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I own a 1996 21 Chaparral with a carbureted 350 inboard engine.

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I own a 1996 21'...
I own a 1996 21' Chaparral with a carbureted 350 inboard engine. It only has about 300 hours on it. I have noticed that it's performance has dropped considerably in the last few years. The maximum rpms that it achieves is about 3,500 compared to over 4,000 before. There is a black smoke that sometimes comes out of the exhaust when it idles. I have... Read full answer
I own a 1996 21' Chaparral with a carbureted 350 inboard engine. It only has about 300 hours on it. I have noticed that it's performance has dropped considerably in the last few years. The maximum rpms that it achieves is about 3,500 compared to over 4,000 before. There is a black smoke that sometimes comes out of the exhaust when it idles. I have had the carb rebuilt and tuned, spark plugs and wires replaced, and the distributer cap and rotor replaced. I have noticed a couple times that moments before the fuel tank runs out completely while I am going full speed that suddenly the motor performs much better and that the RPMs go up above 4,000. Any ideas as to what could be causing the boats lack of performance until moments before it runs out of gas? Any suggestions would be greatly appreciated...
Submitted: 4 years ago.Category: Boat
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Answered in 7 hours by:
9/5/2014
Marine Mechanic: Jason, Marine Mechanic replied 4 years ago
Jason
Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 19,695
Experience: Degree in Marine Technology. Gas and diesel marine mechanic.
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Hello my name is Jason and I will be helping you today.

 

I have noticed a couple times that moments before the fuel tank runs out completely while I am going full speed that suddenly the motor performs much better and that the RPMs go up above 4,000.

 

This is because the engine leans out that last minute of running.

And then phenomenon would be normal. All engines pick up rpms as they lean out. That being said, this really isn't a good practice. If you consistanty suck the tank dry as you run the boat your going to end up sucking up any junk on the bottom of the tank as well. And that junk will end up in the carb, clogging it.

 

What concerns me more is the black smoke. That means incomplete combustion. The very first thing you need to do is a compression test to make sure you don't have low compression on all, or a dead cylinder. Start with a compression test and let me know what you get for a numbers.

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Marine Mechanic: Jason, Marine Mechanic replied 4 years ago
Hello, Jason here again. I'm just checking in and seeing how you were making out. Did you find my answer helpful, or did you need more help with it? A rating has not been put through on my end yet, so I wanted to make sure you were either all set, or see if you had more questions about this. Did the system give you any problems when trying to put through a rating?

Thanks again!
Jason
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