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Engine Surging

tke240

New member
"I have an '86 mercruiser

"I have an '86 mercruiser 260 that continually surges between 800 and 900 rpm in neutral and sounds like it wants to stall briefly as it dips down to the 800 range. There wasn't a manual with this boat when I bought it, so I'm just kind of flying blind with only a little working knowledge of boat engines. So far this season I've put in new batteries, new fuel filter, new starter solenoid, cleaned electrical connections, and replaced oil. I had about 25 gallons of gas that sat in it over the winter months. Poured some Seafoam cleaner down into the tank and dumped a few gallons of fresh gas on top of it hoping that would smooth things out, but so far it hasn't. Could it be a simple matter of completely topping off the tank with fresh gas? Holds about 100 gal. Any ideas what else might cause this?"
 
"I've only had it in the w

"I've only had it in the water a week and have just been doing work on it at the docks, so I really haven't been able to take it out on the water yet. I increased the throttle while in neutral up to about 1500 rpm and noticed that it was still surging a bit. I suspect that it will do it in gear as well, but haven't been able to test that yet. It's a short trip to my dock from the boat ramp and the engine only starts to surge once it's warmed up."
 
"Sounds like the idle circuit

"Sounds like the idle circuit in the carb. Could be an adjustment. If it will stay running for you, fill it up with good 89 octane fuel and try running it on the water for a while. Give it a good work out. Sometimes this will solve the problem. If not it might be time to think about a carb rebuild or replace. Also check the timing, if the timing is flying all over the place at idle there may be some timing chain issues."
 
Thanks for the advice. Hopeful

Thanks for the advice. Hopefully the weather will hold this weekend and I'll give that a shot. It'd be nice if it were something simple for a change...
 
"It definitely sounds like car

"It definitely sounds like carburation. Air fuel mixture screw may need adjusting (do this with the engine warm), your pcv valve or pcv hose may be bad, a vacuum line somewhere may be bad. Check plugs to make sure they aren't black. Try tapping the float bowl of the carburator with a wrench to see if the float is sticking. With respect to the timing, if you have a timing light, see if the timing holds steady. I wouldn't adjust it though, it's probobly good. Just make sure it's steady. Make sure your breather (air breather, flame arrestor, or whatever you call it) is clean. Make sure the carburator bolts at the base aren't loose."
 
Sounds to me like water in the

Sounds to me like water in the gas. Have you changed the separators and filters?

Jeff
 
"I really appreciate all the a

"I really appreciate all the advice, and I'll be sure to look into each suggestion. I did change the fuel-water separator... but as I'm not yet very familiar with engine mechanics and I'm still waiting on the manual to come in, I don't really have a grasp as to where any other filter might be along the fuel path. I didn't see anything like an inline filter between the tank and the pump other than the water separator... but honestly I don't even know how to access the fuel tank on this thing so I guess there could be one at the connection. My first thought was water in the gas or dirty carb. I was hoping the new water separator would do a little more to fix it, but it didn't. What do you guys suggest for water in the gas? That Seafoam stuff seems like a magic bullet... think I'd have any luck using that to remove the moisture in the tank?"
 
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