50 PM
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Sounds like the problem starts when the boat gets out of balance. Check your carb float levels with the boat in the water. Could also be a powervalve or a needle seat sticking open. Probably runs rich long enough and then fouls the plugs. It could also be a coil going bad.
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Old 05-28-2007, 08:05 PM
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Ran it with the spark arrestor off. Lots of fuel dumping from secondaries venturii. Pulled needle seat valve. The o-ring fell apart sealing needle seat to the bowl. Replaced o-ring new plugs runs like a champ again. Now have to change the oil due to the dumping of fuel washing down cylinders. Also found a plug wire #6 which was arcing Replaced, Found a champion plug which crumbled on #8. The center electrode showed a old fracture. Have new ACs now, Wonder I got it home. Gotta love these 454's little will kill them. Lesson learned put oil in gas filter run till she dies to fog the carb and coat bowls prior to putting it away. Tried to expedite things this fall.
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Old 05-28-2007, 10:58 PM
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Glad to hear you got it fixed!
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