Evinrude 15HP will not Rev to WOT rpm low power

[flexfanac]

15hp on a light 12ft boat will not rev up and reach WOT RPM

1977 Evinrude 15hp 15704m Bought on Craigslist for $120 I Had fix recoil starter, tiller handle and kill switch. Compression test 100-110psi both cyl. Rebuilt and cleaned the carb two times. New thermostat, New Head gasket. New spark plugs. New fuel lines. New prop Solas 9.25x10. Set the timming, Had to set throttle cam all the way out so throttle shaft will just start to move when roller is in line with mark on cam. Throttle roller sits in small notch on throttle cam at wot and timing plate turns all the way to the stop. At wot the looking in to carb throttle plate is open 90% of the way. The throttle plate is NOT flat 100% but more like 90% open almost flat... Is this normal? I checked and have the correct carb, throttle cam and throttle roller. Even if the throttle was only open 90% I should get much better performance.

Boat is a 12ft Semi-V Aluminum Starcraft weighs 130lbs 12ft long 48inch wide at transom top. I weigh 170lbs Two Batteries 80lbs. trolling motor 25lbs. Evinrude 75lbs. Trim set all the way down. Both batteries strapped to front seat

I can not get the engine to to WOT rpm even with the light boat and just what is listed above for weight. The highest rpm I can get is 4800rpm at 15mph GPS. I can tell the engine wil not rev up like it should. If I have a second person in the boat (170lbs) I get 11mph

If i apply a bit of choke it bogs down, If i spray some wd40 in to carb it bogs. If i pull either plug wire it runs very poorly. squeezing primer makes no difference. Run with engine cover off no change.

I should be able to get 6000rpm from this set up and i think around 20mph even with two people in boat. If you have any ideas of what could be wrong or what to check please let me know. I have done lots of searching, manual reading and reading leeroysramblings.
Thanks for any help!

 

[flexfanac]

Re: Evinrude 15HP will not Rev to WOT rpm low power

Cavitation plate is 1 inch below bottom of boat hull and even with the center keel rib

Spark Plugs after 4hours of use then removed after a 15second WOT run then shut off

 

[OptsyEagle]

Re: Evinrude 15HP will not Rev to WOT rpm low power

Just some thoughts I have.

Did you do an external spark test to see if your ignition can supply the voltage so that your spark can jump a 3/8" gap? What spark plugs are you using and what are their gaps?
When you cleaned your carb did you use a new carb kit and install a new high speed oriface plug? Did you adjust the rich/lean adjustment of your carb for optimum gas/air mixture? This would probably effect idle more then WOT but the adjustment is easy to do.
Could be your upper/lower crankcase seals, although I would suspect one would go before the other and you would see some difference when running on one cylinder compared to only running on the other.
How did you set the timing? Also, have you tried adjusting the roller cam to touch the throttle cam a little sooner (arrow slightly to the left of the roller when looking straight on from the front of the motor) reducing your spark advance. Your plug pictures do not look all that bad but they might be showing a little pre-detonation or they could be showing a lean mixture or timing and mixture could be just right. Can't really be sure. Not a difficult test to run.

 

[F_R]

Re: Evinrude 15HP will not Rev to WOT rpm low power

You aren't going to like what I am about to say, but I think that's all you are going to get out of that rig with the weight you are carrying. You might get a bit better by fixing the throttle (I don't think the notch is mormal---usually seen on motors with many hours of run-time.) A small boat is actually harder to plane than a larger one because of lack of bouancy (pressed down too deep in the water).

It can't rev higher because the boat is holding it back. Same reason you have to downshift a truck to get RPM to climb a mountain. It is running on both cylinders, or it wouldn't get anywhere near 4800 RPM.

The 15 was notorious for fouling spark plugs, but that is not the case here. Again, if fouled, it wouldn't get 4800.

You might get another MPH by raising the motor a bit, and maybe not.

Just my 2c

 

[TrueNorthist]

Re: Evinrude 15HP will not Rev to WOT rpm low power

Just my two bits too, but I agree with F_R. You may have hit the wall as far as speed with the weight configured that way. Try moving the batteries aft and see if that improves speed or remove all the excess weight and see what happens. You want as much of the hull as possible out of the water at WOT and the batteries on the front seat will keep the bow in the water which will slow you down.

 

[OptsyEagle]

Re: Evinrude 15HP will not Rev to WOT rpm low power

Trim set all the way down.

Have you tried using the next trim holes on your transom adjustment. I know with mine, I need to set it in the next slot to get the nose up so I can really fly and I usually put it another notch out when I have someone sitting in the front. It really helps a lot. I do have a 14' boat, however.

I have a notch in my throttle cam of my '76 15Hp motor, also. It is notched so smoothely it almost seemed like it was designed that way, but logic always told me it must be a defect of some sort. I have thought about using JB weld to create a smooth surface but I figured I might just make a mess of things and I doubt it is hurting my top end all that much. Probably explains your 90% open throttle as opposed to 100%.

 

[OptsyEagle]

Re: Evinrude 15HP will not Rev to WOT rpm low power

Does your notch look like this one. Not easy to see but it is just above the arrow.

 

[flexfanac]

Re: Evinrude 15HP will not Rev to WOT rpm low power

I just set up two screws to 3/8in gap and was able to jump the gap with loud blue spark. I then adjusted the screws to 1/2in and also had no problem with spark. The spark plugs are NGKBZ7HS-10. I think i set the gap when I was at work where my plug gap gauge is. I will double check gap... I think I set them to .030

When carb was cleaned I pulled both welch plugs. Sprayed carb clean through all holes. Used stranded elecrtical wire and poked it in and out if all holes i could find. I removed the main jet ran wire through it and cleaner. The main jet had a #54 on it which looks like the normal jet. Spray through the main jet pick up, spray through hole kinda on side of main pickup. idle needle hole. Blow out with shop air. Installed new needle and seat, set float so it is parallel with carb bowl body. Installed new slow speed adjustment seat. Adjusted needle at idle in gear on boat turn rich slowly till stumble then turn lean till stumble and in the middle of the two was 3/4 out. I then went 1/8th turn richer.

My throttle cam looks just like that picture. I believe the notch at WOT is normal. I set the timing by adjusting the throttle cam with a small wrench. When the mark/line on cam is centered with roller throttle shaft has just moved. The throttle roller touches the cam just 1mm before the timming mark. Im prety sure the throttle cam is set out all the way... I could not adjust it anymore. A WOT the timming plate thing under the flywheel hits the stop... So wouldnt my WOT timming be at the max as well as my throttle position?

The boat only has 3/4 plywood bench seats with foam under seats with 1/4 plywood covering foam sides. Boat Hull is 120-130lbs I checked it. I put the two 40lb batteries on the front seat to keep the bow down. I have the trim pin in the hole closest to the transom. I weigh 170lb and also have the trolling motor in rear. The Bow is not too low or diging in to the water.

When I went out with me 170lb and a second person 150lb and one 40lb battery the boat will NOT plane and only get to 10mph engine RPM unknown

I really think I should be able to plane out with two (normal) 160lb people, battery and a trolling motor. I even think the boat should move prety quick since it is a 15hp.
I also think the boat should go around 20mph or more with just me and two batteries up front to help balance the boat. I should also be able to reach around 6000rpm.

Thank for the help, I cant believe how many post i already got

 

[Tim Frank]

Re: Evinrude 15HP will not Rev to WOT rpm low power

You could verify that you have a viable prop on that motor....from Craigslist you really have no way to tell what is on your prop shaft.
Small point, but many people (I am one) have not had the best of luck running NGK plugs in OMC engines. The OEM plug is Champion, I have always believed that you should have a compelling reason to go your own way. (I've never heard one )

 

[flexfanac]

Re: Evinrude 15HP will not Rev to WOT rpm low power

New prop Solas 9.25x10 Ran the same with the OEM 9.25x10
I will get some Champ plugs, why not they are cheap

 

[TrueNorthist]

Re: Evinrude 15HP will not Rev to WOT rpm low power

How have you been checking your rpm? One thing you could get that will help you a lot is a vibratach. It's a simple - and cheap - tool for testing rpms that actually works fairly well. You just place it on top of the engine and feed a wire out until it vibrates the most and a dial indicates the rpms. Also, the 9.25x10 prop may be a bit too much pitch for your setup. But I would look into filling that notch. All the top end power comes from throttle and the notch may be a problem depending on how deep it is. Hard to know without seeing it.

 

[OptsyEagle]

Re: Evinrude 15HP will not Rev to WOT rpm low power

Again, where have you place your trim peg. If it is in the first position (closest to the boat) the motor will push the bow into the water and reduce your RPMs accordingly. I would suggest the next spot or maybe even the middle one for the best trim. Try a few others. It makes a very noticeable difference on the top end speed. You will know when you have gone too far out, since the boat will start to bounce up and down (bow rising up and then immediately slamming back into the water) when you open up the throttle on it.

 

[Tim Frank]

Re: Evinrude 15HP will not Rev to WOT rpm low power

New prop Solas 9.25x10 Ran the same with the OEM 9.25x10
I will get some Champ plugs, why not they are cheap

Assuming that you have an accurate way to check RPM, the question is why did you change props....and use the same spec?
A quick check would be if you could borrow a 9.25 x 7 and see what you get.

 

[flexfanac]

Re: Evinrude 15HP will not Rev to WOT rpm low power

I am using a Hardline electronic tach that you wrap a wire around the plug wire. I got the Champ plugs and gap to .030inch the engine ran the same same rpm and speed.
I also pushed the throttle lever when at WOT to see if opening that last bit 10%...No change
I installed the other new prop that i had not used yet. The second new prop is a Solas 9.25x9 With the new prop here were my readings... Also note that I removed one battery (40lb) from the boat.
lowest trim =rpm5450 15-16mph Bow out of water seemed good
2ne hole up trim = rpm5490 15mph Bow out of watter a bit higher than first hole
3rd hole up =5350 13mph Bow was getting too high

120-130lb sem-V Starcraft aluminum boat. 15hp evinrude 75lb. ME=160lb. One 40lb battery straped to front bench seat. Trolling motor 25lb on transom.

Any ideas what could be wrong. I still can not reach the proper rpm (6000) range with very little weight and a lower pitch prop? Overall I think this boat and engine should be able to plane out with 2 (160x2) people in it. It should also be able to hit WOT RPM and 20mph with just me... right?
I have seen other post on this forum and others aswell as youtube videos that proves my engine is not performing well.
Thanks for the help

 

[Tim Frank]

Re: Evinrude 15HP will not Rev to WOT rpm low power

The change in pitch has moved the RPM in the right direction.
If it is working....keep doing it....

......
A quick check would be if you could borrow a 9.25 x 7 and see what you get.

 

[flexfanac]

Re: Evinrude 15HP will not Rev to WOT rpm low power

yes the rpms did go up about 500rpm but i did not see any change in top speed. I am closer to peak HP rpm yet had no gain is speed even with removing 40lb from the boat.
Considering all of my results and what im working with Im not sure that prop pitch is my problem. Most 9.9hp can plane a 12ft boat with two normal sized people. 9.9hp can also reach speeds over 15mph with just one person. I am using a light stock aluminum boat that was made for a 10-15hp max with a 15hp engine. If a 9-10 pitch wrong for my boat how come thats the stock universal prop that came on 9.9-15hp? Props for this motor under 8 pitch seem to be mostly for kicker motors on large boats?
I am open to ideas and very well may be wrong. I really thought this 15hp would be a great engine for my 12 aluminum. Please let me know what you think.. I just thought I had some type of low power problem even though it seems to run well, yet not really rev to max rpm

Thanks for the help and if anyone can give me an opinion or experence with a 15hp or even a 9.9 on a 12ft aluminum semi-V please let me know.

 

[Tim Frank]

Re: Evinrude 15HP will not Rev to WOT rpm low power

See if you could borrow a known good 15HP from somewhere and compare performance?

If you get too much weight into too small a boat, the rig will have too much trouble planing to attain the speed that you are after....IMO.
Re-read FR's post.

 

[flexfanac]

Re: Evinrude 15HP will not Rev to WOT rpm low power


just for refrence this is a picture of the boat 12ft long. max width 48in . Width at transom 45in.

I can buy a third prop i just want to be sure. It just seems like something might be is wrong.

I am curious to hear how other 12ft aluminum boats run with a 9.9 or 15hp.

Thanks

 

[Joshua2]

Re: Evinrude 15HP will not Rev to WOT rpm low power

I have a 1976 15 evinrude on a 14ft 36in...(bottom width) and me and someone else in the front a battery 6 gallon tank full and ice chest with ice and fish in it... Planes the boat no problem and gets up and go... Well I'm not saying its fast by no means lol but there's a pretty wake behind me lol and looks like I'm moving pretty good until I look at the bank lol it's not moving really fast but I'm not sure the mph or rpm but u think you setup should be doing atleast the same if not a lil more but I'm know mechanic so I wouldn't have a clue where to start but I just wanted to say that your not crazy it should be getting on plane no prob but stay with it you will get it dnt give up on that motor I love mine!

 

[ss_rider]

Re: Evinrude 15HP will not Rev to WOT rpm low power

View attachment 165076
just for refrence this is a picture of the boat 12ft long. max width 48in . Width at transom 45in.

I can buy a third prop i just want to be sure. It just seems like something might be is wrong.

I am curious to hear how other 12ft aluminum boats run with a 9.9 or 15hp.

Thanks

I have a 1975 9.9 Johnson on a 12ft aluminum. I am basically having the same problem as you, I feel like it should be going faster than it does. I have also tried everything you have mentioned, although I do not have that notch on my cam that you do. With me, a full tank, and about 20 lbs of gear I can only get 16mph. Theres videos on youtube of guys getting 20mph with the same motor on a 12ft flatbottom, which should be slower if anything. I can't figure it out. Where did you get your tach?