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Slow 43L Mercruiser problem

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vtxtdi

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"I've got a Searay 180 wit

"I've got a Searay 180 with a 4.3L Mercruiser/Alpha Outdrive that seems very slow. I've owned it for about a year now and it's the only boat I've ever owned. I've got a 21 pitch prop on it and top speed is about 25 mph running at about 3000RPM. Under a load with that prop it won't rev past 3000. Take-off is also pretty bad. I've got a 19 pitch prop that I'm going to try which should help take-off but I know top speed is only going to get worse. Something just doesn't seem right to me. It seems to run as if the exhaust or intake are restricted, which shouldn't be the case. The engine doesn't run rough so I suspect it's running on all cylinders...but maybe not???

A little recent history...The first time I had it out this year it wouldn't rev at all. Turned out that both exhaust flappers were down in the bottom of the y-pipe choking the exhaust off. So the entire y-pipe was just replaced.

Since I got it I've always been a little disappointed in the performance of this boat. I was hoping that maybe one of the flappers had been in the pipe all along and that I would be pleasantly surprised by the performance with the new pipe. Such is not the case... Any thoughts or suggestions?"
 
"If you can try a different pr

"If you can try a different prop, that may help but according to the general rule, you'll add about 200 RPM/inch of decrease in pitch. If the redline is 4600, you'd be looking at a 13 pitch and that would be wrong. Do you have trim tabs, and are they down all the way? That could affect things, too. Do you have the right gear ratio for that boat/motor combo? IIRC, you should have 1.84:1 and it'll be marked on the upper gearcase, but it's probably hard to read."
 
"vtxtdi, I have that same moto

"vtxtdi, I have that same motor/boat combination, and my top speed with a 21" pitch prop is about 40MPH at 4500RPM. I suspect therefore that you have a severe problem. I would check the plugs to see if they are uniform in color, and do a compression test to see if you have a problem there. You need to get your RPMs up, but a 19-21" pitch prop is normally perfect for that boat. Is it possible the boat has a ton of water trapped in the hull?"
 
No trim tabs. I'd have to

No trim tabs. I'd have to look for the gear ratio but I'm sure it's whatever came from the factory.
 
How would I tell if there is a

How would I tell if there is a ton of water in the hull? What's interesting is that another one of the symptoms of the problem with the exhaust pipe was that because of the blockage a hole had been "blow-torched" through the top at the bottom of the y. So when the boat was down in the water a small water fountain would spring up from the hole. Of course I didn't notice this until the bilge was about 1/4 way full of water. This was all pumped out though and the boat sat in my garage with the plug out for a couple weeks while it was being worked on. There was no indication during that time that there was water trapped anywhere (i.e. no water on the garage floor). But I'm not sure there would be...

It normally stays on a lift in my boat dock.
 
Figured it out. Just replaced

Figured it out. Just replaced the plug wires and all is well again.
 
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