25H Surging with full throttle

[EBass]

I have an old 1988 but rebuilt (twice) 25H short shaft Merc and when at full throttle it will get on plane then surge. Like the RPM's slow down, then rev back up, then slow, then fast, then slow sluggish/sounds rough for a bit, then fast again. Grr!

It was working fine last summer but towards fall starting behaving like above and I forgot about it until I bought it out yesterday. Has anyone had this issue before?

Gas/Spark/Air?

[BoxMN]

First thing I would suggest is to make sure the vent is open or closed FULLY depending upon what that motor needs. Out 50 will do that if the vent is NOT open.

Second sounds like maybe not enough gas is getting into the bowl to sustain the motors need at WOT. Maybe clean the carb, I bet that might be it.

Or you could have a leak in your bulb or fuel line. That is easy to check if you have a buddy with same hose. Or, if the hose is older than about 6-7-8 years, maybe just get a new one as it is most likely time. BTW, after getting "hosed" with cheap stuff, I only buy quality hoses right from dealers - you pay more, but the actuall merc/rude/etc. hoses ARE better than big box cheapies. Good luck, others are more knowledgable than me though so listen to them

-edit - last year my 50 was doing something similar but only when I would turn (to whip tubers). Found it was a plastic fitting on a cheap hose I was using - I found original metal from an old Merc hose, cleaned them off slapped them on the fuel line and puuuurrrrfect since The plastic cheapy fittings were letting air in when motor turned and put a little pressure on them.

[EBass]

Thanks BoxMN. It's a newer gas line, but an after market but was still $25 and made for Merc, but not a Merc line. And funny, never had this issue until I had the new line, but it worked for about a year then started this surging.

The vent cap is loose, er I thought I'll go check that again.

I also use the metal gas tank and have to use the metal gas tank since I made a custom back deck and the plastic 6.6 tanks will not fit.

On hold with Twin City Outboard now.

[raymondk]

Had the same problem throw the after market one away and get at least the fittings from merk ......................

[DaRangeFinder]

I had the same issue with my 25 horse merc ended up that there wasn't enough gas pressure being that we mounted the gas tank up front. When it is surging is the ball on the fuel line maintaining full pressure? If not there is your problem, either shorten the length of the fuel line, vent the gas can more, or if the tank is old make sure there are no metal fragments floating around and that should fix it, if not my guess would be an engine problem.

[raymondk]

Had the same problem throw the after market one away and get at least the fittings from merk ......................

[raymondk]

Had the same problem throw the after market one away and get at least the fittings from merk ......................

[EBass]

I new fuel line did the trick. It's after market stuff from Fleet Farm, but it works!

Thanks for all the replies