'96 88spl rough at idle, less than full power.

[wcheney]

Purchased this boat last year (Sunbird 170, 17ft bow rider), engine has never seemed right. Engine runs somewhat rough at idle and suddenly smooths out about 1800 rpm. Slow to get on step, maxes out at about 4100 rpm with two adults on calm water. Barely pulls a single 180# skier out of water on two skies, will not pull a skier out on a single ski.

What I've Checked:

Compression, 120-121 psi on all four pistons.

Rebuilt carbs twice.

Replaced plugs.

Changed prop from an Evinrude 13.75 x 17 pitch to a Rubex 13.5 x 15 pitch.

Drained/ replaced fuel.

Pull plug wires one at a time at idle, lower wires engine almost dies, upper wires slows engine a little and runs a little rougher but not as much change as removing lower wires.

Hand over carb mouths at idle. engine slows and bogs, upper and lower.

Suggestions as to what to check next?

Thank you!

 

[steelespike]

Re: '96 88spl rough at idle, less than full power.

I can't be specific about the 88.
I think I would get a spark tester see if it will jump 3/8 to 7/16" using a factory service manual trouble shoot the ignition.
If the carbs are the same you might swap them and see if the plug wire reaction is the same.

 

[emdsapmgr]

Re: '96 88spl rough at idle, less than full power.

When you do the spark drop-test, the rpm's should reduce equally when you pull any of the 4 plugwires while the engine is running. From your comments, the lower plugs may be firing normally, the upper two may be intermittent or weak-firing. You can attached an inductive timing light to each plugwire when idling on the trailer. The flashes from the gun will show you the quality of spark on each plugwire. Compare the flashes from the bottom two cyls with the top two. Sounds like something is not right. Usually the coils are pretty reliable. That leaves the stator, power pack and timer base. All big $ items.

 

[wcheney]

Re: '96 88spl rough at idle, less than full power.

Thanks both for the suggestions. First a spark test. Have to borrow an inductive timing light. If those are inconclusive I'll try swapping carbs.

Is there a referral forum for outboard mechanics? I'm in Dutchess County NY.

 

[wcheney]

Re: '96 88spl rough at idle, less than full power.

OK, linkage is not the same on the two carbs, could swap but they would probably not be in sync.

Put a spark tester on each of the four coils. sparks look good on all four.

Timing/ magneto issues should affect both banks equally no?

I'm thinking of pulling the top carb apart a third time.

Could it be the carb sync? Not real familiar with outboards in general or multiple carbs (had a '69 MGB with twin SU's) as a kid.

 

[V153]

Re: '96 88spl rough at idle, less than full power.

Have you inspected and/or rebuilt the fuel pump?

 

[emdsapmgr]

Re: '96 88spl rough at idle, less than full power.

At idle, the carbs draw fuel through small, dedicated idle passages. If they get plugged/restricted over the years, it will absolutely affect the idle. Once you crack the throttle even a little bit, the carbs will draw fuel from other jets and it may run much better. You may have to tear the carbs down again and concentrate on getting the idle passages clean. An aerosol Gumout type product works well. That engine should run 5500 rpm's at WOT. If it won't, you may have the wrong pitch prop on the engine. What pitch are your currently running?