15hp lacking power

[kbait]

Johnson 15 from '74. Had a buddy pick up a craigslist 'excellent condition' motor. You can guess the rest! I fixed the tiller gears, and cleaned and gapped the 'new' points, replaced the 'new' impeller, and barrel tested. Runs great and idles nice and slow. Tested on my boat, and it acts exactly like a 9.9. Nothing wrong at all, just 9.9 power (barely planed me out on a wide 14' alumacraft). I've had a '80, '75, and another '74 15hp motors on that boat this year, and they all flew, and planed fast. Not this one. I immediately pulled the carb, expecting a 9.9 carb, but it is indeed a 15hp carb (I have a 9.9 narrow throated carb to compare). Pulled it apart, and it was pretty clean.. nothing clogging the main jet. Compression is good, as is spark (for a non-cd ignition). Reeds sound good (I pulled plugs, and mostly covered the carb port w/fingers and it makes that cool sound when I spin it over w/socket wrench on flywheel nut). I'll put the cleaned carb on tomorrow and test again, but I feel I'll have the same result.. acting like a 9.9. I didn't get compression numbers, but pulling it over slowly feels great, just like the other 15's I've used lately. Points are perfectly clean and at .020. New NGK b7hs (they work nice in non-cd ignitions). Lower not binding at all, new grease. Any thoughts? (I'm kinda aiming this at BK THE king!) but I'll graciously accept any insights.. I'll check this tomorrow before I put it back together, and I'll definitely post back after I test it tomorrow. I REALLY like the low speed performance of this one.. It'll troll wicked slow for walleyes, but I don't know if it'd plane out w/my girl in the boat w/me, and it's nice to jet to the next spot (like the other 3 15's I've used always do). Thanks, and GO TWINS!

 

[Aratsman]

Re: 15hp lacking power

Have you tried reseting the timing? Maybe it isn't opening the carb fully. Also is it firing on both cylinders? My 9.9 idled fine but not power at WOT, had a misfiring plug.

 

[Rick.]

Re: 15hp lacking power

Sounds to me like your doing the important stuff. I wonder how old your fuel is? Also next time your in the water with it give the primer bulb a go and see if it picks up a bit. Sounds like a spark issue but you've said you have good spark. One other thing would be to pull your plugs right after a disappointing full throttle run (right on the water) and see if one is so wet it's fouled. You could have a spun prop too which you might want to mark prior to your next run. They are great trolling motors but there are times like you said where you have the need for speed. Best of luck. Rick.

 

[Tim Frank]

Re: 15hp lacking power

I've never had good results with NGK plugs in any OMC I've run.
Have you tried champions?

 

[bktheking]

Re: 15hp lacking power

NGK's on these 74-76 have been thumbed up by quite a few on this motor. Mag plate good on it, i'm sure you've heard enough times about the wear issue (although electric start is way worse). Carb- yes the diameter is bigger agreed, what about orifice, the part numbers are different between the 9.9 and the 15, we sure it's got the correct one? My 76 15 doesn't get the rpm I want either despite running 24mph on my boat, I still think it can go faster. I've often wondered if the orifice is wrong. It just doesn't wind up like a 15, more like a 9.9. I know it's not the carb, been kitted. I know it's not the ignition, I converted it to CDI. I know it's not the reeds, that was cured. It's not compression, it's 100 on each cylinder. The only thing that made RPM gains was the carb, the rest was all the same.

 

[Mork]

Re: 15hp lacking power

Also, check you tank and make sure it is venting. I have forgotten to open the vent cap on two seperate ocassions and did not even notice it until I ran wide open. Also, you may want to check the prop. Does it have the standard pitch for this engine?

Jim

 

[kbait]

Re: 15hp lacking power

Thanks for the replies! In response:
-Link/sync good. Throttle plate just opens at cam mark, and opens fully at full throttle.
-Fresh 50:1 non-oxy fuel mix, tank vented properly. Squeezing bulb=no difference. New fuel line from fuel pump to carb.
-No lateral play in Mag plate (Been there on an e-start BK..)
-Standard prop, not slipping.

I'll put the cleaned carb back on, get some champion plugs (calls for UL81C). Is this the plug I should use, or is there a better champion equivalent?

Thanks for all your suggestions, I'll post back later today after 2nd test run. I guess it could have been a slightly gunked main jet, but I checked before cleaning. I'll put a new carb base gasket on too, just in case it was leaking there, and confirm torque on exhaust plate (wasn't leaking..)
-Thanks again!

 

[bktheking]

Re: 15hp lacking power

If you say you have it off and have a 9.9 carb to compare look at the jet, I suspect the 15 would have a slightly larger orifice. UL81C's are the correct plugs for points.

 

[Tim Frank]

Re: 15hp lacking power

I was the poster who questioned the NGK vs champion plugs, but that is based on my experience with motors that I have owned....and that has never included a 15HP of that vintage.

BK pointed out "NGK's on these 74-76 have been thumbed up by quite a few on this motor" so you might want to put plug replacement at the bottom of your list of steps for now.

Of course, BK doesn't say whether he runs NGK in his....

 

[kbait]

Re: 15hp lacking power

Thanks BK.. I'll compare jets. If they're the same, I'll swap carbs w/my '75 elect 15 that I'm having trouble w/stator play (Thought I had that figured out, until I re-installed the charge coil.. That thing YANKS on the plate!). And thanks for the heads-up Tim. I'll stick w/the new NGK's for now..

Again, I'll repost w/results and thanks again!!

 

[kbait]

Re: 15hp lacking power

Hey BK, just compared main jets. The 15 carb's jet is obviously larger than the 9.9's. I'm still gonna switch carbs w/the electric start motor, since I'm messing with that one also today, and I KNOW the carb works perfectly on that one..

I'll post w/results..

 

[kbait]

Re: 15hp lacking power

I'm back from test 2. Switched carbs, and even did a decarb treatment before running on the lake. It's still lacking some power. Idles wicked slow, and eventually planes me out, but still like a 9.9hp. Definitely running on both cylinders, and it actually runs awesome. Maybe that's just what it is? Even though I know from an identical '74 15hp that it's a bit lacking..
I'm gonna take my girl out fishing tonight. If it will plane out, good enough, I guess..
Thanks.

 

[kbait]

Re: 15hp lacking power

ps.. I've always adjusted timing by the J Reeves method.. .020 point gap. I'm going to try to check the timing with my multimeter, as I've read that that's more accurate. Perhaps, if my timing is retarded a bit (like my brain, haha), that would cause the lack of power issue, and still idle like a dream. I'll search for that meter-timing procedure and try it Monday. I'll post back if it is like a dream come true! Have a great weekend!

 

[jmendoza]

Re: 15hp lacking power

You already have said what I suspect, and that is ignition advance. I use a timing light (power it with a 12 volt battery) and look at the marks on the bottom outer edge of the flywheel with the engine running, this tells you how close they are to each other, and note the large fixed mark on the stator allows for quite a range of timing. Set the timing to fire at the leading edge of the mark for best high speed results. Keep in mind that if you have new points, they will advance for the first couple hours of run time, thereafter they begin to retard as the rubbing block wears. Too much advance will slow accelleration and cause backfires at low speed. Too much retard makes for awesome idling and low end grunt.

These engines are a PIA to set the points on as you cannot check the timing with the flywheel off. OMC makes an arm that goes on where the flywheel went and has pointers that line up with the timing mark on the stator which makes things ever so much easier.

I myself ran small wires from the points out the bottom of the stator plate to my test light, or beep box and put the flywheel on to see where I got a light and beep when the points opened, then removed the flywheel and made adjustments and then re-checked, it takes a while and you have to do both as identically as you can.

I found my 15 was not up to snuff on RPM, felt slower than my 9.9, and found fine tuning the timing, and a de-carb did the trick. It took about a 1/2 hour of running to loosen up. Check your prop, be sure it's the correct size, and not bent.

HTH, Jay

 

[bktheking]

Re: 15hp lacking power

Of course, BK doesn't say whether he runs NGK in his....

It was running NGK's, then I swapped to the Champions then I swapped out the points for a CDI setup so QL77JC4's are in it now.

 

[Tim Frank]

Re: 15hp lacking power

It was running NGK's, then I swapped to the Champions then I swapped out the points for a CDI setup so QL77JC4's are in it now.

I was just kidding....

 

[bktheking]

Re: 15hp lacking power

I know!

 

[kbait]

Re: 15hp lacking power

Thanks for the tips jmen. I'll fine adjust the timing w/wires and my multimeter. Sounds like you've dialed yours in nicely, and it started out behaving like mine. I am optimistic!

 

[Tim Frank]

Re: 15hp lacking power

I read somewhere that that vintage of 15HP is the most-stolen motor around....must be a reason...

 

[kbait]

Re: 15hp lacking power

It's all better! I used jmendoza's wire under the flywheel timing, and found one point set was firing at the retarded end of the flywheel mark, and the other was right at the leading edge (advanced side) of same mark. I opened the point gap on the retarded set slightly and re-tested. It IS a PIA to get it exact, but what a difference. Planes out much faster (and 'normal' compared to other 15's I've used), top end increased too. Thank you all!

So, what I've learned is: Set point gap correctly to .020, and check actual timing point w/multimeter, and adjust point gap from there, if needed.. and opening is advancing.. Always learning something here!! Thanks!