25hp murcury backfire please help

[flounderhound]

I bought a used 25 murcury from a yardsale owner said it had setup for 2 yrs
and had carb trouble so i bought it took it home put fresh gas and oil,new plugs,and it started ok but rough idle and backfired when i reved it up so i cleaned rebuilt the carb and fuel pump with murcury repair kit put it back on and adjusted low speed mix it idles fine now but still backfires when i rev up i checked the timing and spark on both clynders both ok. when it backfires i can see a puff of smoke come from under the carb and out the exoust.can anyone help me please i dont know what else to do.
thanks F.H.

 

[roachblade]

Re: 25hp murcury backfire please help

Backfires are usually caused by either faulty ignition timing, which it sounds like you have eliminated, or an improper fuel/air ratio. Likely causes could be stuck or sticky reed valves if yr model has them, an incorrectly adjusted/faulty/modified carburator, air getting past the carb gasket or poor compression not cycling mixture properly. A can of ether/easy start is a good diagnostic, just squirt a little in to richen the mixture and see what happens.

 

[ENSIGN]

Re: 25hp murcury backfire please help

Ether/easy start is a easy way to screw that motor up !!! Sounds like it's running to lean.Spray some primix in to the carb and see if it helps,If so back out the mixture screw 1/2 turn.

 

[flounderhound]

Re: 25hp murcury backfire please help

Thanks for the info but i would never use starter fluid on a 2cly ,i turned the low mix back 1/2 turn like you said and it seems to help.the motor is a 2003,25hp.it would idle ok but when i rev it up to about 1500to1800rpms it would run ok for about 10, 20 sec.and then backfire and slow down rpms and when i try to rev back up it will caugh or pop I have not put my boat in the water yet only on water muffs i was wondering if that may be the cause?
thanks
Flounderhound

 

[will941s]

Re: 25hp murcury backfire please help

Make sure that carb it sealed. No leaks. Also check the compression and make sure the cylinders are even (within 15psi of each other). If all else fails then it could be the reeds. Reed problems are uncommon but they do happen. If one reed petal won't close all the way then it will let burned exhuast back through the carb and backfire. If that is the problem then it's not a hard thing to fix and really is'nt all that expensive if you do it yourself.

 

[flounderhound]

Re: 25hp murcury backfire please help

compression seems to be good im getting about 100psi on both clynd.I put a new gaskit on after i rebuilt carb and tight it down to spec. in seloc book this is starting to test my nerves and i dont know what else to do /I really want to get it fixed because my 8yr old son is coming to live with me for the summer and i want to teach him how to flounder fish so im greatful for any info i can get LOL
THANKS
f.h.