1989 mercury 90 hp (3cyl) missing

[philipjames5k5]

I have got this ninety horsepower mercury that starts right up and idles fine and pees great. Compression is good with new plugs and a 2 inch spark jump on each cylinder. The problem is that when put in gear and given half throttle the motor misfires and gives loud backfires. Put it back to low rpm or neutral, and it runs fine without a hiccup.

Please any info to help trouble shoot this or narrow down the issue please help.

 

[GreenHornet763]

Re: 1989 mercury 90 hp (3cyl) missing

Have you cleaned your carbs? Seems like your High jet is clogged

 

[philipjames5k5]

Re: 1989 mercury 90 hp (3cyl) missing

I took them apart and cleaned them with carb cleaner, and changed the gaskets. A friend also told me that the high jet is clogged. which is the high jet and how do I unclog it. I am not familiar with carbs.

 

[oldman570]

Re: 1989 mercury 90 hp (3cyl) missing

The high speed jet is a fixed jet and is located under the brass screw/nut on the side of the carb bowl. It should be cleaned and blown out with air to clean it. Have you changed out the spark plugs to see if one of them is bad and will not fire while under compression? Are you sure that a coil or plug wire is not shorting out at higher RPM's?
Oldman570

 

[Texasmark]

Re: 1989 mercury 90 hp (3cyl) missing

The high speed jet is a fixed jet and is located under the brass screw/nut on the side of the carb bowl. It should be cleaned and blown out with air to clean it.

I don't know what carbs you have as Merc lists a lot with that series of engines. On my 2002 90 with 2B2 carbs the plug is at the very bottom of the bowl.

First of all a 2" spark gap fires.....where? Second backfire has to do with spark and possibly a bad timing setup....aka linkage needs a proper setup. What's the maintenance history on your engine?

Mark

 

[philipjames5k5]

Re: 1989 mercury 90 hp (3cyl) missing

The high speed jet is a fixed jet and is located under the brass screw/nut on the side of the carb bowl. It should be cleaned and blown out with air to clean it. Have you changed out the spark plugs to see if one of them is bad and will not fire while under compression? Are you sure that a coil or plug wire is not shorting out at higher RPM's?
Oldman570

I am not sure if the plug wire is shorting out under high rpms, how can I trouble shoot this? The problem started a month or so back, and changed the plugs so I do not believe its the plugs. Ill clean all my jets and see.

 

[philipjames5k5]

Re: 1989 mercury 90 hp (3cyl) missing

I don't know what carbs you have as Merc lists a lot with that series of engines. On my 2002 90 with 2B2 carbs the plug is at the very bottom of the bowl.

First of all a 2" spark gap fires.....where? Second backfire has to do with spark and possibly a bad timing setup....aka linkage needs a proper setup. What's the maintenance history on your engine?

Mark

Maintenance history is:
Bought the motor a year and a half ago, got it tuned up and checked over and ran fine.
Sat for three months, cleaned the carbs and ran fine again.
then one weekend I ran it and when making a run across the lake i felt like the motor was missing so putted back to the boat ramp took it home put muffs on it and cranked her up.
motor idles fine and runs at real low speed any more throttle than 1/4 and it starts to back fire. So I checked the compression 90/90/90 cold and 115/112/117 semi warm. And changed the plugs. problem still persists.

 

[Texasmark]

Re: 1989 mercury 90 hp (3cyl) missing

Backfiring is an explosion in the cylinder on the intake stroke when the intake port is open to the intake manifold. Since the magnets in the flywheel aren't aligned with any trigger coil at that time with the correct timing setup, it could only be glowing deposits probably on top of the piston. Pop the plugs and look at the crown of the pistons. What do you see?

Other thing could be your timing setup, but once set, if set correctly and the engine runs properly I wouldn't think it would just go whacko all of a sudden. You need a cause to get the effect.

High speed problems are usually related to spark plugs I have been taught all my life. Course in that can be the spark producing components to the plug.

Maybe you have a wire chafing that comes from under the flywheel. Might have a look at them as you rotate the throttle and see if any are chafing.

Otherwise I don't know. Maybe one of the professional mechanics will jump on here and give this thread some lift.

Mark

 

[oldman570]

Re: 1989 mercury 90 hp (3cyl) missing

Most times when the carbs have been worked on, a link & sync should be done to make sure that everything is in time. Here is a link that you can use to check it out, http://forums.iboats.com/engine-fre...q/timing-sync-merc-inlines-1988-a-168855.html
JMO
Oldman570

 

[Texasmark]

Re: 1989 mercury 90 hp (3cyl) missing

Most times when the carbs have been worked on, a link & sync should be done to make sure that everything is in time. Here is a link that you can use to check it out, http://forums.iboats.com/engine-fre...q/timing-sync-merc-inlines-1988-a-168855.html
JMO
Oldman570

Great idea. If that year model engine sets up like my 2002 90 does, if the link and sync are off you can have that problem......having he spark plug fire with the input port open. Idle timing is just a few degrees BTDC and WOT is 20 to 22 BTDC depending how you measure it....big difference and link and sync sets it and the carb butterflies up synchronously.

Mark

 

[philipjames5k5]

Re: 1989 mercury 90 hp (3cyl) missing

Thanks guys, I may just take it to a mechanic and get it synced. Seems a bit out of my realm. I can pull a subaru wrx motor and rebuild it with new pistons and rods but this is all new me. I appreciate it. Does any one know of any other syncing write ups or videos that may give me a better understanding ... or was the one that oldman posted pretty straight forward for most of you ??? hahaha