Logo
FULL SITE MENU

1988 90 hp Weird idle

D

donaldboyer

Contributing Member
Hello!!

Mercury 90hp
1988
s/n 0b312461

Recently had our boat on holiday for a couple of weeks and part way through this time a problem started with the motor. Wouldn't idle very well...

Symptoms: Motor stalls at random times (2 - 3 times per min) when at idle (in or out of gear), and needs plenty of throttle to restart again.

When it is running at idle every few seconds or so it does a funny thing thats hard to describe... kind of sounds like on cylinder gets an extra bit of "oomph" and revs jump every so slightly (maybe 50 - 100 rpm) for a second or two then settle back at 900rpm. Then it'll do it again after a few seconds (maybe 7 - 10 secs between), and thats if it doesn't stall!

At revs, this motor runs perfectly.
Idles at 900rpm in water
Has fresh fuel.
Used a can of Power Tune in it less that 12 months ago
Motor was fogged for winter
Battery is less than 12 months old
Motor is producing over 13v for charge

So I'm not sure what to do about this. I haven't touched anything so far.....

Any advice??

Donald.
 
Re: 90hp Weird idle

Whenever an outboard is not performing properly a compression test is in order.------Post the results.----------------Then a spark check to see if the spark will jump a gap of 7/16" on each plug wire.---------------3 or 6 cylinder ??
 
I had a similar problem recently. It kicked my ass for weeks. Long story short, it turns out that the screws controlling the air / fuel mixture just needed to be adjusted. Can't tell you which way to turn 'em in your case, but it's a really easy thing to do.

If you don't know which screws they are, you can't miss 'em. They're flathead screws likely located toward the top of the carb and will have a spring around them.

Start by turning them in until they seat and note how many turns it takes. Mine took about 1.5 to 2 turns. Set 'em back to where they started and then adjust 'em in and out by about 1/4 turn to see if the engine behaves any better.

Hope that helps!
 
what are the results of a compression test ??------------there is no point adjusting carburetors if there is mechanical damage.
 
OK, boat is in at the shop at the moment. Compression came back 110psi on all 3 cylinders. (which was good news)

Carbs were out of sync, and there was some water in the fuel pump.

But the major is that they did the stator test which came back as failed. (resistance of the low speed part was 2.6 when the minimum spec was 3.6)

They have installed a new switch box also, as this may have been fried as the stator has been failing. I have asked them to reinstall the original and test it tomorrow when the new stator is installed to check that the problem was not compeletely the stator (no use paying $NZ350 for a switch box if I don't need one).

Opinions?

So yeah, looking at a rather large repair bill. Not good.
 
Please post results of the repair work.----------------- There is a need for shops to generate revenue , many shops do this in a fair and efficient manner.
 
Last edited:
OK, got her home today. New Stator, new switchbox. Carbs cleaned & tuned, Fuel pump kit done too. Running better I think (hard to tell on muffs) Bill came to $NZD1400 ($USD1030)

However, tell tale water got quite hot within a minute or two (could barely hold my hand under the stream) head was very hot to touch but not burning. Shut her off immediately and am taking her back into the shop as she didn't used to get this hot previously. (any idea why she getting so hot? - I use double sided muffs (water squirting into intake on both sides of lower unit), hose on full with town pressure, I did rev her but only slightly to 2000 - 2200) impellor replaced recently also.
 
Some muffs will do that. I recently tried to run a 70 hp Johnson on them and it got reallly hot. Stuck the LU in a bucket and she started pumping fine. Stuck the muffs back on --in the bucket--and she got hot again!

Jeff
 
Back
Top