rpm drop and rise, rough idle

[banki_21]

I have a 90hp Evinrude just had the carbs rebuilt about two weeks ago, and yesterday all of the sudden I go to get on plane and my engine bogs down to 1200 rpms less than wot then jumps back up, and continues this process, its acting like its out of gas. Also it idles rough, coughs, spits and when I go to from forward to reverse real fast the motor dies or from a high idle to real low it will die.<br />I changed out my fuel filter just got new fuel lines about a month ago and changed spark plugs. <br />Any advice on where to go next?

 

[Joe Reeves]

Re: rpm drop and rise, rough idle

If that up & down surge is a slow type surge, in all probability there is a fuel/air leak between the fuel pump and the fuel supply somewhere (allows the fuel pump to draw air) or possibly the fuel pump is failing.<br /><br />A sharp instant type surge would indicate a ignition problem. Which type problem do you have.<br /><br />Also, it would help if you listed the year of that engine.

 

[banki_21]

Re: rpm drop and rise, rough idle

Its a 1991 and has a sharp surge.<br />I took it on the water today and found that one cylinder isnt firing consistently, is that a powerpack or coil issue?

 

[fireman57]

Re: rpm drop and rise, rough idle

Yes it is. The powerpack is probably shorting to the block. Replace the one you need and you should be good to go. You might check your ground to it first and clean it. The plug wire also might be loose inside it. Was the wire tight to the plug to start with?

 

[Joe Reeves]

Re: rpm drop and rise, rough idle

A easy matter to test the coil. Simply reverse the wires leading from the coil(s) to the powerpack. If the faulty coil now fires, the coil is okay. Be sure to return the wires to their normal position.

 

[banki_21]

Re: rpm drop and rise, rough idle

I replaced a weak coil and replaced the spark plugs and took it for a test on the water the problem still remains. <br />I pulled the plugs and discovered the same cylinder is running lean again, I had the carbs rebuilt by a reputable mechanic and have only had the motor back a couple weeks now. The motor ran great then all of the sudden this week it slowly started dropping rpms now to the point it bounces from 4500 to 5100 every 5 to ten seconds.<br />Any ideas where this continuous problem is coming from?

 

[Solittle]

Re: rpm drop and rise, rough idle

If your mechanic is truely reputable he should stand behind his work. Give him another shot at it.