Johnson 115 Hp only getting 4000 rpm WOT instead of normal 5100

astampfel · Apr 22, 2011

I have a 115 Hp 1990 Johnson. Last week it was running great! At WOT I was getting 5100 RPM and about 40 mph. Yesterday I took it out and I could only get it up to 4000 rpm. It idled slightly rougher, but nothing to make me concerned. I didn't take the engine cover off or performed any maintenance between the 2 runs. I live in a slow zone, so I have to transit out to the fast zone between 1500 and 2000 rpm for about 30 minutes. When I pushed the throttle to WOT, the boat barely responded. It sounded like it was going to take off, but it never did. I could only manage 4000 rpm. Sometimes when I'm at 2000 rpm and push the throttle forward, it will stall. But, it will start right back up. Any ideas on how to trouble shoot it? Do you think it is electrical? Maybe a clogged port in my carbs?

Chris1956 · Apr 22, 2011

Re: Johnson 115 Hp only getting 4000 rpm WOT instead of normal 5100

First take a compression test on all cylinders. Next take a spark test to see if all have spark. Post results back, as well as spark plug color and we can give next steps.

astampfel · Apr 23, 2011

Re: Johnson 115 Hp only getting 4000 rpm WOT instead of normal 5100

Well, I hooked up a timing light to the plug wires and found that #1 cylinder wasn't firing. So, I replaced the power pack to #1 cylinder and that fixed the problem. I just came back from the water and it ran perfectly, 5200 RPM and 40 mph, and great idle. I love it when my boat is cooperating. Thanks for the help.

emdsapmgr · Apr 23, 2011

Re: Johnson 115 Hp only getting 4000 rpm WOT instead of normal 5100

You should consider re-propping that engine. You should be running it 5500 rpm at WOT. You'll get better hole shot and better mileage.

astampfel · Apr 25, 2011

Re: Johnson 115 Hp only getting 4000 rpm WOT instead of normal 5100

I'll look into it. Do you have any advice? I don't even know what type of prop I have?

emdsapmgr · Apr 26, 2011

Re: Johnson 115 Hp only getting 4000 rpm WOT instead of normal 5100

I'd go down one full prop size. If you are running a 19 pitch prop, go down to a 17. If you are running a 21 pitch, go down to a 19.....Your corrent prop size is indicated (stamped) with the size. You should pickup about 300 rpm when you make the change. Keep the other prop in the boat as a spare.

astampfel · Jul 17, 2011

Re: Johnson 115 Hp only getting 4000 rpm WOT instead of normal 5100

Thanks everyone! I actually moved my engine one more hole higher on my transom and trimmed the boat out a little more on the water and now I can get about 5400 RPM and 44 mph on GPS. I'm a happy camper now! My prop is still underwater and my enginer still sucks in cooling water even though it seems like it is pretty close to the surface of the water.

emdsapmgr · Jul 18, 2011

Re: Johnson 115 Hp only getting 4000 rpm WOT instead of normal 5100

That's a good catch. Less engine in the water means less drag, more speed. When you get into raising the engine-you may want to consider adding a water pressure guage to the dash.

astampfel · Jul 18, 2011

Re: Johnson 115 Hp only getting 4000 rpm WOT instead of normal 5100

What is a water pressure gage? What does it sense?

emdsapmgr · Jul 19, 2011

Re: Johnson 115 Hp only getting 4000 rpm WOT instead of normal 5100

The racers that raise their engines on the transom for more speed, add a water pressure gauge to make sure the powerhead gets adequate water flow at full throttle and high trim angles (and tight corners.) A 0-30 lbs gauge is a good addition.

reddogg · Jul 19, 2011

Re: Johnson 115 Hp only getting 4000 rpm WOT instead of normal 5100

Just a thought, with you having to run at such low rpm for so long to hit the fast lane keep an eye on your plugs for fowling.

Red

astampfel · Jul 21, 2011

Re: Johnson 115 Hp only getting 4000 rpm WOT instead of normal 5100

Thanks for all the help! I'll see about a pressure gage and also watch my plugs.

Johnson 115 Hp only getting 4000 rpm WOT instead of normal 5100

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