25 Yamaha not turning enough RPMs (not the prop)

[Blake]

I'm starting to get frustrated...

2000 25 Yamaha four stroke. I'm only getting about 3000 RPMs off of either plug wire, this prop should be getting almost twice that. The motor starts, idles, and runs smooth.

I just had it in the shop and they found a broken magnet on the flywheel that was causing a knock and cylinder not to fire. That was a $750 fix, but it's still knocking and not turning enough RPMs.

Here's a list of everything I've done so far:
New flywheel
New oil and filter
De-carbonized the motor
New lower unit oil
New fuel filter
New water separator filter
New gas with stabil and ring free
New thermostat (old one was stuck open)
New fuel pump
New belt and set the timing
Checked the valve clearance (only one exhaust valve was just a little off)
New plugs (gapped properly)

Literally the only thing I haven't done is rebuild the carb, only because I'm still waiting on the kit. I checked the throttle cable and the carb is wide open when the cable is maxed out. But I seriously doubt it's a carb issue, otherwise it wouldn't start idle and run so smooth. It has to be an ignition issue, right? Think the stator got damaged from the broken magnet? Ideas, thoughts?

 

[DaleH]

Anytime I had an OB low RPM issue that wasn't fuel delivery caused, it was the powerpack ...

 

[Blake]

Hell it might be this piece of junk tach that I have. It's all over the place now. I'll buy a better one.

And the clicking I was hearing was when I had the fuel line disconnected and running the carb outta fuel.

So maybe I overreacted but it's still not running very fast (25 mph on a 1542 alumacraft) and now I don't have a clue what rpm I'm turning. Maybe I should just rebuild the carb and call it good.

 

[RiverBottomOutdoors]

Does it sound like it's running fine?

 

[Pappy]

Depending on the tachometer you may have the wrong setting on the tach and it is reading at half the true RPM. If you have a wasted spark system on the ignition the setting should be at 360 and if it is individual cylinder ignition it would be on 720 depending on the tachometer.
The other indicator is that the boat would be going at barely above planning speed or not planning at all and I notice you did not say anything about that....just the opposite.

 

[Blake]

Yes it sounds fine. I still don't understand why it isn't faster.

 

[Pappy]

Well....if you mention the boat type and what all has been added to it and where then someone may chime in and give you a comparison number to go on.
Since we ain't mind readers all we can say is the more information you give us the better it will be in return!

 

[Blake]

Well.... If you read above you'd see that I said it's a 1542 alumacraft and it's only doing 25.

 

[Pappy]

Just trying to help. Maybe someone with a similar boat will chime in and give you that comparison I mentioned earlier.

 

[RiverBottomOutdoors]

25 mph sounds about right to me for a stock 25 4 stroke. I had a Yammy 25 4stroke on my 1448 and was getting about 27 mph.

 

[Rumblejohn]

Be sure to check the settings on the tach. If it was set up for a two stroke, you would be turning 6K.

 

[AllOutdoors]

Check your tach and power pack
.

 

[Blake]

I'll order a better tach.

Is there any way to test the power pack?

 

[Pappy]

What kind of test? If the engine is running smoothly the pack is doing it's job. If you want to verify that each cylinder is firing at WOT for example you can use a timing light and watch to see how steady the flashes are. Other than that you will need a peak reading voltmeter which is a specialty instrument and a shop manual.
Am thinking, once you get a tach and verify that you are up there in RPM you are probably going to find that your speed is accurate for the boat and engine.
There is no way mathematically that you can be running 25mph at 3000RPM.

 

[Blake]

Yeah you might be right. I was just hoping for 30 plus out of that set up. I've got it on a mini jacker, so I would think it'd be a little faster than a regular set up.

 

[Pappy]

On these small engines a jack plate alone does not ever give you much. It just allows you to start working with engine heights and propellers.
Here is an example. Gheenoe Classic 15'6".....same jack plate as you I think.

First engine is a modern 25hp 3-cylinder 2-stroke. Note the jack plate height. Prop is a 15" pitch stainless that has now been modified twice to allow me to run at this height and provide added performance.



Again, same boat and plate. Second engine is a Vintage 22.5 horsepower direct drive Evinrude engine. Note the jack plate height I can run with it. Much higher.
Different gearcase, different style propeller and different water pick-ups and location.



The loads in my boat stay pretty steady. When testing this is important.
Keep gasoline within 3 gallons or 18#. Keep onboard weight the same and in the same location. Any changes re-run your baseline. Start increasing your jack plate height until the speed stays the same and the RPM goes up. You have just increased your slippage rate. At this point you are at the end of what the engine will do without modifying the propeller or taking weight out of the boat or changing weight location, usually aft. One change only at a time. Record your numbers. If you are serious about it...this should help.

 

[turbotodd]

25mph is about right, assuming it's got a factory style 10 1/2" pitch prop. I think 26.3 is about where mine ran, on the limiter (at 6300 RPM). Sounds like your tach is reading 1/2 the "correct" RPM, which at 25mph, should be in the 6000 RPM range.

Don't know what prop you have, but I always had very good luck out of the old 2 stroke prop. 9 7/8" diameter (same as the 4 stroke prop) but 11 1/4" pitch. Goes right on and looks factory, and picks it up about 3 mph, with very little loss of low end. I have also tried and still have a Turbo hotshot 11" SS prop, and like it, but it's not any better than the factory 2 stroke aluminum prop which is 1/2 the cost. The turbo and the white props both perform almost identical on my boat, both top out at 5980 to 6180 RPM (depends on weather and water conditions), both about the same speeds, both about the same holeshot, etc. The only real difference is that when I hit the bottom of the river, it doesn't break the blade off of the SS prop. Bend prop shaft? Yes...but the blade didn't break or bend.

Funny thing about these motors, they're so quiet and because they're 4 stroke, they really don't sound like they're running 6000 RPM at full tilt. To someone who's used to hearing the scream of a 2 stroke, they sound like they're idling, so it's a little misleading in that sense. That was my very first observation on a F25 when I ran mine the first time. I was mad at it, thought it was gutless, heavy turd. Few weeks later after trying different props, I found out that it runs identical, to the tenth of a MPH to the 2 stroke 25 that it replaced. About 29 mph, both at 6000 RPM and change. The 2 stroke just sounded like it was going to spew it's guts at that speed where the 4 stroke was just puttering along.