1991 90HP Mercury won't idle on 3 cylinders

[Chun-Mu]

Hey all,<br /><br />I have a 1991 90HP Mercury that I Cannot get to idle. I have to start it with the throttle half open viewing the fact that this is THE ONLY way it will start. As soon as it starts (usually takes various attempts) I warm it up at 1500 RPM’s. After a minute or so I try to easy throttle to check if it idles but it ALWAYS stalls. Yesterday I was checking the plugs for spark when I grounded the number two plug; it fired right up!! And had a smooth idle, I’ve read in other posts that the 3 cylinder loopers idles better on two cylinder instead of all three but this lead me to believe the plugs are not firing in the correct order.<br /><br />Previous to this (when I started the engine at high idle) it always had this strange sound from the carbs like someone is doing a gargle . But yesterday when running on cylinder 1 and 3 this disappeared. <br /><br />I’ve done all the basic maintenance: rebuilt carbs, checked the reeds, lines, filters, fuel pump, new plugs, cleaned wires, decarb. I also checked the electrical components; all tested out ok. I have the red stator kit. Timing is also ok at 4 BTDC<br /><br />I have the Clymer manual; the swithbox is wired to the coils as follows: <br />Top-cylinder 3<br />Middle-cylinder 2<br />Bottom- cylinder 1<br />The rest is color coded so no problems there. <br /><br />Any help is greatly appreciated and sorry for this long story it’s just that I’ve tried EVERYTHING with NO results.

 

[Laddies]

Re: 1991 90HP Mercury won't idle on 3 cylinders

It sounds like a switch box problem, ck the stator and trigger with a OHM meter, if you have a DVA you can ck voltage with the engine running (thats a more relieable tet but most people don't have a DVA)

 

[Chun-Mu]

Re: 1991 90HP Mercury won't idle on 3 cylinders

WOW that's almost an instant reply; thank you laddies.<br /><br />I already Ohm tested the stator & trigger; both are fine. <br />Can I switch the wires from the trigger on the swithbox and to the coils to check if the problems shifts to another cylinder?

 

[Chun-Mu]

Re: 1991 90HP Mercury won't idle on 3 cylinders

I swapped my switchbox with one of the older mercs inline six cylinders to no avail, it's good that I had the chance to try before buying a new one. My next suspects are the coil and wire of No. 2 cylinder.<br />Am I on the right track here?? <br /><br />Any suggestion is greatly appreciated

 

[Chun-Mu]

Re: 1991 90HP Mercury won't idle on 3 cylinders

Hey All;<br /><br />I tested the plug and coil; they are both ok.<br />Still thinking it’s the firing sequence that may be at fault.<br /><br />I tried looking it up in the manual (Clymer) but couldn’t find it<br /><br />When facing the Power-Pack there are 3 leads that go to the coils. <br />Top goes to coil for Cylinder # 3<br />Middle goes to coil for Cylinder # ??<br />Bottom goes to coil for Cylinder # ??<br /><br />Any help is greatly appreciated

 

[ricksrster]

Re: 1991 90HP Mercury won't idle on 3 cylinders

Here is a wiring diagram. http://www.home.earthlink.net/~richard.palmer2/90 diagrm.jpg

 

[Chun-Mu]

Re: 1991 90HP Mercury won't idle on 3 cylinders

Hi Folks:<br /><br />It turned out to be the rebuilt fuel pump that leaked fuel passed the vacuum line and into the # 2 cylinder. It guzzled the fuel like crazy!!! <br /><br />Is the oil inlet supposed (@ the fuel pump)to be free flowing or restricted, the reason I'm asking this is that I have a spare pump; on the spare I can blow thru it fairly easily and on the rebuilt pump the inlet is somehow restricted; which on is correct?? <br /><br />The motor idles smooth now...But am afraid that with the restricted oil inlet the motor is not getting suficient oil..<br /><br />Will launch the boat this weekend to check how it runs.