Back firing outboard

[Moonshiners]

We have a 8hp 2/s outboard that runs perfectly, ideling, forward and reverse gears. But after a recent viewing/test (the boat is for sale) -

It was found to back fire while revving at around 1/2 throttle in neutral every 20 or 30 seconds when left in this position for a few minutes, this worried the prospective purchaser, does anyone know technically why the engine should backfire while doing this, to be honest i don't usually sit on my mooring spinning my motor over at half revs for minutes on end for no reason, but if i can find out a technical explanation it may ease the prospective buyers worries.

It is just that 2 strokes are not designed to rev freely while not under any load for "long" periods of time?

Recently serviced 90's engine which in all other accounts runs perfectly IMHO.

 

[dukes4monny]

Does it backfire when run under load? My guess is that it is running a bit weak (mixture) which maybe because it is running under 'no-load'.
Is it an actual 'backfire' or a 'pop'?
If it's a 'pop' and it doesn't do it under load then I wouldn't worry about it......if it's a backfire then it could be a plug or timing problem............

The 'pop' occurs because if the mixture is weak, there isn't enough fuel to cause combustion on every cycle.....this causes a build up of unburnt fuel in the exhaust.......the next time you do get combustion in the cylinder, this ignites the fuel in the exhaust which causes the 'pop'.
A backfire however, is caused by an extremely late / early spark.......

HTH

 

[VicS]

The first thing to do is check/clean/reset but preferably replace the spark plugs (You carry a spare set anyway don't you?). You don't tell us the year but if it is old enough to have points ignition do the same for the points.

If it is backfiring and spitting back through the carb it may be what is known as a lean sneeze. This is due to a slightly lean mixture due to dirty jets, incorrect mixture adjustment or low fuel level on the float chamber, in turn due to flow restriction or faulty pump. Pumping the priming bulb gently should help diagnose pump problems or flow restriction. Dirty jets is the most likely cause, the solution to which is clean, clean and clean again.

Read these two recent threads HERE and HERE

 

[Moonshiners]

Sureley both above conditions would effect the running while under load?

It only misses or "pops" I don't think it's backfiring thropugh the carb, just popping in teh exhaust. When running mid revs in neutral.

It runs perfectly when under load in Fwd, Reverse and ideling.

Sure I carry spare plugs, but if it was fouled plugs wouldn't it run madly when in gear also?

I'm convinced it's a "load" issue, but not sure how it mechanicaly equates to the problem as i'm used to bike and car engines.

 

[VicS]

OK its misfiring. I'd still start by replacing the plugs. If you know 2stroke bike engines well you will know all the likely causes.

The best place for outboard advice is The iBoats forums. Post your question on the appropriate board there. There are separate boards for different makes but fully identify the model and year when you post. You did not even tell us the make!

 

[DRANNIE]

why don't you offer £200 off the asking price or a service if its a deal breaker?

 

[Alfie168]

Check you have the right plugs..and as others say replace them (i'd always recommend NGK), This sorts out lots of 2 stroke problems, and you have to eliminate them as the possible source of your problem anyway, so its not money wasted. Also check your fuel mix is reasonable accurate (My Suzukis had automatic oilers so I never had to mix it myself)

After you've done that and reappraised, come back to us..then we'l tell you to get it serviced....most likely /forums/images/graemlins/wink.gif

Tim

 

[VicS]

[ QUOTE ]
i'd always recommend NGK

[/ QUOTE ] I'd say fit the type the engine manufacturer recommends. Equivalents are sometimes not perfect matches. In some case there are no equivalents. Fair enough though NGK are AFAIK a good brand and if the manufacturer lists an NGK plug use it by all means

 

[Kawasaki]

Asessing a 2 stroke outboard whilst in Neutral is not good.
They will muck about and run a bit erratically whilst not under load.
VicS knows His outboard stuff and what He says rings true.
NGK is a good brand.
If the eng runs OK whilst under load, ie performing OK whilst shoving the boat.
Then it is OK.
Testing in a Tub or on the transom in Neut will give a false 'report'.
Running a 2 stroke on a 'fixed' throttle setting not under load, will 9 times out of 10 create the symptom you have found.

 

[Shuggy]

I am no expert on outboards, but I had a 2004 Mariner 90hp 2 stroke for a couple of years, and it was beautifully quiet and smooth while under load in the water, but always popped when out of the water & being flushed through. My old 1990 80hp Mercury 2 stroke was exactly the same.

 

[Moonshiners]

Thanks guys, I though it must be pretty normal and not worried at all, nor really looking for a solution, just wondering why it happens mechanically, what is it about putting it under load that changes it?

 

[cliff]

Concur. Runnig a 2t in neutral normally causes it to "misfire" - weak mixture due to throttle opening and no load.

Running under load in a drum also not ideal. It does not sound to me as if there is anything wrong with the engine if it runs well under load fwd and rev and ticks over nicely in neutral.
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