Engine Backfiring Every So Often

[kingfishercam]

Gday raiders , i bought a 2004 mercury with very little hrs on it , olmost new rescently , engine runs great and loads of power but when in idle , every 3 mins or so it will cough and splutter for a second (like a backfire noise) then the idle seems to slow back down and it continues to run smooth again. Please help to explain to me what this would be , Is it bad or normal? If it needs attention what ways can i fix it , i was using premium fuel in it and ive only used it the once so far. Cheers and happy fishing to all. cam.

[pelican]

Add fowardfiring module to even it out!!!

Be having a quick look at the condition of spark plugs and confirming plug leads are fitted firmly. I've assumed it is a 2 stroke premix? Idle jet could be the other guess?

Edited July 10, 2008 by pelican

[kingfishercam]

[ yes , its a 25 two stroke.

[jewgaffer]

Add fowardfiring module to even it out!!!

Excellent suggestion Pelican! Maybe we can invent a forward firing module for flatulence and sell the rights to the toothpaste companies.

Cheers

jewgaffer

[caine]

could it be the cold water??

and the engine is not getting up to temp?

[a boat]

Hi Cam, if your engine is backfiring or coughing at idle you are running too lean in the air/fuel mixture or the carby butterflys are been held open slightly causes the same situation.

You will see on the your carby you have a brass coloured straight blade screw that is your low speed mixture screw. This screw adjusts the amount of fuel, in relation to air, the engines gets at idle. If all is as designed, that is good compression, good spark/fuel and the butterflys are fully shut at idle, just screw that screw counterclock wise about a 1/4 of a turn which will richen the engine up and should solve your problem.

Ideal to do this in the water with the engine at operating temp and hopeully you do have the thermostat in and operating correctly becasue if the engine is running cold that is also a cause of bad idling.

What you want is the engine to be idling as lean as possible but without it sneezing/backfiring and also no hesitation upon acceration.

If you are not comfortable trying this, bring the outboard in and we can put in our test tank and do it for you. It will only take about 10 minutes.

Cheers,

Huey.

Edited July 11, 2008 by Huey @ Huett Marine

[pelican]

One other thing. If that motor has sat unused for a long while and was stored with fuel in the carby bowl the idle jet may have picked up a small bit of gum / sediment loosened up by the new premium mix you have put in.

It would be very good insurance to have the motor checked out as above and then you know you will get lots of trouble free hours.

With a motor that has only done few hours and has been stored dry it is also a good idea to check under the cowl all the fuel line fittings for weeps and leaks and seals / gaskets at the carby. Unfortunately some (not the sponser) dealers are lazy with smaller motors and they can leave the shop never having been checked so the above offer would be really worthwhiile and the few maintaince tips you pick up will be well worth the trip.

When after being on a run stop and lift the cowl and put the back of your hand around the powerhead and it should all be warm to hot but not so hot you can hold it there briefly. If it is too cold or hot you may have a thermostat or waterpump not working correctly. Water pump impellers can also dislike sitting and for the small cost it is worth inspecting. I would also do a compression test so you know your baseline compression and then at every service do one as it can show the first signs of many possible problems along with colour of plugs. If you look after it with good oil , fresh clean fuel, flush it it will take a very very long time to wear out.

[peterd]

Hi Cam

I think as I said to you before that the motor has always been run with a very rich mixture.

It could be all the above! However the motor was not tuned for premiun fuel and is at the moment cleaning the residue out of the motor. I would, before you spend money on it, give it a good run on normal fuel and it might come back to normal. Or it might be a good idea for its first service.

The motor was always stored with out fuel in the engine.

PeterD