15hp lacks top end power

[akwormy]

Wondering if any of you have experience with a small outboard that lacks some power at the top end. 1984 Force 15hp.

If I choke it a tiny bit (not to the first detent) it picks up and seems to run as it should...push the choke back to normal run position and it picks up a tiny bit more before quickly bogging back down. Runs fine there, but just lacks that extra bit of the power band.

I cleaned the carb - no varnishing or anything ugly. The main jet is a fixed size, not adjustable. Adjustment screw for idle mixture. Replaced fuel pump diaphragm and cleaned filter screen. Fuel hose/bulb is new. Points have been cleaned and adjusted. Plugs are new and gapped correctly.

I assume that it's running lean (by reducing airflow with the choke it runs better - too little fuel or too much air??)

Thanks in advance.

 

[roscoe]

Re: 15hp lacks top end power

Wow, you've been busy working on that motor.

Check to see that the throttle shutter is fully opening at full throttle, but not going past full open.

Did you replace the gasket between the carb and intake? Sounds like it may be sucking in some air from there.

Also, what elevation are you running at, and do you remember what size jet you have in there?

 

[akwormy]

Re: 15hp lacks top end power

Admiral Roscoe! Yep...would be nice to go drop a couple grand on a shiney new one, but my time is about the only thing I can afford, so I'll keep trying to make a gem out of this old tub!

I'm only a few hundred feet above sea level here in Fairbanks Alaska. I don't remember what size that jet was - I'll look. Wouldn't hurt at all to A) replace the jet and B) buy a couple varying sizes. I'll look into that. What size jet should I have at near sea level elevations? Any high recommendations for a place to find a couple jets? I imagine they should be fairly cheap and have cheap shipping as well!

As for the throttle - I can't seem to find an adjustment or anything to do with the opening of the throttle. Although the handle doesn't quite seem to rotate fully. There is a plate (26 in the picture herehttp://www.mercruiserparts.com/Show...0&bdesc=IGNITION+SYSTEM+(84A+THRU+87A+MODELS)) underneath the flywheel that has a cam type ramp that moves the butterfly on the carb - it doesn't go all the way to the end nor does it go off the end, but from the looks of it (mark I eyeball) there isn't much if any increase left on the remaining part of the "ramp". Again I don't see a way to adjust that - I'm thinking it must advance timing, run throttle, etc...all off that plate and it sure looks fixed. There is an adjustment for idle speed, but that's all I see.

I did not replace the gasket and don't have one handy, but I'll try the starting fluid trick (spray around the gasket - if the engine picks up then the ether is being sucked in and you've confirmed a leak). Does that sound like a good enough plan or do I need to replace the thing?

Thanks for the good suggestions!

Could a leaky cylinder be the problem? Is a leakdown test the only way to find out if I have a problem? I don't have the tools for that...

What about reeds? Are there reeds in this engine somewhere? I think maybe there is a "reed plate" in the fuel pump assembly, but not sure if that's something I can clean and how it might present itself as a problem.

 

[roscoe]

Re: 15hp lacks top end power

The ether test on the intake will work.

That mercruiserparts website is the place for jets. They have several sizes listed and the corresponding elevations.

The carburator page has the jets and gasket if needed.
The fuel system page has the reeds, but I don't know if that is the problem.

It sure seems to be a fuel delivery problem. Hmmmm. (thinking).

Next time you run it, take a small spray bottle and put some premixed fuel in it.
Squirt it in the carb throat a little at a time while at full throttle. See if it helps, or floods out the engine.

Also, maybe would help to run some Seafoam Deep Creep or equivalent Mercury or Yamaha product through the engine to decarb, possibly loosen up a stuck ring.

 

[akwormy]

Re: 15hp lacks top end power

been a little disapointed with local shops on products like that - had a hard time finding hypalon patch material, hard time finding large oar locks, hard time finding decarb fuel treatment...

I've tried pumping the fuel bulb while at WOT and it doesn't have a noticeable impact, but I'll try the little fuel direct to the carb trick tonight.

I found the jets on that site after I asked! It's a good site - although way to many "not available" listings on this unit! I haven't done the spray test or pulled the carb yet (it's been sunny with salmon in the river so guess what I've been doing instead of working on the outboard!) but I'll be real surprised if it's not the 0'-2500' unit....the engine has been in AK since new and shouldn't have ever needed a high altitude jet.

It did sit for a number of years before I got it. When I cleaned the carb I just took it apart and soaked it. Should I jam a small piece of wire or something through the jet and other passages in case there is a small blockage?

I keep hearing more and more that it is likely a leaky cylinder/crankcase...someone explained to me that when you choke it you not only reduce airflow, but you in a way increase the vacuum (proper term for the compression on the down stroke?) and would pull more fuel through the vent - which further corrects the lean problem...you reduce airflow - increase fuel flow and bam...running like a champ.

I'm hopeful that the jet is gummed up...doesn't get much cheaper or easier than that!

Hope to post back a bit later with results. I guess if none of this works I'm in the hunt for a local shop to do the leakdown test - I guess that's the only sure way to detect a leak that isn't picked up on the compression test.

I've made a lot of progress on this engine...from not running at all with a broken recoil starter - it's come a long way. Starts reliably If I can get it this last little bit fixed I'll be my own hero!

 

[imported_LakeRunner]

Re: 15hp lacks top end power

You can order oar locks from West Marine online. Overton's and West Marine carry Sea Foam/Deep Creep on the marine side and Pepboys, Carquest, Napa and Advanced Auto parts carry it on the automotive side. It's cheaper at Advanced than the other places. It is out of stock on the iBoats site so that should tell you how good it works. If you can't get it locally, you can order it thru the manufacturer at http://www.seafoamsales.com/

 

[imported_LakeRunner]

Re: 15hp lacks top end power

Might need those oar locks if you can't get that engine running!8)

 

[imported_LakeRunner]

Re: 15hp lacks top end power

Rats! I forgot, they don't sell it online but they will help you find a source as close as possible to you. Just email your address to customerservice@seafoamsales.com.

 

[imported_LakeRunner]

Re: 15hp lacks top end power

BTW, how did the reeds look? If the are worn or out of adjustment, this could also cause the motor to lose top end. The reeds should be inside the intake manifold just behind the carb.

 

[akwormy]

Re: 15hp lacks top end power

no experience with reeds at all - do you need to regasket if you pull the intake off? Am I looking for anything special or just general scummy vs. clean? What about "out of adjustment" any basic info on that?

Is this a dummy-type thing that you just pull off and look at or is it a complex inspection/adjustment? I'm no engine dummy, but have zero experience with reeds so don't want to just go around pulling stuff!

Thanks!

(re: oar locks - that was definitely the case early on! But now she runs fine except the lack of top end power needed to jump up on plane empty and the power needed to climb on plane with a load. With it choked a hair it climbs on plane at pretty much near max weight...without it - it just plows....starts easy and overall runs pretty decent for an old engine!)

 

[imported_LakeRunner]

Re: 15hp lacks top end power

You need to have a good service manual to do this. But the only special tool you will need is a gap feeler. It's a tool that has small blades at different gauges. You need to get the specs for the gap opening of the reeds. You can visually inspect the reeds and check for wear or breakage once you have removed the carb and intake mainfold. See your service maunual for the proper procedure.

 

[akwormy]

Re: 15hp lacks top end power

ok - got plenty of feeler gauges in the box. I'll swing by the library and borrow their manual again...

lots to do and no time to do it - (unless I skip fishing tonight, but the motor runs good enough to get me down river and back, so that ain't happening!)

 

[roscoe]

Re: 15hp lacks top end power

Every outboard manufacturer has their own branded version of a decarb spray. If you have a Merc, Yamaha, or Evinrude/Johnson dealer near by, they have it.

........."although way to many "not available" listings on this unit! "

Those are all factory parts on that site.
If you need something that is "no longer available", the aftermarket manufacturers may have it.