15hp Evinrude won't reach max RPMs

[gabebp]

Hello All,

I have a 1979 15hp evinrude on a 14' aluminum boat and the motor tops out around 5,200 rpms max with a light load (me and gear and battery ~ 250 lbs) which is around 16 mph at the very most. I posted a comment on an existing thread, but didn't get any response, so I thought I would start a new thread. The most concerning thing is the lack of rpms. It should be 1,000+ higher to get to the rated hp.

So far I've done or will do the following:

---compression is 110 on both cylinders
---replaced plugs
---readjusted throttle throw to open the throttle plate a bit past WOT to make sure I was all the way open
---replaced the existing 9.5x10 prop with a new 9.25x9 prop. (RPMs didn't go up at all, speed went down by 2 mph)
---de-carbed the motor with seafoam in the gas, sprayed in the carb and in the spark plug holes. Idled a bit better and started a bit better but no affect on top end rpms.
---Removed the carb and verified that it is clean (previous owner replaced float and the metal float bowl was virtually free of rust and grime). I ordered a carb kit and will install that when it comes in. The high speed orifice plug didn't appear to have any obstructions, but I cleaned it anyway.
---I'm considering custom drilling my own plug at 0.052" as I'm at 4,000 ft.
---Fuel pump leaked fuel when I took it off the crankcase. It appears this is the original because it is missing the rear gasket that was added to kits later. It doesn't appear the diaphrams are leaking into the crankcase. The pump was leaking out of the two small vent holes. The pump worked enough to operate the boat. Sometimes at WOT I would pump the bulb to see if there was low psi on the pump. This didn't increase the rpms at all.

Anything I'm missing? I know I should get more out of this motor. I had a similar 14' aluminum boat that would go 14 mph with a 5 hp newer motor and just me in the boat.


Thanks!

gabebp

 

[racerone]

Re: 15hp Evinrude won't reach max RPMs

Check the flywheel key.------------Look for a reed valve issue.-----Look for crankcase compression issues , like a bad lower crankshaft seal.----------Picture of where the fuel pump was leaking ?----Hold off on the custom drilling bit for now. You need to understand what drilling a jet to be larger means at 4000 ' elevation.

 

[Bosunsmate]

Re: 15hp Evinrude won't reach max RPMs

Do a drop test, see if one cylinders working noticeably less than the other

 

[raczekp1]

Re: 15hp Evinrude won't reach max RPMs

maybe seals(upper and lower one) are not holding pressure when you hit 5500 rpm

 

[gabebp]

Re: 15hp Evinrude won't reach max RPMs

Re flywhee key -- I haven't checked the flywheel key, but would think if it were sheared it wouldn't run at all. Is the timing on these motors fixed? Or when you adjust the throttle timing on the throttle cam, does that have some bearing on the spark timing? I assume the spark is fixed by the position of the flywheel and the spark occurs at the same location relative to TDC at all RPMs?

Re Fuel pump was leaking (when fuel pumped by hand) out of the holes labeled D here http://www.leeroysramblings.com/OB%20images/FPump5.jpg

Re the orifice, I was going to drill out a 0.045" that fit in the main jet threads to a 0.052". this would actually reduce the hole diameter by 0.002" to lean it out slightly.

Re Crankcase seal - Would my compression test not have checked this?

 

[Bosunsmate]

Re: 15hp Evinrude won't reach max RPMs

No, compression test doesnt check crankcase pressure

 

[Bosunsmate]

Re: 15hp Evinrude won't reach max RPMs

If timing key has gone and flywheel has moved a bit, timing will be out a bit so it wont get up to enough power again to move flywheel further out of timing for it to be drastically noticable

 

[gabebp]

Re: 15hp Evinrude won't reach max RPMs

I'll do some searching on how to check these items, but if anyone has any quick tricks, I'm all ears:

- Crankcase seal - What's an easy way to test this? Does putting a card in front of the carb while running help check this?

- Timing Key- I assume I'd need to pull the flywheel? Is it obvious to tell if the key has slipped?

- Drop Test - What's the best way to see if I'm dropping spark. Does a "static" spark test help with this (1/2 in gap jump)?

 

[raczekp1]

Re: 15hp Evinrude won't reach max RPMs

seals are allready 35 years old

 

[racerone]

Re: 15hp Evinrude won't reach max RPMs

The plate under the flywheel rotates to change the timing of the spark !---Compression test is done to check condition of pistons / rings / cylinder wall / head gasket.---The compression in the crankcase is totally different / seperate from that !!

 

[Humperdink]

Re: 15hp Evinrude won't reach max RPMs

run your engine at wot for a few minutes and stop it abruptly. Look at the plugs. Decide if you want to modify your main jet according to their color. if you got a nice tan brown then dont mess with it.

The timing is static, can only be thrown off by a sheared key. A key that was damaged but not completely sheared could alter timing slightly. Any other adjustment to the timing is all done by the armature plate.

I suggest you do the spark plug test to see if you are getting a good burn then check the crank case seals and flywheel key, etc. As someone else said, check those reeds.

Have you ever had this engine reach full rpm on this setup before?

My 13'8" semi v runs 21 to 22 mph with similar weight. 18 with 2 ppl. It has 1980 johnson 15 with stock 9.5x10 prop. I haven't taken an rpm reading but plan to on my saturday trip.

I will link a video to it running if you want to see it.

 

[Humperdink]

Re: 15hp Evinrude won't reach max RPMs

also, i forgot to mention, the tilt setting on my motor can change my top speed by 4 mph. Have you adjusted the tilt?

 

[gabebp]

Re: 15hp Evinrude won't reach max RPMs

also, i forgot to mention, the tilt setting on my motor can change my top speed by 4 mph. Have you adjusted the tilt?

Thanks for the suggestions all. Right after I typed it I realize I asked a dumb question regarding checking the crankcase seal and doing a compression test. I was off in my terminology thinking that the "crankcase" extended above and below the cylinders...
I can tell all the gaskets except carb are original as they have the factory blue paint still undisturbed on them and the bolts.

Humperdink,

I've only had this setup for a few months and never had it run above 5,200 rpms before except when I when I was first playing around with the boat and motor and raised the trim up to the second from the highest notch and began to ventilate for a second. The tach went just above 6,000, but it wasn't a real load on the motor. Needless to say I didn't run it like this for more than a second.

I have adjusted the tilt and found second from bottom pin to be optimal. I also built a transom plate to raise the motor 1" so that the anti-vent plate is about 1/2" above the bottom of the boat. Even with this setup if I have two guys in the boat it goes down to about 12 mph and 4,800 rpms with trim optimized at third hole.

One thing I didn't mention - the guy I bought it from said his last trip the motor would bog down when giving it full throttle from idle after getting warm. It was also cutting out periodically when he had it at full throttle. He took it in and they put a new power pack on it and said that the problem was solved. I had the idle to full throttle "bog down" happen to me a couple times but I adjusted the idle jet and it seemed to fix that issue. It seems to want to cut out if I crank the throttle all the way down immediately from WOT, but this may be because the mechanical idle stop knob is broken on the side of the motor (not concerned about idle this cause I don't use it for trolling)

If there was an issue with the ignition/spark/timing, wouldn't there be some sputtering or erratic behavior like sputtering or missing? When I leave it at WOT it runs very smooth, just not as much power as should be...

 

[Humperdink]

Re: 15hp Evinrude won't reach max RPMs

A strobe style timing light and a spark tester can answer your spark questions. A weak spark can cause erratic behavior. A complete failure of an ignition component would drop a cylinder. Does it run smoothly at mid throttle?

I might back out the screw on the cam follower to sync the carb back. the timing is retarded if the cam follower is engaging early. Have you checked to make sure there is no leak around the carb gasket? spraying starting fluid or the like around the carb when its running should not change the rpms.

My rig weighs at least 250 with me in it. Its a semi v. what kind of hull do you have?

edit: have you tried running with the cowl off to make sure its getting fresh air?

 

[Eyeballsdeep]

Re: 15hp Evinrude won't reach max RPMs

Check to see if the throttle cam is worn. Had the same issue with one that I rebuilt. I just made another out of a scrap piece of aluminium.

 

[gabebp]

Re: 15hp Evinrude won't reach max RPMs

The engine runs smooth at 1/2 throttle and what is now "WOT" as long as I leave it at a constant setting. When I go from WOT to idle it will sometimes cut out.

throttle cam is a little worn and the throttle plate starts to move a little before the timing mark, but as long as the cam reaches the end of it's travel and the throttle plate hits horizontal by that time it should be able to hit max rpm, right?

I do need to back out the screw a bit so that the plate doesn't go past WOT, but before I adjusted the cam follower it was getting just to WOT.

Mine is a 79 crestliner superseaman semi v with a 16" transom. Weighs around 200#.

 

[gabebp]

Re: 15hp Evinrude won't reach max RPMs

I have tried running with cowl off. No change.

 

[gabebp]

Re: 15hp Evinrude won't reach max RPMs

At the risk of taking off on a “low-deflection tangent,” I have a very closely related theoretical question I'd like to pose to some of the more seasoned boat guys who use kickers a lot. I want to preface the question by admitting I don't have enough experience with different engines and boats to have a good feeling for what affect boat weight and hull design has on rpms and slippage for small outboard engines. The answer to this might give me some insight into whether I have just a motor problem or if I’m asking too much from the motor or maybe a combo.

If these little 15 hp motors are running in top condition with the stock 10" pitch prop, what is the max rpm one can expect to achieve on a fairly heavy boat, say a 18' fiberglass? Kind of like a kicker with an overpitched prop.

The slippage would be very high, but I'm just wondering if some small motors with stock props can get close to the low end of their rated WOT range when they are pushing a large boat at lower speeds. In other words the prop is moving a bunch of water backwards (slippage) and can power up close to the operating power band?

One way I'm visualizing it, taken to an extreme, at some point with a very low pitch prop you could hook the motor up to the dock and reach the rated rpms without allowing the motor to move. What would this pitch be?

 

[racerone]

Re: 15hp Evinrude won't reach max RPMs

A good shop would have a test prop to do exactly that.-----Lets the motor rev to a specified RPM in a large test tank.........If it does not do that there is something wrong with the motor !!---Call around and see if there is such a test prop on hand at your favourite shop.-----I have one for the 9.9 / 15 hp models.

 

[gabebp]

Re: 15hp Evinrude won't reach max RPMs

Tried calling a couple places, with no luck locally. It looks like there are some for sale on Ebay, but it's hard to know which one goes with my motor. Any website recommendations on where to find this info?