90HP Honda rough idle and misfire

[akhamsi]

I have a 2004 90HP Honda outboard with 100hrs that runs rough at idle and intermediately misfires at idle and hire revolutions. I have installed new spark plugs, fuel filter inside the outboard, water/fuel separator filter and cleaned the carburetors; and I still have the same problem. Any suggestions/recommendations on what to check for? Thanks.

[Sea Wolf]

How old is the gas coming into the filters from the tanks? Is there any indication of water in it?

Have you checked the compression on all the cylinders? What was it and was it even from cylinder to cylinder?

Did all of the plugs you took out look the same?
Were any wet, carboned up or covered with oil, etc?

If you run the motor at night in the dark with the shroud off, can you see any evidence of shorting on the ignition wiring or plugs from observing blue flashes?

Did the carburetor clean-out involve a complete cleaning out of the jets, draining of the bowls, and replacement of any necessary gaskets? How about setting the float levels and adjusting the carburetor synchronization with vacuum gauges?

Does the missing occur only with the engine in gear (with a load on it), or in neutral as well at low and somewhat higher rpm?

I wonder if you could up a timing light on the motor and test to see if each plug is firing by moving the plug lead from plug to plug to see if the misfire can be seen as a non-flash in the sequence of flashes; e.g., when the misfire occurs, does the light not flash?

Have you run any carb cleaner and/or carbon cleaner through the engine, such as SeaFoam?

Do you have a Honda Shop Manual for your model? How difficult is it to check the engine timing?

Have you allowed the motor to warm up throughly to get any condensed water out of the electrical system, etc?

Many of our fellow C-Brast will also chime in on this, I'm sure!

Here's another source to ask:

Ask a Honda Mechanic
Honda.JustAnswer.com 8 Honda Mechanics Are Online! Ask a Question, Get an Answer ASAP.

Good Luck!

Maybe someone with a manual for your motor can suggest some more leads to follow up on...

[T.R. Bauer]

[Almas Only]

Any 2004 with just 100 hours has been sitting around a LOT, and can be expected to have some gunk in the fuel system. Here's what I suggest you do to begin with. Where you end up depends on what you find.

Remove the motor cover, and with the engine at idle, check to be sure that there are no major fuel leaks around the motor head. There probably aren't, or you'd have noticed a large explosion. But, check just to be sure.

Then, put on a pair of thick leather gloves, and beginning at the top, and working down, pull the spark plug caps from the plugs, one at a time, separating the cap from the plug by a couple of inches so the spark doesn't arc across the gap, and fire the plug.

If the rpm begins to fall off, put the cap back on right away - that plug was firing, and the fuel in that cylinder was burning. If the rpm is not affected by a cap being pulled, or if the rpm drop off is significantly less than that of the other cylinders, that cylinder was not firing anyway, and you have a spark or fuel problem associated with that cylinder.

Hopefully, once you've done this drill, you will know which cylinders are firing, and which are not. Repeat this drill a few times, and you will know whether the problem is consistent with one or more cylinders. Now, you can form a few conclusions.

First, the gas you're pulling from the tank is good. It is consistently burning in three of the four cylinders, and should be burning in the fourth.

If you changed the plugs, and are observing the same condition, you probably don't have a problem with the plugs. Although you could have an electrical problem with the spark to the affected cylinder or cylinders, I would probably look elsewhere, first. My reason is that you have a near new motor, and electrical problems don't generally result from low use. Fuel problems, on the other hand, seem to come with low use.

Assuming you have a fuel problem, I'd crack the drain screw at the bottom of the float chamber of the affected cylinder or cylinders, and let the fuel drain out onto a rag. Then retighten the screw. While you're there, you probably should drain the other three, given the low usage hours. Then reprime and restart the motor. If the problem goes away, you had some gunk or water in the bowl.

If the problem persists, figure you've got some plugging in the carb. Dump some Stabill (sp?) into the fuel, and run the motor for a few hours.

Let us know whether the problem goes away, and we'll go from there.

Good luck.

[Matt Gurnsey]

We have a customer with a smaller carburated Suzuki with a similar problem. ANother low hour engine that had sat a lot. Corrosion in the carb was the culprit, and the carbs had to be replaced, as they could not be cleaned out.

We recommend running Sta-Bil marine (the blue stuff) all year long. Sta-Bil has a regular concentration and a winterization concentration. We figure it's cheap insurance, along with a fuel / water separator to maintain that motor on the back of the boat.

[cbull]

my 2004 honda is doing the same thing. After a complete carb. rebuild still have the same problem just not as bad. Easy fix is fuel injection.

[Sea Wolf]

[breausaw]

Might want to see if Corn ethanol is mixed in with your gas; it’s not recommended for outboards.

[cbull]

I should have said a brand new shiny fuel injected 90hp suzuki. The 10% ethanol is causing lots of problems. Star tron they say is helping. The shop says with my 90 they can not get the vacuum the same on all carbs. They also said ethanol causes any water is the tanks to suspend in the fuel causing it to by-pass the f/w separator. I do not buy this. I think there are many shops out there using the fuel issue to rebuild carbs prematurely. Always try sea foam treatment before taking to shop. This could save $$$$$$$$$$$$$$$$$$.

[MikeMac]

If it will run, I'd dump a can of SeaFoam (available at most auto stores, about 9 bucks) in 6 gallons of fresh gas and run it. If this treatment is going to work, you should get some indication of improvement after 15 or 20 minutes, but it may take a few hours running time to clean up completely. My 75 is pretty finicky if it's been getting little use, or sitting for more than about three weeks. The SeaFoam can reads like one of those impossible miracle cures, but amazingly it does work!

Mike

[T.R. Bauer]

[T.R. Bauer]

[Sea Wolf]

[Wefings]

Check the tiny emulsion tubes in the carbs . When they clog , they split and the motor wont run right even after a "carb cleaning" Its not obvious unless you deal with alot of Hondas . Then you always look for it .Once its running right draining the carbs is the right answer ! New fuel sucks .
The Suzuki carb problem with the small ones [25s and 30s]is the needle and seat in the carbs [I know, I have one and it took a long time and our super sleuth and carb master Alan to figure it out] They were bad on a brand new set of replacement carbs right out of the box
Marc!

[T.R. Bauer]