Mercruiser 260 engine performance question
 

Hello all, Its that time of year again I took my boat out for the first time this year. I have a small nagging problem that I need some advise on. After installing new plugs for the year changing oil and fuel filter I took it out for a shakedown run. Initially I was able to go 53 mph with the engine running smoothly, this is the normal top end speed for my boat everything good there. But, after a couple hours with much time spent idling I opened it up again and noticed a top end miss and the boat would only go like 47. There is no misfire under acceleration when planing, only at WOT, and if I pull it back it clears right up. I have a faulty tachometer in the boat that reads like 2500 at WOT can the tachometer be causing this problem? I should add that it started doing this last year and if I replace the plugs it will run good for an hour or so then I get this issue. Is it possible that the mixture is too rich in the secondaries? I'm not sure where to start it is a nagging problem...

I guess i should mention the boat is an 84 Scarab 1 and has electronic ignition.

Make sure your flame arrester is good and clean , choke is opening all the way and adjust your air idle screws . When is the last time she had a good tune up , cap rotor wires .

Yeah I cleaned out the flame arrestor, the wires and cap and rotor might need to be replaced, but I was thinking if that was the problem I would get a miss under load like when trying to plane right?

Time to get the carb overhauled. Sounds like you are running rich and fouling out your plugs. What is odd that you are not having any missing under load to get up on plane. Have you checked your timing, if stock with TBIV you should be around 8-10* at idle.

My guess is the carb, though. I am thinking you are fat at idle and lean on the big end.

Does your motor have a knock sensor?

My guess is you're lean on the top end which will cause a miss in addition timing maybe reduced if a knock sensor is present. These two items, lean and retarded timing, will hurt the motor in short order.

Same thing happened to me, but mine was pinging also. Thought bad gas, timing, carb etc. Vacuum gauge showed a fluxuation, so I pulled the heads and found a tuliped exhaust valve and piston started to burn.:poopoo: It only had 540 hrs on it, and other than that it looked nice inside.

Thanks for all the responses guys, I'm thinking the motor is overly rich at WOT causing fouled plugs, the motor is originally a points style ignition system so its not TBIV it has a Mallory electronic distributor installed.

ck compression before you go any further...tuliped valves very common on marine small blocks...

If you think its atuliped valve pull the valve cover and rockers, run a straight edge across the stems, if there is a bad one you will know. If not, well at least this way you didnt yank the head.

My money is still on carb issues

check 5 and 7 wires makesure they are not twisted around each other or swapped. go along with checking cap/rotor and plug wires. breaking down wires, against each other 5 and 7 can induce spark into the other cylinder.