Looking for answers

Mur386

Cadet
Joined
Jul 10, 2011
Messages
8
Hi everyone.
First boat for me.
I have a 1998 tidecraft wildfire with a 2s 1998 150 yamaha, it was sold to me with a 14x28p SS yamaha propeller. The engine would not get anywhere near the reccomended rpm range of 4500 to 5500. Wot run tests resulted in less than 4000 rpms. Looked at yamaha's website and could not find anything up to 300hp that reccomended a 28 pitch prop. So first stab at a replacement prop was a generic prop, alum 3 blade, at a 14x23pitch, and wow, its made so much difference, its like a new boat. With the new prop, 5000 rpms at full throttle. Middle of the reccomended rpm range for this engine. This generic prop is uncupped and probably not of the best quality of the props that are available.

But I noticed this and want some input if possible. With new prop had to hold back on throttle for first 10 minutes or rpms would go over 5500 max range. After the warming up, we could not get rpms over 5000 regardless of trim or throttle.

My question is: is this normal for a 2s engine? Or does the lowered rpm after warmup indicate the carbs may be to rich at WOT? Engine seems to run ok, What do you all think? Not asking for a fix, I just want some opinions on 2s engines and how they operate at wot. Relating to engine temperature.
Thanks for any opinions on this question.
Bill
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Looking for answers

You should get better info in the Yamaha section.
imo its highly unlikely the motor would run normally as it warmed if it was rich and slow as it warmed.I would be more inclined to think the plugs may be the wrong heat range,or the timing set wrong.A bit of a stretch but you may have a carb or carbs running a little lean that might show up as the motor warms.If it is running rich the plugs should show it.
You might do a compression check.
 

craze1cars

Lieutenant Commander
Joined
Dec 26, 2004
Messages
1,822
Re: Looking for answers

That's strange. I'll take a wild guess/stab...

When cold, the engine or intake tract (after the carb) has an air leak somewhere and is therefore running lean/hot/strong/fast. As it warms up, thermal expansion closes the air leak, properly enrichening the mixture, and therefore keeping your rpms and combustion temps in check.

This means your 2 stroke has the potential to run extremely lean/hot when it's sucking air and could burn holes right thru your pistons....

Let me assure you that my guess above is just that...a guess. In any event this is not normal, and something needs to be fixed.
 
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