Loss of power/surging at WOT

[boozer1966]

1984 Regal Medallion, Mercruiser 140, Alpha M/R, 2bbl Mercarb.

The following fuel system parts have been replaced or rebuilt: fuel tank filler hose; fuel tank vent tube; fuel tank pickup cleaned; anti-siphon valve replaced; fuel line from tank to fuel pump replaced; fuel pump and internal filter replaced; rigid fuel line from fuel pump to carb replaced; carb rebuilt.

The problem before and after parts replacements is identical: Boat starts and idles fine. Runs up to about half throttle without trouble. When going to WOT, the boat accelerates great, then starts to have a slight miss, then loses power and starts surging and lurching. After that, I have to take the boat back down to idle and then it will eventually cough its way back to running smoothly at half throttle.

I have also replaced and/or checked all ignition parts, timing, dwell, etc. The boat starts and runs perfectly at idle on the muffs. It seems like a fuel problem for sure, but I have run out of parts to replace. Any ideas?

 

[Mischief Managed]

Re: Loss of power/surging at WOT

Sounds like fuel starvation and clearly you have already tried to address that, but perhaps it's a lack of air that's causing it. An exhaust leak could fill the engine compartment with gases that displace the oxygen. WOT might overwhelm the engine compartment ventilation and half throttle might not. Just a thought.

 

[dkellogg3]

Re: Loss of power/surging at WOT

Check that you didn't mess up the firing order when you did the ignition parts.

Don't ask why I mention this...

 

[boozer1966]

Re: Loss of power/surging at WOT

I can guess why you mention that. Ha! It runs great for brief periods, so I'm pretty convinced its a fuel problem. If i hadn't just replaced the anti-siphon valve and fuel pump, I would think that either is experiencing a failure under heavy load.

 

[kbsunlovers]

Re: Loss of power/surging at WOT

Here's an easy check: Broken flapper partially blocking the exhaust.

 

[NHGuy]

Re: Loss of power/surging at WOT

Just cause you replaced the vent tube doesn't guarantee ventilation. Try it with the gas cap off or loose. If that does not help, try running it off a separate tank.
Did you verify timing advance at high RPMs? Is the carb exactly correct for the motor. OE brand, model and setup? Are the ignition wires isolated from one another to prevent cross firing.
If you are positive everything else is perfect change the cap and wires (again?)

 

[dkellogg3]

Re: Loss of power/surging at WOT

I ran around for 2 summers chasing what I thought, and all troubleshooting said was a fuel problem.

 

[jimbo1229]

Re: Loss of power/surging at WOT

Put a water fuel separator on and change out the old fuel filter as well. I added some berrymans carb cleaner also and now I don't have the surging or power loss it runs like a new boat

 

[boozer1966]

Re: Loss of power/surging at WOT

It was the COIL!!!!!!!