2000 Mercury 75hp 4 stroke carb running rough

[tdagner]

Hi everyone,

I have a 2000 model mercury 75hp carb motor that runs kind of rough sometimes. Seems to be getting a little worse.

I cleaned the carbs out really well last season and it ran fine, not as smooth as I would like, but fine. This year it seems to be getting back to its same old ways, even though I have 2 fuel filters on the boat. I also didnt let it sit all winter, we would take it out occasionally to keep fresh gas running through it. I also had my vacuum tester on the carbs and synced them up.

Anyways, I keep reading that you can swap out the jets in these engines and it will run smoother. Something like the small jets that they put in these engines were meant to meet EPA standards, but clog up very easy and make the engine run crappy.

Does anyone know what carb jets I need to switch to. I cannot seem to find any info on what jets I need, or where to buy them. If someone can help me out with the source, im sure I can get this engine running fine.

I also recently changed out all the spark plugs. Also, the tach goes a little crazy on occasion, IDK if that is part of the underlying issue also.

Any help is much appreciated.

 

[Texasmark]

I wasn't aware Merc built a carbureted 4 stroke in 2000. I have a manual on Merc. 1998 and later 75-125 hp 3 and 4 cylinder 2 strokers, updated in 2015. Before you go diddling with jets, maybe doing a "DeCarb" might help. Won't cost much, not a lot of time to do it and may solve your problem. It's in the archives, or just Google the term.

 

[tdagner]

Thanks, I dont think this is the issue. From what I read, decarbing is mainly for 2 stroke engines. My engine doesnt smoke.

I really think I need to rejet the carb at some point in the near future. Im gonna pull the boat out of the water this week and re-sync the carbs to see if that helps at all. Really getting annoyed at this thing. Im also going to check the fuel pumps to see if they have any cracks.

the engine is either a 1999 or 2000 model 4 stroke 75hp carb engine. I still have the sales manual for it.

Any other help would be appreciated.

Thanks

 

[bugelingkayaker]

you said you have two fuel filters on it. is it supposed to have 2? if not its possible the extra filter may be causing too much resistance in the fuel line that the suction through the carb cant overcome, and starving the engine of fuel as a result.

 

[tdagner]

I’m not sure if that could be it. One of the filters is a water separating filter, the other is the stock filter under the cowl. I wouldn’t think they are causing enough restriction to cause that.

im gonna look into rebuilding my fuel pumps to see if that helps. Cheap and easy process that could be the fix. Just hve to get the engine serial number so I know which ones to order. From what I have read there are two pumps on this engine.

 

[Texasmark]

...........the other is the stock filter under the cowl. I wouldn’t think they are causing enough restriction to cause that.

Do yourself a favor. Get a new filter and install it blind, or better yet, pull your old one out and back flush it. If you find little black specs of rubber or tan, crisp, pieces of matter, you have fuel line problems due to age and they and the filter need to be replaced all the way back to and including the fuel line from the tank to the engine...including the bulb.....BTDT!!!

Edit: You will also find them, at least the black specks, past the filter, in your carb, or other type fuel control device.