Mercruiser 3.0 surging, running rough

1985 3.0 liter mercruiser. The boat starts right up every time when cold. When I first leave the dock the boat runs smooth at idle speed. When I give the boat more throttle it will get up and go to wide open throttle. If I throttle down at all, the boat will start surging and running very rough. The boat surges at most speeds from this point, except WOT. The boat will run like this for about 30 min. At this point the boat starts running even more rough to a point where it will stall, will not even run at idle speed. I can get it re-started with a little trouble. When I get it re-started and try to put it into gear it will stall 5 times out of 6. I can usually limp back to the dock by re-starting several times.<br />I have tried a new coil, rebuilt the carb., new points and condensor, new spark plugs. I have checked the anti-siphon valve, seems clear, tried running with the gas cap off for venting.<br />After reading the many posts on this board I am thinking it is the fuel pump. Why would the fuel pump act up after running for awhile? I am trying to eliminate other things before trying a new fuel pump.<br />Any help would be appreciated.

Re: Mercruiser 3.0 surging, running rough

Could also be a vaccuum leak.<br />check carb to spacer (if equipped)<br />Spacer to manifold gasket<br />intake gaskets.<br />You can spray water around machine surfaces and<br />throttle body shafts.<br />while engine is running.<br />Any drop in rpm indicates a problem. (vacuum leak)<br /><br />Might also double check the fuel air mixture jet adjustment and the choke setting.

Re: Mercruiser 3.0 surging, running rough

You have just describe the problems I have on my I/O. I, like you rebuilt the carb figuring it would solve the problem. Then slowly replaced all the other parts (points,cond,coil,wires,cap,rotor,etc) thinking they were the cause. I think my mistake was that I used a automotive carb kit and not a marine kit. <br />Did you use a marine rebuild kit? Have you tried to idle the boat around for 20-30min without going beyond 1/4 throttle to see if the problem reoccurs? Did you use carb cleaner and spray into every little hole then follow up with compressed air to make sure all passages are clean?<br />After mine has sat for about a day it will idle for almost an hour before loping and stalling, but if I go to WOT or 3/4 WOT first thing after the engine warms up then throttle back to an idle it will start loping almost immediately. I am guessing it has something to do with the accelerator pump circuit due to the fact it only happens after that part of the carb is utilized. I have not had the chance to re-rebuild the carb to give you a for sure answer but sometimes new parts can be just as bad as the old ones (learned this lesson a couple different times). Good Luck.

Re: Mercruiser 3.0 surging, running rough

Following Kevin's idea, a friend of mine had his engine rebuilt and fought with it for the next year. It turns out that an automotive gasket was used to mount the carb. The marine gasket blocks one of the heat riser holes. The heat riser is used for emissions control but on a boat it just messed things up. Essentially it acted like an internal vacuum leak. Good luck.

Re: Mercruiser 3.0 surging, running rough

Had almost exactly the same symptoms. Tried everything. Ran for an hour and died consistantly. Loose carb mounting bolts. Stupid but true.

Re: Mercruiser 3.0 surging, running rough

Why would this only happen when the engine has been run for 45-60 min? It gets continually worse as time goes by until I can bearly make it back to the dock. I used marine parts to rebuild the carb.

Re: Mercruiser 3.0 surging, running rough

You sure that there is no problem with either the gas tank vent or the anti-siphon valve??<br />You might try running it on an outboard portable gas tank (be careful with the hookup!!!) to narrow it down.<br />Other idea is that the coil could be breaking down under heat.<br />Lastly you say it starts to run rough when you throttle back, I wonder if the exhaust flapper failed, allowing it to suck water up the exhaust.

Re: Mercruiser 3.0 surging, running rough

Not an easy problem. I would start by trying to isolate the problem as either fuel or ignition. Get it good and hot ( and running badly) and have a syringe of gas handy to dribble down the carb. You could try pulling a spark plug to look for a good spark. You will get much better advice if you can isolate which system is the problem.