low rpms and low speed at wot

[Alex Pullen]

ok, so heres my story; i own a 1984 16ft trophy bass boat(fiberglass made by bayliner)and i bought a 1984 90hp johnson last year that was rebuilt by a reputable and established dealer. During the 20hr breakin period the tech told me not to run it over 4500 rpm and to move the throttle around so as not to keep it in one place for too long. So now that all of this is long gone i am having trouble getting this motor to go anywhere near 4500 it runs out at about 41-4200 and only has a top speed of 27mph(speedo and gps) some other shops said it could be my prop but i am spinning a 13x15 now,my other problem is at w.o.t i start to lose rpm and power(like it is missing). I can pump the fuel bulb and it will stop the sputtering, but as soon as i let off of it it starts losing power again. The stamp on the motor says i should have 90hp at 5500 rpm but i cant get anywhere near that. I have changed plugs And even adjusted my throttle stop and all that seems to do is make it idle higher and i even lost 2 mph. this is my first motor and the tech knew this when it was purchased, could there be something that he did to "tune it down" so as to protect the motor from being damaged during break in? Sorry to be so long winded but i cant seem to pinpoint any kind of specific ansewer if there is one. Thanks Guys.

 

[RAFaris]

Re: low rpms and low speed at wot

Brother, I know your pain. I just went through something very similar with my Evinrude 88. I got some good advice from JB about a week ago. Go back to around July 24th, he's got a good response to the problem. I'm no mechanic, but the problem was solved very simply. Basically, I cleaned out my fuel pump, got clean lines, and made sure my fuel tank was vented. She ran like a top after that. The Title of my question was "Evinrude 88SPL, no power". You're going to have to get your hands dirty but the extra speed and power (and piece of mind) are worth it. Good Luck...RAFaris.

 

[12Footer]

Re: low rpms and low speed at wot

As RAFaris says, it is a fuel delivery or quality problem. This condition is not very good on the motor either, so don't say, "I'll dig into during the winter",and run it like that.You may end up with holy piston syndrome, or friction failures (if it isn't getting fuel, it isn't getting oil)...<br />Unless it is oil injection.<br />Suspect the fuel.If it is older than 3 months old, it could be turning into varnish in the tank. You will be able to smell this.It will smell like varnish instead of gas.<br />Also,the entire fuel delivery system, from the tank pickup,to the fuel pump discharge will give you these symptoms.<br />Make sure your tank's vent is venting.<br />Check all these, and you will be back in the shallows again.

 

[mbb]

Re: low rpms and low speed at wot

you should be flying with a 90 on that boat. You can count on needing a higher pitch prop than you have AFTER you get the fuel problem straight. Prob 19-21"pitch.

 

[Alex Pullen]

Re: low rpms and low speed at wot

I went to a johnson shop near me today and i explained the problem to the tech there and he said from the sound of it that it seemed like it was a syncronization problem and told me to bring it in.( for $50.00 per hour of course). Just a little more info on my case I have a 19 gallon inboard tank and have tried it as well as 6 gallon portable cans and have had the same problem with both. Should i try running it with the cap rempoved completly? I have also noticed that many people here recommend not using 93 octane gas so i will stop doing that immediatley. Also to get more speed do i need to go up in pitch or down(and what will me rpms do?) this is really confusing me again the techs want me to keep switching but none of them seem to have any free props *L*. Thanks for your help guys and i appologize for being so ignorant on this subject i just dont want to seriously damage something or get screwed by the dealers.

 

[crazyray2]

Re: low rpms and low speed at wot

Ok you need to get some details before you take this to a shop so you will know what you have and what it may cost ya....try and check <br />1. Compression on all cylinders<br />2 spark plugs on all cylinders....compare do they look the same?<br />3. fuel to all carbs?<br />4. can you run it with cover off and look down throats of carbs at speed, do this while someone else drives. Is there fuel in carb throats... all look the same<br />5. Check ignition timing.<br />6. On dockside with engine off move control to full throttle and see if everything is opening up all the way...<br />do these thingsthen you know what you have...then you can have some idea in what you are in for......