1991 3.0 mercruiser runs rough/stalls

[BPrickett]

Ok, I'll try to explain this as best I can, bear with me.

First off, we had the engine and outdrive overhauled/tuned up and it has ran great for 3months, skiing every weekend. about 2 weeks ago we started having problems.

The boat will start, takes a couple cranks when cold but starts right up when warm. It will idle but the rpms will putter down to like 5-600 then just before it stalls it will rev up again. If I give it throttle it sounds like it powers up normally but if I leave the throttle forward it will do the same thing as at idle, rpms will go down then up, but not down low enough to almost stall. When the rpms are down the carb makes a hissing noise, then it revs back up. I've adjusted the idle back and forth and it doesn't seem to solve the problem but that's the only thing I know how to adjust on the carb.

I obviously don't have much experience with engines but I'm trying to learn, if someone could point me in the right direction I would greatly appreciate it.

 

[chiefalen]

Re: 1991 3.0 mercruiser runs rough/stalls

Clean all the fuel filters. And the separator, if you have one dump it into a clear glass what do you see.

One filter in the fitting into the carb.

One in the fuel pump if you have one.

On top of the gas tank there is a anti-syphon valve could be stuck closed take off and blow thru it.

 

[BPrickett]

Re: 1991 3.0 mercruiser runs rough/stalls

cool, thanks, I'll try those things. I looked for any filters I could find but you said they are in the actual fitting so I'll take a look. One question I have is, I have two things going from the fuel pump to the carb, one is the yellow tube, I'm assuming it's the fuel line(with the filters I need to clean on each end of it?) but what is the other thing? It's a stiff metal piece, I have a pic from google and I put arrows pointing to what I'm talking about. What is it and what does it do?


a newbie question but I know nothing and I dont have anyone around to help me so iboats is all I've got!
Thanks everyone

 

[imported_TheMan]

Re: 1991 3.0 mercruiser runs rough/stalls

The stiff metal piece is the actual fuel line. THe yellow tube is the overflow safety device on marine carbs.. The filter is in the metal piece.

 

[JustJason]

Re: 1991 3.0 mercruiser runs rough/stalls

can you define a hissing noise better???
Does it sound like there is air coming out of the carb, or air entering the manifold???

 

[BPrickett]

Re: 1991 3.0 mercruiser runs rough/stalls

well I screwed that right up. trying to disconnect the fuel line from the carb I grabbed where the blue arrow is with channel locks then tried to undo where the red arrow is with a wrench. it was really hard to undo and when finally budged the line twisted with it. I only twisted it like a 1/4 turn or less so the line didnt brake but its probably close. You can sort of see the bend in the line, where the green arrow is. also the bigger nut with the blue arrow is loose going into the carb now, it doesn't seem like it's threaded in, how does it connect? I now have fuel dripping from this area when i turned the key.


In this next picture i was able to get apart where the red arrow is but didn't find any filter in the line or the fitting. I was also able to get the hose and fitting going from the tank to the pump out and there was no filter. I actually unscrewed the fitting from the pump because i couldn't get the hose off the fitting. I also couldn't get the hose off the fitting coming out of the gas tank.


so what should I do now? I probably need to replace the line, right? How does the connection at the carb come apart? hopefully i can deal with the leak once it's apart.
I guess this is part of learning what the hell i'm doing.:redface:

 

[chiefalen]

Re: 1991 3.0 mercruiser runs rough/stalls

Jeez you need a new line. And I feel awful about it to. It was so simple I didn't think someone would grab a line with a channel locks.

The good news it's a common part for a common engine.

The fitting that goes directly in the carb has no screen?

Soomeone here might even have one laying around on a parts motor maybe e-bay has it.

 

[JustJason]

Re: 1991 3.0 mercruiser runs rough/stalls

you need to get a set of flare nut combination wrenches for those line fittings...... the sears ones work ok.
even a high quality snap on regular combination wrench will round those off if your not using a flare wrench.... channel locks are about the worst thing you can use on those...

 

[imported_TheMan]

Re: 1991 3.0 mercruiser runs rough/stalls

Alright.. Couple things.

You have a mechanical fuel pump. On the PUMP SIDE, the fuel filter is inside of the pump. You have to undo the safety wire and remove the retaining cap. Then you will see the filter. If it is the stock filter it will be a blue ceramic. If it is non-stock it will be the paper filter. Go to NAPA to get a replacement filter (paper).

On the CARBURETOR side you will find a filter and spring. You did not need to disconnect the fuel line from the pump side.

You should be using a manual to do all of your boatwork. See the link below.
http://forums.iboats.com/showthread.php?t=251571

 

[imported_TheMan]

Re: 1991 3.0 mercruiser runs rough/stalls

By the way, both of those nuts you were trying to turn are threaded and although you didn't use the right tools, you were doing it somewhat right. First the gray nut is threaded into the carb, then the brass nut is threaded into the gray nut.

 

[BPrickett]

Re: 1991 3.0 mercruiser runs rough/stalls

ok thanks everyone.
I didn't grab the line with channel locks, I grabbed the gray nut, then tried to turn the brass nut and the line twisted with it.

Do I need to replace the line with a specific replacement or is there something available at the auto parts store? I'm guessing it has to be a specific one since you were talking about ebay and parts motors.

You said sears for the wrenches, any idea what sizes I'll need or do I need to buy a whole set?

thanks again

 

[JustJason]

Re: 1991 3.0 mercruiser runs rough/stalls

buy a set, standard and metric and then you'll have them all. It's craftsman club days so you may even get a deal!!!
you can buy new line at NAPA, it's just steel brake line. It comes in straight sections and you make them up yourself. So your going to need to buy a tube cutter, a tube bender, and a Flaring tool to make up your own line.

Yes it gets expensive in tools.... and that's just for 1 itty bitty line