1996 Merc 25 HP – Carb adjustment help needed.

Sheldon (Iowaboy), hoping you weigh in.

OK, a Canada trip is 2 days away so naturally my dad’s 96 25 HP Merc is acting up. I used it last week and it was fine, but of course he had to take it out one more time and he forgot to open the fuel tank vent and ran it until it died a couple of times, restarting it again and again to get back to the ramp, and it hasn’t run well since.

The motor wouldn’t run well at idle unless choke was on after that. Can’t get it into gear without choke on at all, it just dies.
I found a small fuel leak at a hose clamp, so fixed that, cleaned fuel filter, gas is fresh. I adjusted the idle up about 50 RPM. Checked the plugs and

Everything seems fine now, but now the prospect of going to Canada with a dubious motor has me spooked.

Question: Where is the bleep is the mixture adjustment on these motors?

I’m an OMC guy when it comes to this vintage of motors and Mercs are much more complicated. OMC’s had a giant main adjustment right in the front of the carb that said what it did. Merc is a mass of linkages and cables.

Pictures posted, but can someone tell me where the mixture adjustments are on these outboards? I just feel like I want to know how to adjust something besides the idle.

Many thanks.

Grouse

Pics in case its easier to sho vs tell.

Sheldon will get ya fixed up. I would assume (silly me) that you are staying at a resort in Canada? If so, even if your Merc takes a dump they should have a motor for you to rent. The trip won’t be a loss so don’t fret the small things. Like you said it’s running good now.

oh boy,another victim,er,uh,someone needing help lol
basics first,with the fuel line from the tank disconnected at the motor,push the ball back with a small screwdriver or pick of some sort and placed over a bucket.
squeeze the primer bulb several times to make sure you are getting an ample fuel supply from the tank,if not,find out why,ie,something stuck in the pick up tube opening,collapsed hose etc.

if you are good to go there,see the cover plate with five screws in it??
thats your fuel pump,I am betting there is a chunk of crap under one of the check valves,or the diaphragm has ruptured trying to overcome the huge vacuum it would take to draw fuel from a closed tank vent.

that carb is simpler to work on than it looks,take a few pics as you disassemble things to get to it,no big deal.
as far as any adjustments there should be an idle mix screw on the left side of the carb closest to the motor that allows for idle mix adjustments only.
the main jet is fixed,and one idle speed screw,thats it.

there is nothing scary about that carb at all and super easy to work on.
if you decide to tear it down and clean it,the worst part is making sure you get the gaskets,check valves,and the diaphragm in their proper place in the fuel pump.
again,take pictures as you go.
I use sierra marine parts available from any Napa or Orielly’s auto parts store,never had any problems with their parts.
be forewarned though,they include parts for many different carbs of a like model in one kit so you may have more parts than what you took off,no worries,only use identical parts to what you removed.

also check to make sure that there are no cracks in any of the fuel lines between the carburetor clear back to the tank,I have seen in the past when a weak hose rears its ugly head when a larger than normal vacuum is applied from either a closed vent or plugged line somewhere.
you can have an air leak,or a collapsed line restricting flow.

hope this gets you on your way to a happy merc again,keep me posted.
sheldon

So maybe,,,,

forgot to add,the check valves in that pump are part of the stamped diaphragm gasket ,they look like little tongues,one of them could be sucked into its bore that it is supposed to cover.

I have that
same Merc simplest solution take my motor my boat and me!

I forget exactly what it was, but when we were up in the northwoods of Canada my dads 25 Johnson acting up similar. Found out to be the hose connection, I think the ball was engaging, not engaging and made it very infrequent. Would work great for few hours then act up, then nothing again for a day then back at it after lunch. Cant remember what I did to rig it up but a new hose and fitting fixed that problem.

Good luck to you guys up there

We’re staying at a private cabin in the sub arctic, so there’s no backup.

Thanks Sheldon, that confirms why I can’t find the mixture adjustment.

So I don’t think he hosed up the fuel pump diaphrams because I have the thing running on the hose now and it idles pretty well. I tweaked the idle and as I mentioned, we had an obvious gas leak that was letting down the fuel pressure. Once that was fixed, the fuel pressure stays up and whether or not that was the whole issue, air leaks are never good.

Tim, it’s funny you should mention taking you. For the last 5 trips up to this place, I couldn’t find anyone to go. I asked guys, my dad asked guys, relatives, etc, etc, etc they all had something better to do than go and catch a few hundred walleyes. So now this year we’ve FINALLY got a group of 5 guys to go! So sorry, we’ll take your outboard, but you’re going to have to stay home. Maybe next trip.

Thanks guys, let’s hope this solves it.

Grouse

Another simple test to see how clean the carb is, Is if you turn the rich/lean mix screw and you hear the motor change rpm’s it means the carb is fairly clean. If it doesn’t respond to turning the screw it needs overhauled. If you do this remember which way the screwdriver slot is facing, make the turns in 1/8th turn adjustments. Their very similar to a lawn mower carburetor with just a few parts, not many. Mercury carbs are simple to begin with, their not over your head Grouse. Most carb linkages on Mercs are more simple then Evinrude/Johnsons.