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Low top speed
https://www.smwebhead.com/phpBB3/viewtopic.php?f=3&t=15774		 Page 1 of 2
Author:  1baddoggy [ Mon Oct 23, 2017 9:43 am ]
Post subject:  Low top speed
Hey everyone.

This summer we spent a fair amount of time on some big water, and I started realizing my top speed is quite a bit lower than what is listed in the manual. I have the carbureted 5.0 with the Alpha One. The manual calls for 49-52 MPH Top Speed. At WOT, the highest I've seen has been 38MPH (By GPS) on glassy water with virtually no wind. I know the specs in the manual are under perfect conditions, but this is with only myself in the boat and less than 1/2 tank of gas. My RPMs appear to be within spec at WOT, but I'm definitely not hitting the speeds it lists at the various RPM's. Where should I start looking for my lack of performance?

Thanks!
Author:  chris268 [ Mon Oct 23, 2017 11:38 am ]
Post subject:  Re: Low top speed
I would start with the prop. What type and pitch is it?
Author:  babbot1 [ Mon Oct 23, 2017 11:58 am ]
Post subject:  Re: Low top speed
looks like you are on a trailer so you could start by weighing the boat to make sure you don't have any water soaked foam.
How far are you from the top end of max rpm range and are you using the same prop as specified in the four winns fast facts? If you are under propped with an unhealthy motor you may be hitting proper rpms but not getting full potential.
Author:  MichiWinner [ Mon Oct 23, 2017 12:27 pm ]
Post subject:  Re: Low top speed
Just went through the same thing this year... Replaced the plugs, distributor cap, rotor, and ignition coil - and now it runs better than ever. Out of all those parts, I believe that the coil was the largest contributor. My boat would only get up to about 3600RPM at WOT until the replacement.
Author:  1baddoggy [ Mon Oct 23, 2017 1:05 pm ]
Post subject:  Re: Low top speed
Boat, with gas and a 2nd battery I added weighed in at 3540lbs. Right now, I have a 17p aluminum 3 blade prop on it. I hit about 4500RPM at WOT. Prior to that, I had the factory 21p aluminum 3 blade prop that I was only hitting 3900RPM at WOT.
Author:  babbot1 [ Mon Oct 23, 2017 1:17 pm ]
Post subject:  Re: Low top speed
Sounds like a a sick motor. Fast facts states 14.25x21 AL prop. If you had to drop 3 degrees to hit proper WOT then you lost power somewhere since boat weight seems correct.
Get the carb cleaned and do a complete tuneup. If you still aren't getting full WOT with the 21p prop then you should check compression.
Whats proper WOT range for your configuration? Is it 4400-4800?
Author:  1baddoggy [ Mon Oct 23, 2017 1:24 pm ]
Post subject:  Re: Low top speed
We just bought the boat last year, and I really don't know what, if any, maintenance (other than oil changes) the boat has had done. Probably due for a tune up regardless.

I'm embarrassed to say that I didn't realize changing the prop also changes what the proper WOT is supposed to be. I thought WOT RPM was supposed to be around 4500 regardless of the prop I'm using, understanding that different props cause it to run at different RPM's. The 17p prop put it where it's stated it should be with the 21p prop.
Author:  babbot1 [ Mon Oct 23, 2017 1:38 pm ]
Post subject:  Re: Low top speed
The prop doesn't change the proper WOT rpm range. That range is stated by the mercuiser and your engine should fall into this range no matter what. If you run too low a WOT it means the boat is overloaded and can prematurely fail. If you run over the proper range it could me catastrophic failure.
The general rule I learned about props is 1 degree lower in pitch will increase WOT by 200rpms. This seems to line up to what you did going from 21 to 17 and increasing from 3900 to 4500.
Four Winns designed your boat/engine configuration with the 21p and you only got 3900 so it sounds like the engine needs some attention. I would definitely start with a full tuneup and carb clean, especially since you don't know history and is probably due anyways. It is all possible the PO changed the outdrive ratio and a prop change is needed. But without knowing everything about the boat, start with the simple stuff and work your way to the drastic.
Author:  rpengr [ Wed Oct 25, 2017 9:57 am ]
Post subject:  Re: Low top speed
As others have said, to drop to a 17p prop should not have been necessary. The 3900rpm with the factory spec prop tells the story that your engine is not putting out proper power. I assume it still sounds like it's running smoothly or you would have said otherwise.

You could be running on 7 cylinders instead of 8 and it would still sound smooth. So an easy check is to pull the spark plugs and see if they all look the same. They should look dry and dark grey to brown without buildups. Post pictures if you need help. A bad ignition wire, bad plug or bad distributor cap could cause one cylinder not to fire.

Next step is a compression check, which is easy to do yourself if your borrow a compression checker. If one or more cylinders has very low compression, then it is likely you have valve damage. This could lead to having a head rebuilt, expensive but it's not the end of the world.

Get this solved and you will deserve the name 1baddoggy again :lol:
Author:  1baddoggy [ Mon May 07, 2018 5:19 pm ]
Post subject:  Re: Low top speed
Nothing like waiting a year to circle back to this :D

This spring I had the local marina replace all the spark plugs, which they said were fairly fouled up. I switched back to the 21p prop and I'm now hitting 4500 RPM at WOT (Was 3900 before the new plugs). Manual shows WOT above 4500, I'm guessing 5000 based on the increments they're using. At WOT, 4500 RPM, I'm doing about 43MPH. This is going down river with the wind at my back and trim all the way up.

Should I consider this acceptable? Other than the low top speed and low RPM's, there are no issues. No weird noises, no chugging, stalling, etc. Am I correct to assume that WOT should be 5000 RPM with the factory 21p prop?

Image
Author:  babbot1 [ Tue May 08, 2018 8:27 am ]
Post subject:  Re: Low top speed
Did the mechanic have any feedback? Also, did you change cap and rotor? That's an easy one and they get corroded quickly in the damp environment. Did they say why the plugs were fouled? The carb may be too rich or the choke is slightly closed. Fouled plugs may be your first indicator. One more thing is to check the air cleaner is not plugged.
Sounds like you are barely hitting the recommended WOT which is not optimal. This means the engine is slightly overloaded. Usually those Four Winns tests are pretty close.
Author:  1baddoggy [ Wed May 09, 2018 8:04 am ]
Post subject:  Re: Low top speed
No, the mechanic didn't really have much feedback. I believe they just changed the spark plugs and nothing more because that's what I mentioned to them. They did not change the cap and rotor. I had a sheen behind the boat last summer, and they thought that may be because the spark plugs weren't always sparking and I was seeing unburned fuel. I'm not a mechanic by any means, so I'm just kind of going by what you folks have suggested.
Author:  babbot1 [ Wed May 09, 2018 8:52 am ]
Post subject:  Re: Low top speed
Sounds like its running a little rich and fouled your plugs making it worse. Replacing the plugs helped but you're still running rich so it will probably foul again. I would start by checking cap and rotor. If they need replacing check how it runs after and if its still fouling plugs. If still fouling then you need to address the carb, maybe just needs a good cleaning. That's just where I would start(simple first). Others may have some other suggestions.
Author:  Surface Interval [ Wed May 09, 2018 4:08 pm ]
Post subject:  Re: Low top speed
With the table you provided, I would assume the Max rpm range for the carbureted 5.0liter engine is 4400 to 4800. On the table each 500 rpm increase is 5 mph. From 4500rpm to full throttle is only 2 mph. It does seem that you are close to 5 mph slow. If possible you could use a digital tachometer/timing light to check the tachometer accuracy. If you are not getting the speed for a given rpm the tach could be inaccurate. The book speeds are normally with a fairly light load in the boat. This might be 2 people and half tank of gas or similar. I am not sure if the boat has issues yet or not. It is a lot closer to right. I would also verify speed with a GPS, maybe even on your smart pbone. Boat gauges can be inaccurate. Hope this helps, and have a blast this summer.
Author:  1baddoggy [ Tue May 15, 2018 8:01 am ]
Post subject:  Re: Low top speed
My speeds/rpm are with 2 people in the boat and 1/2 tank of gas. Going downstream with the wind at our back. The speeds are by GPS. RPM is just the built-in tachometer.

Forgive me, I'm not very mechanically inclined. What do the cap and rotor do, and how can they affect performance?
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