mercruiser 260 HP

[amazing2926]

hey all... Need some help here. I have a 1981 Searay 260. Has a Mercruiser 260 on it. About a month ago, started to have issues with the boat dieing out after running on low RPM's for too long. Would happen everytime i popped it into neutral to dock. We have quite a drive down our ditch to the mouth of the lake, so we have to run at no wake speeds. Anyway, I changed a number of things and its just not working.

I changed out the points, put a dwell meter on it and they are running perfect.
I changed the rotor, cap, plugs and wires.
I took out the carb and had it rebuilt.
Tweaked the carb so that its set just right
Checked with a timing light and everything is right on the mark.

Take the boat out, pull up to the gas dock, boat is running great and did not stall... take out on lake, ran great for a bit then I pumped it to full throttle, heard what sounded to be back firing. Backed off and ran at 3500 RPM's... Noticed the boat starting to decline in RPM's and act sluggish. Backed her down all the way and it died on me. Took me upwards of 30 minutes to get it started again but when i did and put in gear, it would stall out. Had to get towed in.... Any recommendations? This boat has never ran harder then 4200 RPM's and I just bought it in March, so I have very little history on it...

Any suggestions as to what to check next? Im at a loss.... thanks.

 

[amazing2926]

Forgot to add... At the dock under throttle, the motor sounds awesome, runs great and HARD! Just under load does this issue begin.

 

[sdarc]

Not sure of the distributor system but it sounds like the distributor is not advancing. This I say in reference to your 2nd post. Other thought is the float level. Just because it was rebuilt doesn't mean that was set though it should have been. One last idea is to look for vacuum leaks. that can cause the backfiring noise mostly thru the carb.

 

[JVM225]

You might want to do a compression check. Don't ask how I know......
Anyway; Assuming the carb and ignition system are in good shape you might have other internal problems in the motor.

 

[moparlvr4406]

sounds like it's loading up the cylinders with fuel at low rpm and fouling plugs which will break down under a heavy load.

 

[mysticowner]

Sounds like a fuel pump to me.

 

[tdschafer]

On my old 260 I had a few similar experiences to what you explain. More than once it was my lower shift cable. This connects to the momentary kill switch that engages when you shift gears. It causes a little "stutter" in the engine that breaks the inertia at the gearbox in the outdrive allowing the gears to disengage, or something like that. When the lower cable starts to stick it keeps the kill switch engaged killing the motor and making it difficult to start without moving it in & out of gear a few times.

The other time it was a combination of things. My carb was running rich and loading things up with too much gas and my engine was at the end of it's servicable life. The heads had so much blow by that it was actually choking the engine out. There was 2 hoses from the valve covers that ran up to the spark arrestor that just pumped this smoke in. The engine had about 1100 hours on it at that time and I ended up replacing it with a brand new crate motor.

I don't know how many hours you have on yours. I hope it is a less costly repair in your situation.

 

[SCORPIO]

+1 on the shift cable comment. If the backfireing started after the tune up, you may have a wire crossed. Re check your firing order to make sure you have them connected to the correct spark tower on the cap and the correct plug.

 

[penguin]

Also make sure that all the major grounds are clean and corrision free and tight! It might be one of those electrical gremlins that only shows after some heat builds up in the bilge. Look for grounding cables and that the negative terminals on the batteries are spotless.