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Running/idling issues 2004 25hp merc outboard

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willys_wonka

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Hi all, I got this 13' Boston whaler as a tender for a motoryacht I just bought, and it came with this mercury 2 stroke 25hp outboard. The first thing with it was leaking fuel from the priming pump, so I replaced the primer pump diaphragm, as well as the fuel pump diaphragm. I also tore it all down and blew out all the ports and orifices. But it's still hard to start, and while most of the time it idles fine once warmed up, after a few minutes it'll eventually stall unless you pump the primer, and most of the time when you crack the throttle it seems starved for fuel and will stall unless you rapidly pump the primer or squeeze like hell on the fuel tank primer bulb. A lot of backfiring. If you can manage to get it past the throttle opening bit, you can get it wind right up and run like a top. But letting go of the throttle after a good run will result in a stall. I've checked that fuel is getting into the filter screen bowl, and into the carb input. I've cleaned the float bowl needle valve and seat. The gas is old but not rotten or stinky (the boat came with a couple of tanks, both filled with fuel) and I plan on replacing the plugs but haven't done so yet. Not sure where to go from here.
 
Sounds like a fuel delivery issue and or air leak still in system, Pull plugs and have a look,they can tell you if she's running rich or lean, dump that fuel and clean or flush tank properly,lines ,filter,pump, etc.
 
Turn the low speed screw in the carb out 1/8 of a turn and see if that helps. You may need to play with it and open it more.
 
It totally is a fuel delivery issue. Played with it a bit more. Wide open it seems there's enough oomph in the pump to keep the fuel bowl filled but not at idle. I can tell because after idling awhile when it starts slowing down I go to press the prime button I can feel there's no back pressure, so little fuel in float bowl. But if I squeeze hard on the bulb it comes back. Yet when I pull the fuel line off the carb and squeeze primer bulb, tons of fuel comes out of the line, so I don't think the problem is before the carb. There's nothing between the fuel inlet and float bowl besides the pump itself and I'm certain I put it in correctly, spring, diaphragm and gaskets. Besides, it was doing this before I changed those parts. Ive adjusted the idle screw and can get a nice idle as long as it will idle.
Plugs didn't look too bad, a little black from cold running.
I tried switching tanks and no real difference.
 
I don't have a manual but the only carb adjust was right above the air horn, which I assumed was a mixture adjustment. Took it out for a run this morning and as long as I squeezed the primer bulb hard it kept running really well, idling well.
 
If you are sure the carb is perfectly clean. One thing to check is the carb base gasket make sure it is not upside down. The pump gets it's pulse through the carb itself through a tiny hole in the intake and the gasket has a maching hole for that..
 
Yes, I blew spray through all the passages to make sure everything was clear. I was aware that the carb gasket goes in only one way, and again it does work under high RPMS so the pump pulse is getting through. Plus it was doing this before I cleaned the carb. I'm wondering about the two metal flapper one way valves mounted on the plastic intermediate piece between the carb body and pump diaphragm. If they were not sealing properly the pump wouldn't work right. Or a vacuum leak in the crankcase?
 
Hell and death. The new gasket I put on had the hole misaligned to the crankcase. It was like 1/4" off. I cut it out into a slot put it all back together and now it keeps the fuel bowl full. What a PITA. The only thing remaining is that it doesn't idle well unless I pump the primer or I hold closed the small solenoid on the top of the carb. What's it for? It doesn't seem like it ever closes.
 
After a little research I see it's an enricher solenoid. Does the same thing as the primer. So what I have is that unless I use the manual primer or electric enricher, it doesn't idle well and will still stall, even when warm. I"ve backed out the mixture screw a couple of turns from where it was before but doesn't seem to help.
 
I think you may have the wrong carb base gasket on. If you got the kit there are 2 or 3 in there no way you should have had to cut a slot in it. The correct gasket would have lined up and maybe have a bigger hole.
 
There were only two flange gaskets and one was clearly not correct. This one was exactly the same as the old one but with the hole offset. On the carb side there's a slot leading away from the passage to the pump, and on the crankcase side the hole lines up with the uppermost edge of the aformentioned slot on the carb. But the gasket hole is offset from the crankcase hole such that it was plugged. I cut the hole so it matches with the carb slot and it now works. All other ports and passages seem to line up fine.

I went for a run tonight with a piece of tape covering the port that the enrichment solenoid covers and the engine ran perfectly.
 
That enrichment port is normally not covered. That is for an electric choke which covers that hole as the choke is activated. When the choke is let off that port is open to the air. You still have something wrong.
 
I know, and can't figure out what might be causing it. Carb cleaned out, lots of fuel in float bowl, but idle too lean. Does anyone know how the idle circuit works on these carbs? I physically blew out every port/orifice with spray solvent.

What is the basic setting for the mixture screw?
 
Your carb. isn.t going to get cleaned throughly with a spray can solvent, It needs to be soaked overnight in a commercial cleaner,blasted with high pressure water,followed with dry filtered compressed air,reassembled and reinstalled. Turn idle mixture screw clockwise,counting revolutions, Start at 1and a 1/2 turns counterclockwise,1/4 turns from there in or :rolleyes:
 
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Take the top plate off of the carb, leave the idle mix screw in and you will actually see where it seats. That is the port that needs to be cleaned if you just sprayed in the screw hole itself you never got close. Under the plate is pretty wide open so carb cleaner will not get to the hole.
 
willys_wonka said:
I'll check that out tomorrow and make sure there's a clear passage through to the crankcase.
Click to expand...

That hole for the idle mix does not go to the crankcase it goes to the float bowl. You need to tear the carb down and clean it and put a good quality rebuild kit in it. That spring in the primer also needs to be replaced when you rebuild the carb.
 
But where does the idle exit the behind the throttle plate? I cleaned the port from the float bowl to that location and it's definitely clear. I looked but didn't see an idle port in the carb throat, and I recall there was an orifice above the throat on the carb flange, and I assume that's where the idle fuel flows into the crankcase.
 
There are 3 ports on the base of the carb. The first big one is for the fuel pump pulse. The second smaller lower one is for the primer. The 3rd small one at the very top is for the idle mix. If you look at the intake throat it has an odd shape as it has cutouts for the 2 small ports. Thats why the base gasket has to have the same odd shape as the intake not the round port of the carb.
 
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