# Mercury 60/40 4 Stroke 4800 RPM WOT right?



## MrTotty (Jul 11, 2016)

I've got a Lowe 1655 with a 60/40 Jet. I'm showing a WOT of 4800 on the tach which seems on the low side. I'm getting around 24-26 MPH which also seems a bit low. River was way up and moving fast last weekend and I got it up to 28.5 downstream, but still never turned more than 4800. Does this seem correct? Dealer claims it's because of the heat. I'm not buying that.

-Chris


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## momule (Jul 14, 2016)

There may be a connection to the heat and high humidity that you're experiencing but your 4800 RPM is not that far off what you should normally expect which should be around 5000 RPM. You could drive yourself crazy (and broke) trying to squeek that last 100-200 RPM out of the motor or you can check obvious things and then just let it go. You might find some non-ethanol, premium and also run an external fuel/water filter but it's not like you're operating a race boat with that 40HP 4 stroke jet.


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## BigTerp (Jul 15, 2016)

Just for reference I get around 5200-5300 with my Johnson 50/35 on a 1648MV. When loaded properly it tops out about 26-28 down river. My boat is pretty light though with a probably .080" hull or even less. It does run a bit better in the cooler times of year, but nothing significant and no difference in RPM's that I've noticed. IIRC, I did loose a few RPM's when I switched from an aluminum to stainless impeller. What does your manual say for WOT RPM's? Mine is supposed to fall between 4500-5500. One simple thing to check is your reverse gate. Make sure it is fully locked down when you throttle past neutral. If it doesn't that could knock your RPM's down, although I'd imagine more significant than 4800.


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## MrTotty (Jul 15, 2016)

Manual says 5500-6000, Outboard Jets says 5300+, Mercury is still trying to figure out what it's supposed to be. The dealer told me they ran an identical boat to mine at 4850 RPM's with a top speed of 27 which is right about what I am getting. The motor produces top HP at 5k, I don't see why Merc would have put that motor out not turning at least 5. I did find out the impeller is stainless and Mercury provides it. I also found out they back cut the trailing edge of the impeller to get more RPM's. Mercury is supposed to call me back once they talk to a product manager. I'll send an update once I hear back. It may be what it is, but I still feel that motor should open up a touch more.


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## mphelle (Jul 15, 2016)

You may have one of the old style impellers made for the two stroke, they found that the four stroke was unable to spin it to a reasonable rpm and had to cut them back. This thread has some good info on page 2.

https://www.tinboats.net/forum/viewtopic.php?f=85&t=40480&start=15


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## MrTotty (Jul 18, 2016)

Interesting you say that. I spoke to the third person at Mercury and explained I needed to talk to someone very familiar with that motor. This person agreed that the RPM's were low, and talked to a product specialist who also agreed they were low. His fix was to order a new Merc Impeller, that most likely the one that is on the boat was an older model that had not been modified for the four-stroke. More to come...


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## handyandy (Jul 18, 2016)

If you get one of the new impellers I would be interested in your old one I have a two stroke 60/40 and would rather have the old style impeller. I can pm you my contact info if you'd get the new one and find out you do have the old style impeller and want to sell it.


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## MrTotty (Jul 19, 2016)

I'm pretty positive that Mercury is going to want this one back. I'll see what happens. It's a brand new boat/motor and this would be Warranty.


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## handyandy (Jul 26, 2016)

If it's possible for you to keep it try to if you don't mind.


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## LarryMc (Oct 20, 2016)

MrTotty said:


> Interesting you say that. I spoke to the third person at Mercury and explained I needed to talk to someone very familiar with that motor. This person agreed that the RPM's were low, and talked to a product specialist who also agreed they were low. His fix was to order a new Merc Impeller, that most likely the one that is on the boat was an older model that had not been modified for the four-stroke. More to come...



Any update on your motor's performance with a new impeller? I ask because I'm going through the same hassle that you have had. I also have a Lowe 1655. About a month ago I replaced my 2005 Mercury 2 stroke 40 Jet with a new Mercury 4 stroke 40 Jet. When I got a chance to get the boat with the new motor out on the water, max RPM at WOT was 4500 and top speed was around 22 mph. It would just barely get on plane. I took it back to the dealer. After about a week and a visit by a Mercury Tech Rep to check it out, they finally gave up on trying to find the problem with the motor and shipped another one from the factory and put it on my boat. The new replacement still doesn't run up to the RPM specified which is around 5500-6000. At best it runs around 5200 at WOT, usually less and the top speed is only around 26 mph on still water. The dealer says that the motor will run at a higher RPM after it is completely broken in. I have doubts about this. 

I replaced my old 2 stroke because the powerhead needed rebuilding and I didn't want to put a lot of money into it. I'm beginning to think I would have been better off going with the rebuild. Back when it was running right, the 2 stroke ran 5250 RPM at WOT and got around 28 MPH top speed.


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## MrTotty (Oct 21, 2016)

The issue with your first motor was most likely the impeller. The impeller on mine from the factory was as I thought an uncut 60HP 2 stroke impeller. Mercury backcuts the 4 stroke version to get more RPM's out of the motor. Your speed is dead on what mine is as an average. I get between 5200 and 5400 now which gained me 2 MPH. I'm still not happy with the performance and I'm looking into going back to a prop on the boat. I do really like the boat. I think that boat is way underpowered with a 40 Jet and would be much happier with the 65 or 80 (90/115) motor. I'm trying to figure out what the performance specs would be with the 60 with a prop. I'm going to guess high 30's maybe even low 40's.


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## handyandy (Oct 21, 2016)

MrTotty said:


> The issue with your first motor was most likely the impeller. The impeller on mine from the factory was as I thought an uncut 60HP 2 stroke impeller. Mercury backcuts the 4 stroke version to get more RPM's out of the motor. Your speed is dead on what mine is as an average. I get between 5200 and 5400 now which gained me 2 MPH. I'm still not happy with the performance and I'm looking into going back to a prop on the boat. I do really like the boat. I think that boat is way underpowered with a 40 Jet and would be much happier with the 65 or 80 (90/115) motor. I'm trying to figure out what the performance specs would be with the 60 with a prop. I'm going to guess high 30's maybe even low 40's.



Did mercury take your old impeller? I'd still be interested in it, as I have a two stroke and hell will have to about freeze over before I run four stroke outboard with a computer on it. 



LarryMc said:


> MrTotty said:
> 
> 
> > Interesting you say that. I spoke to the third person at Mercury and explained I needed to talk to someone very familiar with that motor. This person agreed that the RPM's were low, and talked to a product specialist who also agreed they were low. His fix was to order a new Merc Impeller, that most likely the one that is on the boat was an older model that had not been modified for the four-stroke. More to come...
> ...



You should have kept that two stroke a good clean one with a propperly rebuilt power head, with some minors mods is will walk the dog around the four choke, and is worth about the same as a used four stroke. But I'm obviously biased to two strokes, and in jets the 3 cyl omc two strokes were the best.


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## LarryMc (Oct 21, 2016)

MrTotty said:


> The issue with your first motor was most likely the impeller. The impeller on mine from the factory was as I thought an uncut 60HP 2 stroke impeller. Mercury backcuts the 4 stroke version to get more RPM's out of the motor. Your speed is dead on what mine is as an average. I get between 5200 and 5400 now which gained me 2 MPH. I'm still not happy with the performance and I'm looking into going back to a prop on the boat. I do really like the boat. I think that boat is way underpowered with a 40 Jet and would be much happier with the 65 or 80 (90/115) motor. I'm trying to figure out what the performance specs would be with the 60 with a prop. I'm going to guess high 30's maybe even low 40's.



I think you are right about the impeller being the problem with the first motor. It had a manufacturers tag on it with "15" on it. I assume that means it was made in 2015. The replacement has a "16" on the tag. Seems strange that even the Mercury rep couldn't figure out what was wrong with the first motor. Like you, I'm still not happy with the performance of the 4 stroke 40 jet on this boat. But I guess I'll live with it. With the rivers I fish, a prop is not an option and I really don't want to put a 65 jet on it. 

I'm going to pull the intake off of the jet lower today and check out the impeller and see what it looks like. I've never seen a backcut impeller. Is the difference between the old type and the new type obvious?

handyandy, I still have the 2 stroke. Depending on how things go with the 4 stroke, I may have the powerhead rebuilt and put it back on my boat.


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## MrTotty (Oct 21, 2016)

The year of the motor should not matter however, did you ever take the impeller out? If you did you would clearly notice the top end of had some shiny grind marks where metal was ground off. It's something you would not be able to miss. Here are pics of each. the shiny one is the right one for four stroke.


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## LarryMc (Oct 21, 2016)

Thanks, appreciate the photos. I haven't taken the impeller off yet. Plan to do it today. The dealer's mechanic took the impeller off the first motor and checked the numbers and said it was the correct impeller, but I didn't look at it. From what I've read online, Mercury didn't change the part numbers when they changed the impeller design, so I don't actually know which type was on the first motor.


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## handyandy (Oct 21, 2016)

MrTotty said:


> The year of the motor should not matter however, did you ever take the impeller out? If you did you would clearly notice the top end of had some shiny grind marks where metal was ground off. It's something you would not be able to miss. Here are pics of each. the shiny one is the right one for four stroke.
> 
> View attachment 1




MrTotty I'd be glad to take that uncut impeller off your hands if you still have it


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## LarryMc (Oct 21, 2016)

MrTotty said:


> The year of the motor should not matter however, did you ever take the impeller out? If you did you would clearly notice the top end of had some shiny grind marks where metal was ground off. It's something you would not be able to miss. Here are pics of each. the shiny one is the right one for four stroke.



Took the impeller off and checked it out. It has been cut back, although it is not as obvious as the one shown in your photos. The first photo below is the 4 stroke impeller alongside the 2 stroke impeller. The part numbers are identical. The second photo is a closeup of the 4 stroke impeller. I'm tempted to put the 2 stroke impeller in the new motor and find out just how much difference it would actually make.


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## handyandy (Oct 21, 2016)

The two stroke one won't work as well in the four stroke. Your rpm will decrease at wide open throttle with the old style two stroke one in the four stroke. I wish they changed the part number and still made the old style two stroke ones.


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## MrTotty (Oct 21, 2016)

You have to buy a new 4 Stoke to get one since apparently they didn't know they were doing it. The one I have has to go back to the dealer. It was a warranty part and require it back. 


The Cut impeller looks just like mine but instead of being ground it looks like they casted a new mold. 

Ever heard any performance numbers on a 1655 with a prop?


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## LarryMc (Oct 21, 2016)

handyandy said:


> The two stroke one won't work as well in the four stroke. Your rpm will decrease at wide open throttle with the old style two stroke one in the four stroke. I wish they changed the part number and still made the old style two stroke ones.



I know you're right and I probably won't go to the trouble to do it, but it would be an interesting experiment.


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## JL8Jeff (Oct 23, 2016)

Just for reference, I have a Lowe 1652 jet tunnel and with my mid 90's 60/45 Mercury 2 stroke I was getting around 28 mph upstream. I'm not sure what the rpm was or if my linkage/timing is correct at WOT because the linkage was broken when I got the motor. I don't normally run close to WOT but I found a small GPS so I did a test. The batteries died before I could get a downstream speed check.


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## LarryMc (Oct 23, 2016)

Sounds about right. I was getting about the same speed with my 2005 Merc 60/40 2 stroke. With the 4 stroke 60/40 I'm lucky to get 26 mph upstream, usually it's around 25.


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## handyandy (Oct 26, 2016)

MrTotty I'll get you a new impeller to send back to them to make merc happy if you'll hold on to that old style for me.


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## JoshKeller (Dec 6, 2016)

LarryMc said:


> Sounds about right. I was getting about the same speed with my 2005 Merc 60/40 2 stroke. With the 4 stroke 60/40 I'm lucky to get 26 mph upstream, usually it's around 25.



what is the weight difference between the 2 stroke and 4 stroke? that may be some of your speed missing.


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## LarryMc (Dec 7, 2016)

According to Mercury, the 4 stroke dry weight is 267 lbs. The 2 stroke weighs 233 lbs. Not enough weight difference to make that much difference in the speed. The 4 stroke motor is just not strong enough to run the standard 6 7/8" impeller at 5200 rpm which is what I got with the 2 stroke. Mercury had to cut the trailing edge of the impeller back in order to get the motor to rev up to 5200 rpm. I tried running my old impeller in the 4 stroke and it would only get 4900 rpm. The hole shot was a little better, but max speed was only around 24 mph in still water.


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