# (Noobie here) What causes an outboard to backfire?



## KAI

ok hello guys and gals,

I think i got a 1990 2 stroke 15HP mariner outboard motor, havent started it for like 4 months. i just started it, it runs good but seems to backfire when in high idle or when i throttle up. just wondering what cuases it to backfire? i am new and have no knowledge on outboard motors
my guess is: "could it be the spark plugs?" any help would be apprieciated!

on another note: how often should you start your outboard if you dont use it for a long time? once a month?

i can post pictures or video if need to!

THANKS TO ALL THAT CAN HELP.........

KAI


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## ben2go

Could be fowled spark plug(s),bad ignition parts,ignition timing could be off, if it has adjustable timing.If,major if,the exhaust has unburnt residual fuel in it,it may back fire.Lean running condition caused by dirty carb(s).Has any work been don to the motor recently?I don't run my motor over the winter.I do drain the fuel out of the carb(s) and fuel lines.I also spray WD-40 in through the spark plug holes.I also demount my outboard and store it in my building.


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## KAI

thanks ben2go for the quick response!

it very well could be unburnt residual fuel in it, a dirty carb, and old spark plugs! 
could it also be old fuel mix? because the fuel mix i have is pretty old like over 7 months to a year old. how long can you keep the mixed fuel for?

how long should i run it for? i think i ran it for like 10-20 mins. im going to run it again tomorrow to get all the old unburnt residual fuel to see if the backfire stops. 

does it hurt to run it too long? whats the longest should it be ran? 
so, everytime i run it i should disconnect the fuel line and let it burn the left over fuel in there?
to answer your question: no, there has not been any work done to it at all.

also it has been stored in my garage the whole time.

thanks again


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## ben2go

Don't run a 2 stroke motor out of fuel.It requires the oil gas mix to lubricate the internal parts.Gas is a shady subject.I don't use gas over 2 or 3 months old.Some people have kept gas a lot longer with no problems.Your motor is water cooled.If you have water muffs or a garden hose adapter to connect a water source,it's ok to run the motor when it's not in a body of water.Only run the engine for short periods,10 to 15 minutes.I'm no specialist but this has worked for me.Obtaining an owners manual may help.Since you have kept your motor stored inside,it shouldn't need much work.The things I would do,would be replace the spark plugs,make sure the gap is correct.This is very important on 2 strokes.Drain and refuel with fresh fuel and new oil.Try to avoid ethanol blends if possible.Ethanol is hard for a 2 stroke to burn,it eats up rubber parts and gaskets,it also has a tendency to absorb water.Hope this helps.


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## KAI

thanks ben2go for the quick responses!

is there any one else who would like to give me more input?

i just want to hear from more than one person to answer my question.
ben2go dont get me wrong or anything i love that you respond so fast and with knowledge so keep it coming if u want. i would just like to hear from more people.

thanks


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## ben2go

I understand.It always good to have a members advice or opinion backed up by another member.


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## ACarbone624

I have the same sort of problem with mine. I haven't used my boat much in the past 4 months and when I started it up over the weekend it was kinda backfiring/stumbling a little. It was a PITA to get started too. It never did it before so I'm guessing its the ethanol and old gas causing my problem. After it warmed up it ran better but still had the same problem. My plugs still looked good. I changed them this year. I guess I will see in the spring when I fire it up.

Over the weekend I fogged it, changed my lower unit lube and put it away for the winter. We have a freeze warning for tonight. The cold weather is coming fast!


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## papasage

sounds like it needs a carb cleanning .ifit has a idle air ajustment try opening it a little counter clockwise . that sounds like a leanpop . i have run gass 6 monts old . never let a outbord sit that long


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## KAI

thanks guys i feel alot better now! hopefully when i change the spark plugs, use new gas mixture, and clean the carb, the backfire will go away.

ACarbone624- yes it never happened to me before so i think we got both the same problem.


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## FishinNC27549

I would have to say that it is good idea to run the fuel out of a 2 stroke or 4 stroke after a day of boating or before storage. When the fuel evaporates from the carb of any engine with fuel remaining in the carb. The unburend fuel will leave biuld up of gunk. This is why most of the time a carb cleaning is needed. :?:


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## ACarbone624

How would I go about cleaning the carb? Do I need to dismantle the entire carb?


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## ben2go

FishinNC27549 said:


> I would have to say that it is good idea to run the fuel out of a 2 stroke or 4 stroke after a day of boating or before storage. When the fuel evaporates from the carb of any engine with fuel remaining in the carb. The unburend fuel will leave biuld up of gunk. This is why most of the time a carb cleaning is needed. :?:



The engines internals need the lube from the fuel oil mix for the next start.So running a 2 stroke out of fuel may cause excellerated engine ware.Yes,it's true that fuel evaporating leaves behind residue,but 2 strokes use oil.The oil doesn't evaporate and does gum up the carbs internals.Most boat motors have carb bowl drains with a single bolt/screw that can be removed to drain the fuel.I can winterize my motor in 10 minutes.loosen the bowl screw,let the fuel run onto a rag,while I pull the spark plugs to fog the cylinders with WD-40,and check the lower unit for water in the gear oil(1 screw in plug).


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## ben2go

ACarbone624 said:


> How would I go about cleaning the carb? Do I need to dismantle the entire carb?



This is what I try first.I remove the carb(s) from the engine.Then I take the float bowl off,usually this is one screw in the bottom of the carb bowl.The float and needle have a small roll pin holding them in,it pushes out with a finger.Next I turn the carb upside down in a coffee can filled with enough clear kerosene to cover the carb.Cover the top of the can.Let set over night.The next morning I sit the carb right side up in the kerosene and cover the can up.That afternoon I shoot WD-40 through the little orifices and dry it really well.Then I reassemble in the revere order it came apart.If you try this,don't used kerosene with red dye.It will leave a red residue in the carb.don't put a tight lid on the can.If it goes through a quick heat cool cycle it can blow the lid off or suck it in.This may allow contaminents to get in the kerosene and carb.This is worth a try.If you disassemble a carb you may damage a seal that may require a rebuild kit to replace it.The bowls have a cheap common O ring and the bolt has a copper or aluminum washer.Some have a paper type washer.No big deal most auto parts or hardware stores have these.


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## willfishforfood

This might not be related but 2stokes have fixed ring so they will build up carbon. I use a additive called ring free from Yamaha that keeps this problem to a min.
A cleaner motor will last a lot longer as my 86 Suzuki 8 hp runs all day trolling with out any hickups. I also only keep my gas for 2 months max if it sat.
My poor lawn mower has only run on mix for 10 years and still runs good. My main power is oil injected so I just run that gas in my truck.
WFFF


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## ACarbone624

willfishforfood said:


> My poor lawn mower has only run on mix for 10 years and still runs good. My main power is oil injected so I just run that gas in my truck.
> WFFF



I do the same thing.....It seems to foul the plugs if I use it straight.....I have to mix mine with fresh gas.


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## bobessary

just fyi about fuel the ethenol gasoline mixes only have a shelf life of about 90 days but the regular 100% gas last longer in most cases


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## papasage

you can never run all the fuel out of a carb . you run it till it quits there is just a little left in the bottom of the carb. bol. it will dry up in a few dayes . if you leave it full it will take weaks to dry up .if storing for a long time drain the bowl.for a few monts just pump it full evey month add staybill .i have herif using marvle mystery oil .


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