Fuel issue on Mercury 25 hp two-stroke outboard

[Harold Lee Miller]

Hello all,

I'm having an issue with my 25 hp Merc that seems to be fuel related. Started running rough, surging, dying on idle. Replaced spark plugs, cleaned carburetor and replaced gaskets. Runs about the same.

I now am considering whether it is a problem with the fuel bulb and fuel line from the tank and its connectors, or the fuel pump. Here's what I've noticed: I start the engine with the cowl off, and observe the fuel filter reservoir. Pump it full of fuel with the bulb. As engine runs, it runs smoother at the beginning, then as the fuel level drops in the filter reservoir, at some point as it gets halfway or lower, starts running rougher, then dies. If I quickly pump the bulb and refill the reservoir before it dies, it runs better until the fuel level drops to halfway or lower, and then quits again.

I'm concluding I either have a problem with the bulb and fuel line to the engine not holding enough of a vacuum to draw fuel into the engine; or the fuel pump isn't pulling fuel into the engine. When running on the water, it doesn't die like it does on idle, although sometimes it surges. Seems to get enough fuel to stay running when at higher RPMs.

I tried to replace the fuel line connectors for the tank and the engine, and for some reason, they don't fit at either end -- although they're supposedly for Mercury, they're slightly too narrow at the fuel connector opening. Bought two different sets and had the same result. Not sure what that's about, did the size change on the fitting since I got this engine? It's about ten years old.

The existing bulb and line I have seems to work but I can't tell if there's enough of a loose fitting on either end to let air in, and perhaps kill the vacuum required to keep fuel running. So far, haven't been able to rule out the gas line and bulb.

Regarding the fittings, can anyone advise as to what I'm missing regarding the proper fittings?

Regarding the engine performance, any thoughts about what is causing this? Also, I don't know where the fuel pump is on this motor, nor do I know how to check to see if it's running OK. Any suggestions on that are appreciated.

Thanks for your help --

Harold

 

[GA_Boater]

Welcome aboard.

Fuel filters do not stay full of fuel as many incorrectly assume.

It sounds like you should find the fuel pump and rebuild the fuel pump. What is the serial number of the motor to lead you to the right rebuild kit? It is easy to do on some motors, slightly more work on others. The serial number will help.

 

[Harold Lee Miller]

Hi GA Boater, SN is OT963419. I'll look online for the kit.

 

[GA_Boater]

Here ya go. The fuel pump is on the side of the carb. The diaphragm/gasket is available separately or part of a carb full kit. http://www.iboats.com/Carburetor-Se...876685--session_id.412935132--view_id.1695939

 

[Harold Lee Miller]

Thanks!

 

[flyingscott]

Buy the whole kit because you are going to want to service the primer also.

 

[Harold Lee Miller]

Well here's what I've done so far, with no change in the engine performance: new carb gaskets, rebuilt fuel pump/primer and new gaskets, new tank gas line, bulb and connectors.

It starts easy, runs OK on idle for about a minute, then dies. It's not getting fuel. If I pump the bulb before it dies, it'll keep going. Unplugged the fuel line running into the carb and it did exactly the same, so what it's doing is getting filled up by the bulb, then runs on the fuel that gets pushed into the carb, but doesn't seem to be drawing any more in, so it dies.

Not sure if it is producing any vacuum. I tried to feel if there was any vacuum at the opening where the fuel line runs into the top of the carb, and couldn't feel any, but it may be that the proper vacuum is not strong enough to feel anyway, so can't conclude anything from that.

I didn't use any gasket seal liquid or paste when I replaced all the old gaskets, didn't thing I should, but if I was supposed to use some, that may be part of the problem?

Open to suggestions here. I'd take it to a shop but everyone's busy for the next two or three weeks and I've got a fishing trip coming up this week :grumpy:

 

[Harold Lee Miller]

Well this has been resolved, and as I suspected, user error -- I put on the wrong carb gasket to the block. There were two included in the kit. I used the one that was most like the original. Unfortunately I didn't notice that the small opening in it to allow suction to draw fuel had been moved a bit and now the gasket covered the opening. The other gasket didn't look exactly like the original, but the openings were all in the right place. So I had prevented the engine from creating the suction in the carb that drew in the fuel.

 

[MH Hawker]

that has bitten us all at one time or another