1990 evinrude 120 4600rpm WOT enough?

[archcycle]

I bought the motor from a shop propped a bit large, 14.25x19 on my 16' cape horn, and it is maxing out at somewhere around 4600. I usually run around 3000 to cruise around 25mph in cheap non gas-guzzling territory and WOT runs are typically short and I do a good bit of trolling too. As a side note decarboned last week after reading about the dangers of low speeds and low rpms and whoa smoother

I have zero complaints about the performance, but am wondering if this is a bad thing that it never runs full out 5500 or so. I found some writings stating min wot as 4500 max as 5500, but would appreciate some varied input on if this is OK.

So now I am considering a re-prop but if I do it would be more for concerns about a low WOT than performance.

 

[nymack66]

Re: 1990 evinrude 120 4600rpm WOT enough?

Tricky question since a PROP is base on weight and type of Boat etc, with that said its recommend you at lease see 5500 at WOT on your Boat with this engine. Mine is a 140 v4 with 13 x 17 SST turning close to 6k at WOT ...

 

[archcycle]

Re: 1990 evinrude 120 4600rpm WOT enough?

Yeah it is. My question is really whether this WOT RPM is too low, the process of prop selection wouldn't be an issue if I were to replace it. I had a 13 3/8 x 17 on the busted 90hp I gave them but they insisted I would need a larger one and traded me for this prop Should have stuck with my gut. Going smaller.

 

[wilde1j]

Re: 1990 evinrude 120 4600rpm WOT enough?

5800 should be target for your motor. If set up is OK, (motor height, etc.), you should start with trying a 13" pitch prop, since you need to pickup ~ 1200 RPM @ WOT.

 

[nymack66]

Re: 1990 evinrude 120 4600rpm WOT enough?

How many splines does your current prop have? If 13 then I have 2 spares one aluminum and one stainless (13 x 17 respectively) Where are you located? We can arrange a deal where you can use my prop of your choice as a test. PM me if you like ..

 

[archcycle]

Re: 1990 evinrude 120 4600rpm WOT enough?

How many splines does your current prop have? If 13 then I have 2 spares one aluminum and one stainless (13 x 17 respectively) Where are you located? We can arrange a deal where you can use my prop of your choice as a test. PM me if you like ..

Thanks for the offer I'm in NW Florida. I just spoke with the pretty crooked shop who put this together for me and they said if I bring the boat in they'll work something out with me. Last time I did that it didn't go so well and this boat will never enter that lockable fenced in area again. It was memorable...

So I'm bringing just the prop in to them tomorrow. As said earlier they told me I needed the larger prop , but it turns out that's because (I just pulled it off and counted the splines and wire brushed the garbage out of everything so I could actually read the stuff on it clearly) they put a different lower unit on it that takes a 15 spline, and I've got a 14.75x19 part no 389924 off an OMC outdrive on here.

So I will see what they have that will fit, if anything.

 

[nymack66]

Re: 1990 evinrude 120 4600rpm WOT enough?

Thanks for the offer I'm in NW Florida. I just spoke with the pretty crooked shop who put this together for me and they said if I bring the boat in they'll work something out with me. Last time I did that it didn't go so well and this boat will never enter that lockable fenced in area again. It was memorable...

So I'm bringing just the prop in to them tomorrow. As said earlier they told me I needed the larger prop , but it turns out that's because (I just pulled it off and counted the splines and wire brushed the garbage out of everything so I could actually read the stuff on it clearly) they put a different lower unit on it that takes a 15 spline, and I've got a 14.75x19 part no 389924 off an OMC outdrive on here.

So I will see what they have that will fit, if anything.

Just so you know a 15 spline prop equates to a v6 Lower unit not a V4. I went through this 2 years ago with my engine or mentioned what I told you.
The first thing I would do at this point is get a rough idea of your gear ratio. Take the cowling of the engine off and remove the spark splugs from the engine. Rotate the flywheel by hand until the #1 top dead center mark is lined up with the timing marker on the front of the engine and mark the 2 points. Now put the engine in forward and mark where the propeller is at. Now rotate the flywheel by hand counting the revolutions of the flywheel and propshaft until the prop makes 1 full revolution. If you rotate the flywheel just shy of 2 full turns to one rotation of the prop you have a V6 lower unit. If you get 2 and 1/4 turns of the flywheel to one revolution of the prop, you have the V4 lower.

 

[archcycle]

Re: 1990 evinrude 120 4600rpm WOT enough?

I just did some googling and found boats.net lists there being

http://www.boats.net/propellers/Evinrude/120/props.html
Motor Manufacturer: Evinrude Props
Horsepower: 120
Select Model: 2-Stroke (13 Spline)
V4 Large Gearcase (15 Spline)

Interestingly before rebuilding this gearcase the other month I passed up some attractive deals on the larger gearcases thinking I didn't have one. Probably a good thing because there is a lot of confidence in this one's internals now.

They don't list the size prop I'm looking for but there is at least a 14.5x15 which is substantially closer to what I need compared to the 14.75x19 so this gives me some hope.

 

[nymack66]

Re: 1990 evinrude 120 4600rpm WOT enough?

I just did some googling and found boats.net lists there being

http://www.boats.net/propellers/Evinrude/120/props.html
Motor Manufacturer: Evinrude Props
Horsepower: 120
Select Model: 2-Stroke (13 Spline)
V4 Large Gearcase (15 Spline)

Interestingly before rebuilding this gearcase the other month I passed up some attractive deals on the larger gearcases thinking I didn't have one. Probably a good thing because there is a lot of confidence in this one's internals now.

They don't list the size prop I'm looking for but there is at least a 14.5x15 which is substantially closer to what I need compared to the 14.75x19 so this gives me some hope.

I do know the large gearcase was indeed shipped on a few with the correct ratio to compensate its very confusing..Is your engine a 25" or 20" ? I bet 25" ?
What is IMPORTANT is the correct ratio, to verify follow the instructions I posted and save yourself some time and money. If the wrong gear-case is on this 120 HP it will never run correctly, low rpm will be the lease of your problems.

 

[archcycle]

Re: 1990 evinrude 120 4600rpm WOT enough?

I do know the large gearcase was indeed shipped on a few with the correct ratio to compensate its very confusing..Is your engine a 25" or 20" ? I bet 25" ?
What is IMPORTANT is the correct ratio, to verify follow the instructions I posted and save yourself some time and money. If the wrong gear-case is on this 120 HP it will never run correctly, low rpm will be the lease of your problems.

Nothing is ever easy is it. This motor, not known to me when I bought it, seems to have been put together from the remains of other motors. The motor has been painted so I can't really match it against the foot. The shift shaft was cut and welded (although very professionally welded I have to say) so was it a shift shaft from a 25" they had lying around and cut to fit a random foot? Was it foot off a 25" and they cut that shaft? I don't know! Crazy, huh? I can tell you that Today it is a 20" however it got that way. Maybe it is even the original foot and it just needed a new shift shaft. Possibilities are apparently endless and the shop guys are liars and crooks so there's no sense even asking. They actually sold it to me as a 90hp which I later realized was wrong when no parts I bought fit then noticed the heads were for the 120/140hp block when I went to change the head gaskets after the water pump they lied about changing died on me. Who knows maybe it's a 140... But I really don't think it is a 140 it's incrementally more powerful than my previous 90 but not a huge jump. Then again I am on a way wrong prop. End rant.

I have the prop with me today and was going to go by there at lunch and see if they will swap it for me but will instead verify the gear ratio this evening.

 

[archcycle]

Re: 1990 evinrude 120 4600rpm WOT enough?

Out of curiosity though, what would be so bad about having the wrong gear ratio? Wouldn't there just be a prop setup out there somewhere that could accommodate for that?

 

[metriccrescentwrench]

Re: 1990 evinrude 120 4600rpm WOT enough?

Out of curiosity though, what would be so bad about having the wrong gear ratio? Wouldn't there just be a prop setup out there somewhere that could accommodate for that?

Yes, pretty much.

In theory it's more efficient to run higher pitched props and turn them at lower rpm's via a lower gear ratio, but that doesn't amount to much difference on most normal everyday boats.

No matter what though, you want to get your WOT rpm's up to around 5700.

Another thought - have you verified that the AV plate is out of the water when your boat's on plane? If not then that would help explain the apparent crappy performance.

 

[archcycle]

Re: 1990 evinrude 120 4600rpm WOT enough?

Yes, pretty much.

In theory it's more efficient to run higher pitched props and turn them at lower rpm's via a lower gear ratio, but that doesn't amount to much difference on most normal everyday boats.

No matter what though, you want to get your WOT rpm's up to around 5700.

Another thought - have you verified that the AV plate is out of the water when your boat's on plane? If not then that would help explain the apparent crappy performance.

Is that gear ratio going to be more of the deciding factor in how much change I get by going down a few inches in pitch then? It seems I'm not going to have many (if any even) options for reducing diameter below 14.25. But yes, my AV plate is out of the water and actually I do get good performance out of it, planes in 4-5 seconds when weight is properly distributed and tops out in the low 40s even at that 4600. It is painfully slow to plane if there are 3 or 4 of us in the back and nobody in the front, or else if going from stopped to plane in heavy chop.

 

[wilde1j]

Re: 1990 evinrude 120 4600rpm WOT enough?

Don't get hung up on gear ratio. The reason your boat is so slow to plane is pitch ... the motor needs to turn much higher RPM. Forget diameter ... it has negligible effect on performance. The AV plate is where it needs to be. Get the pitch right and you'll be happy.

Don't be too inclined to change more than one thing at a time.

 

[nymack66]

Re: 1990 evinrude 120 4600rpm WOT enough?

If you have a V6 lower your engine will NEVER run right. LAND N SEA did a nice write up on prop curves vs engine power curves and how you have to match the 2. If your engine isn't at it's peak power point when the prop is at it's peak efficiency point, then it runs like crap. Simply changing your prop pitch won't give the desired effect as changing the gear ratio will. Make sure your of what your ratio is first, and then go from there. Your cost to determined this is zero $
If it is a V4 lower, it is pretty much certain to be a 2:1 ratio which is proper for that engine. The 1.86:1 ratio in the V6 lower would KILL the V4 looper by taking it out of it's power band vs prop speed. Your prop sizes also sound like a V4 lower.
As for if its a 120 vrs a 140 remove the air cover and look at the butterflies if 2 screws 120, 3 screws 140 !

 

[wilde1j]

Re: 1990 evinrude 120 4600rpm WOT enough?

Even easier, look at the top aft of the block. You should find a core plug stamped with the block serial and model nos.

 

[d.boat]

Re: 1990 evinrude 120 4600rpm WOT enough?

OP, I feel your pain. I had a franken motor that was very tough to figure out if/when it needed any repair work. But when it worked, it worked great. Unfortunately, when I had any problem, I could only feel comfortable using a specific local shop because he knew the engine and I'd spend hours ($$) of shop time if a new shop had to try to figure things out. If he was backed up, I had no options and it was frustrating.

 

[archcycle]

Re: 1990 evinrude 120 4600rpm WOT enough?

Even easier, look at the top aft of the block. You should find a core plug stamped with the block serial and model nos.

I wish it were so easy.. the previous owner had gone to very great lengths to make sure it was not easy to identify as a 120. The plug has no useful information on it. He must have really expected the coast guard to do a forensic inspection below the cowl to determine if it was really a 90.

 

[archcycle]

Re: 1990 evinrude 120 4600rpm WOT enough?

If it is a V4 lower, it is pretty much certain to be a 2:1 ratio which is proper for that engine. The 1.86:1 ratio in the V6 lower would KILL the V4 looper by taking it out of it's power band vs prop speed. Your prop sizes also sound like a V4 lower.

Well I've been running it hard since January, so hopefully I haven't been murdering it since then! I will definitely be checking this as soon as I can.

In the meantime, I did end up going by the shop just for the heck of it and they traded me a 15x17 SS for my 14.75x19 SS. This isn't much of an improvement, and isn't at all what I was looking for, but the one he gave me is in excellent condition and the other one was a solid fair (had seen a reconditioning then rough treatment again) so I figured might as well. Incremental step in the right direction and .25 isn't so much larger really?

As for if its a 120 vrs a 140 remove the air cover and look at the butterflies if 2 screws 120, 3 screws 140 !

Great info, will be checking this as well tonight.

I really appreciate all the good input I've gotten in this thread

 

[nymack66]

Re: 1990 evinrude 120 4600rpm WOT enough?

Please note the rule of thumb give or take a few RPM's is every pitch decrease will increase your RPM's +200 . Base one the numbers you posted this reduction in pitch will increase your RPM'S to 5k not even close to the 6k this motor should be showing.
Whilst I will not recommend you Boat at 6k the engine should be around 5500 at or close to wot.
FACT since you now mentioned this motor be bastardized lets identify the LU first.
DO NOT run at WOT just yet if you like I can work with you on the identification of this engine.
Who knows at this point maybe your WOT timing is off? Wrong HS Jets? Point is we need to start from somewhere. I tell you what the LU is the correct place for an unknown engine since every tree shade mechanic can really mess these engines up and send everyone in all directions.
Welsh plug , I swear both of my blocks will tell you its 120 HP its NOT I will explain later lets stay on track.
Here is a peek at my 87 140 v4 Looper I am working on as learning curve plus its my backup engine.


140 v4 Looper on my Boat , I love these engines,simple to own and maintain, cheap parts, COO is the best in the industry.
140HP weight is 370 lbs and tons of torque nothing comes close ..