2003 mercury 8hp two stroke backfire pop smoke

donfielder

Recruit
Joined
Jun 20, 2011
Messages
4
i have a 2003 mercury 8hp that i use only a few times a year. i have always flushed the engine(salt water) after use and run the fuel out of the engine after each use. this year when i first used the engine it ran great and started easily. as i used the engine starting and stoping it seemed fine. however, when i started back to the dock on a fairly long run (1 mile) the engine ran a short distance fine and suddleny felt like it lost a cylinder. the engine was extremely down on power to the point that i felt that if i let up on the gas it would likely die. i made it back to the dock and found that it did not die but did not recover. i inspected the plugs and they looked fine. i replaced them anyway. later i took the boat back out but stayed close to the dock. it seem to run fine. the next morning i went back out and found the plugs must have helped but now the motor would still loose power but would backfire and seem to recover. i found that if the engine was cool it would run fine. when it warmed up it would start backfiring.
i took the thing home and have replaced the coils and the switch box. in a barrel it runs fine cold but when it reached operating temp is started backfiring again when i reved the engine. i removed the carb and cleaned and rebuilt it with a new fuel filter. the motor idles smoother(i thought it idled ok before but i could hear it idle smoother) the carb was clean but the fuel pump diaphram was streached out. the manual says this diaphram must be totaly flat. i then got out a timing light and checked the timing. it says at wide open in gear it should be at 36 degrees. the timing is good. i did not check the base idle timing. maybe this is part of the problem. i do not know.
i finally got the engine to stop backfiring by turning the idle screw out another full turn( 2 1/2 turns). if i turn it back in to where it should be it starts backfiring again. with it turned out to 2 1/2 it feels like the engine is running rich at higher rpms. it feels like is has a slight drag or bog at higher speed(half throttle). it is tollerable but you can tell it is not a smooth clean high speed.
i have owned this motor for several years and it has NEVER done this. i have always used the quicksilver oil 50:1 and 93 octane fuel. the motor has always and even now started very easily. many times a half pull on the rope and it is running.
i have read two other posts about this. one talked about the spark advance and carburator opening timing. maybe this is the problem i do not know. the manual does not tell you how to check the carb opening timing. i have never bothered any of the linkage adjustments.
the only thing i have done to this engine is i replaced the waterpump/housing and the main shaft(rust) i replaced these parts before i used the engine this year. maybe this has something to do with the problem. the other thing i have found is this year when i used the motor at times when it was in neutral the safety lock to prevent you from starting the motor in gear would dragging the flywheel. it would make a clicking clatter noise when i started the engine. when i took the engine home i found that i had to adjust the linkage to prevent it from dragging. why did i have to adjust this? i have never had to before. why suddenly now? the adjustment to the rod has corrected the arm from contacing the flywheel.
i have spent a small fortune on this motor with no luck. the carb looks fine. it was clean and had no corrosion. the reeds looked fine also.
i cannot be the only one with this problem. i could sure use some advice. i appreciate any help on this. i am going back to st. andrews park in panama city fl. in two weeks.
thank you,
don
 

Jake_MN

Petty Officer 2nd Class
Joined
Jun 1, 2010
Messages
106
Re: 2003 mercury 8hp two stroke backfire pop smoke

I didn't read your entire post as it's fairly long. Generally a backfire indicates a lean condition. If slightly backing out the carb needles does not address the situation you're likely looking at a carb cleaning.
 

donfielder

Recruit
Joined
Jun 20, 2011
Messages
4
Re: 2003 mercury 8hp two stroke backfire pop smoke

thanks for the reply. i know my story is long but i stated i have rebuilt the carb already.
i will also update that i put the motor back in the barrel and checked the base timing. i did have to adjust set the base timing. the base timing is now at 8 degrees. i set the carb back to 1 1/2 turns. cold it ran great. when it reached operating temp it started backfiring again. i turned the carb back out to 2 1/2 turns and it stoped backfiring but i am sure it is running too rich at higher speeds.
i do not know what else to do.....buy a new carb? the original one is spotless.
still looking for advice,
don
 

rickjm1001

Seaman
Joined
May 6, 2011
Messages
53
Re: 2003 mercury 8hp two stroke backfire pop smoke

Don, I'm not doubting you but is it possible that the carb float was not installed correctly.
By that I mean put upside down when installed to make sure it's straight?
This all sounds like a carb issue. maybe pull it all apart again, soak it all night in carb cleaner then use a compressor and blow air through all the passages.
 

donfielder

Recruit
Joined
Jun 20, 2011
Messages
4
Re: 2003 mercury 8hp two stroke backfire pop smoke

outstanding suggestion!!!! thank you. i did not see any difference it the float and just stuck it back on...... since it did not hang on a hanger i just put in on. i do not know which end should be up but i will flip it over and test it tonight.
THANKS!!!!
 

rickjm1001

Seaman
Joined
May 6, 2011
Messages
53
Re: 2003 mercury 8hp two stroke backfire pop smoke

When the bowl is off and you are looking at the float upside down is should be straight and now at an angle.
This way when it's full of fuel the needed is seated and will not allow anymore fuel to pass this way you are not flooding the motor.
That's all that's for but for the miss I would defiantly re clean it.
 

donfielder

Recruit
Joined
Jun 20, 2011
Messages
4
Re: 2003 mercury 8hp two stroke backfire pop smoke

I have fixed the engine!! i wanted to update close this case because i know others have or will run into this problem.
first the fuel bowl was on correctly. i turned it over and checked it to see what it would do. the float would not stay straight when you turn the carb over to simulate a full bowl condition. i cleaned that carb at least 3 times. you could eat off of it and i found nothing wrong with anything other than the fuel pump diaphram was streached.
i ordered a new carb. when i installed it i did not adjust anything on it. i just put it on and tested it. no more backfire...no more holding back at high rpm. the other thing i noticed is that the engine does not start as easily as it used to. before after starting the engine the first time the motor would start with less than half a pull of the cord. i now have to do about 3/4 to a full pull to start it. i also now have to choke it to start it ( just like the manual says you should). also the engine puts out almost no smoke. before it would always smoke a little when you first started it. even if you had run it a good bit the day before it would smoke on the first start.
for what it is worth the carb i ordered was no longer available. it superceded to a replacement part number that was on backorder. maybe the new number carb is a improvement from the original.
the bottom line. the motor was running very rich for some time. that was the reason it started so easy, that was the reason that it was smoking( even though i thought, 2 stroke, some smoke normal). the motor runs great, idles smoother, no smoke, and NO BACKFIRE!!!
thank you to those who stoped and offered help to me.
don
 
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