I have an '86 Mercruiser 140 (fresh water lake) which is exhibiting an intermittent rough engine problem across the entire rpm range.
The engine starts and runs great, though it has had a bog under quick acceleration for the past couple of years which I think might be due to the 24 year old unrebuilt carburator...already changed accelerator pump...already changed fuel filter...already changed fuel pick-up hose from fuel tank...none of which improved the acceleration bog issue. If I accelerate moderately I can get past this one bog point to run at full speed smoothly with absolutely no issues.
I upgraded the points/condenser last year to the Pertronix "Ignitor" electronic ignition with wonderful results. The typical smell of unburnt fuel in the exhaust disappeared, the engine started much easier, and I notice a drastic improvement in fuel economy. I also replace the spark plugs, plug & coil wires, and distributor cap during this upgrade. The boat ran great for the rest of the season.
This year, during our first run of the season, everything started out as typical, however after running on plane for 10 minutes, the power dropped by 25% and the engine started running like it was either starved for fuel or not firing on all 4 cylinders. After running like this for 5 minutes, the problem disappeared and the engine ran back up to its typical smooth operation. After another 20 minutes of smooth running, the power again reduced by 25% for 5 minutes and then rebounded back to smooth operation again. The boat ran fine for the rest of the day, and I attributed the bogging to 6 month old over-winter fuel (though I did add Sta-bil fuel additive prior to the winter storage).
The boat has run fine since (though still with the acceleration bog which had been typical for the past few years). A few people recommended the Starbrite "Star Tron Gasoline Additive" for the acceleration problem. The instructions say to double the additive in the first tank, and that overdosing with the additive is not possible. The additive is advertized to perform the following:
-Reduces all emissions
-Prevents fuel oxidation and stabilizes fuel chemistry for at least 1 year
-Lowers fuel system maintenance
-Eliminates carbon build-up in fuel delivery system and on exhaust components
-Increases fuel economy, especially in older engines. Instantly cures fuel jelling caused by cold weather and E10---alcohol fuels.
Immediately after adding this additive, the original problem of the season came back! The engine started fine, ran smooth for 10 minutes, then ran rough for 5 minutes reducing power by 25%, then ran smooth for another 20 minutes, then ran rough for 5 minutes reducing power by 25%. When it runs rough, if I back off on the throttle, the engine stalls below 1500 rpm as if it runs out of gas. I pump the throttle several times, restart the engine to 2000 rpm, but have to quickly drop it into gear and pump the throttle several times to give the engine enough gas to stay running in gear. This sometimes takes 15 attempts before I can get the engine to catch in gear and get back up above 2000 rpm to keep it running. I get it back up on plane, it runs just fine for 30 minutes, then I take it down to idle without a problem and can let it idle to cool down before I shut off the engine as if the problem went away.
I burned 1/2 a tank of fuel in case the additive was causing issues with the fuel system and hoped that replenishing 1/2 the tank with fresh fuel might dilute the additive to the point where the problem should be less apparent, but today I went out again, and the same problem repeated itself.
I did notice that the battery voltage indicator needle is sometimes solid at 14V at 3000 rpm, and sometimes it dances between 13V and 14V at 3000 rpm. This was intermittent, but I could not pin down if this was happening only when the engine was having issues.
One other interesting phenomena which happened today (I thought this was a fluke, but it happened twice), when I put the engine in reverse, it clears up the problem completely. The second time, it was running rough on the way into the ramp at 1000 rpm, and I put it in reverse, and the engine then ran as smooth as ever at 500 rpm idle. I had complete control when jockeying the boat to the trailer with no problems.
This has me absolutely baffled. If it was a fueling issue, it should not clean up magically on its own. If it is an electrical problem due to heat, it also should not clean up on its own. If it is an electrical problem or bearing problem, why would putting the engine in reverse temporarily solve the problem?
I started to look over the wiring on the engine to see if there is a possibility that putting the engine in reverse might be moving a wire to make a better contact. The ground wires look good and all of the wiring connections look clean across the entire engine (no corrosion), though I did notice the ground wire attached to the rebuilt alternator does not have a good crimp, and I can move the wire in and out of the crimp connection 1/8" with little force. I am going to re-crimp and solder that connection tomorrow, though it does not show any indication of overheating or sparking. The engine starter cranks just fine when the boat stalls during this phenomena, so it does not seem to be a battery voltage problem (though the battery is over 12 years old).
Anyone think of anything else which might be causing this issue? Any possibility the shift interrupter switch might be intermittent and might adjust if the engine is reversed?
Thanks for any feedback!
The engine starts and runs great, though it has had a bog under quick acceleration for the past couple of years which I think might be due to the 24 year old unrebuilt carburator...already changed accelerator pump...already changed fuel filter...already changed fuel pick-up hose from fuel tank...none of which improved the acceleration bog issue. If I accelerate moderately I can get past this one bog point to run at full speed smoothly with absolutely no issues.
I upgraded the points/condenser last year to the Pertronix "Ignitor" electronic ignition with wonderful results. The typical smell of unburnt fuel in the exhaust disappeared, the engine started much easier, and I notice a drastic improvement in fuel economy. I also replace the spark plugs, plug & coil wires, and distributor cap during this upgrade. The boat ran great for the rest of the season.
This year, during our first run of the season, everything started out as typical, however after running on plane for 10 minutes, the power dropped by 25% and the engine started running like it was either starved for fuel or not firing on all 4 cylinders. After running like this for 5 minutes, the problem disappeared and the engine ran back up to its typical smooth operation. After another 20 minutes of smooth running, the power again reduced by 25% for 5 minutes and then rebounded back to smooth operation again. The boat ran fine for the rest of the day, and I attributed the bogging to 6 month old over-winter fuel (though I did add Sta-bil fuel additive prior to the winter storage).
The boat has run fine since (though still with the acceleration bog which had been typical for the past few years). A few people recommended the Starbrite "Star Tron Gasoline Additive" for the acceleration problem. The instructions say to double the additive in the first tank, and that overdosing with the additive is not possible. The additive is advertized to perform the following:
-Reduces all emissions
-Prevents fuel oxidation and stabilizes fuel chemistry for at least 1 year
-Lowers fuel system maintenance
-Eliminates carbon build-up in fuel delivery system and on exhaust components
-Increases fuel economy, especially in older engines. Instantly cures fuel jelling caused by cold weather and E10---alcohol fuels.
Immediately after adding this additive, the original problem of the season came back! The engine started fine, ran smooth for 10 minutes, then ran rough for 5 minutes reducing power by 25%, then ran smooth for another 20 minutes, then ran rough for 5 minutes reducing power by 25%. When it runs rough, if I back off on the throttle, the engine stalls below 1500 rpm as if it runs out of gas. I pump the throttle several times, restart the engine to 2000 rpm, but have to quickly drop it into gear and pump the throttle several times to give the engine enough gas to stay running in gear. This sometimes takes 15 attempts before I can get the engine to catch in gear and get back up above 2000 rpm to keep it running. I get it back up on plane, it runs just fine for 30 minutes, then I take it down to idle without a problem and can let it idle to cool down before I shut off the engine as if the problem went away.
I burned 1/2 a tank of fuel in case the additive was causing issues with the fuel system and hoped that replenishing 1/2 the tank with fresh fuel might dilute the additive to the point where the problem should be less apparent, but today I went out again, and the same problem repeated itself.
I did notice that the battery voltage indicator needle is sometimes solid at 14V at 3000 rpm, and sometimes it dances between 13V and 14V at 3000 rpm. This was intermittent, but I could not pin down if this was happening only when the engine was having issues.
One other interesting phenomena which happened today (I thought this was a fluke, but it happened twice), when I put the engine in reverse, it clears up the problem completely. The second time, it was running rough on the way into the ramp at 1000 rpm, and I put it in reverse, and the engine then ran as smooth as ever at 500 rpm idle. I had complete control when jockeying the boat to the trailer with no problems.
This has me absolutely baffled. If it was a fueling issue, it should not clean up magically on its own. If it is an electrical problem due to heat, it also should not clean up on its own. If it is an electrical problem or bearing problem, why would putting the engine in reverse temporarily solve the problem?
I started to look over the wiring on the engine to see if there is a possibility that putting the engine in reverse might be moving a wire to make a better contact. The ground wires look good and all of the wiring connections look clean across the entire engine (no corrosion), though I did notice the ground wire attached to the rebuilt alternator does not have a good crimp, and I can move the wire in and out of the crimp connection 1/8" with little force. I am going to re-crimp and solder that connection tomorrow, though it does not show any indication of overheating or sparking. The engine starter cranks just fine when the boat stalls during this phenomena, so it does not seem to be a battery voltage problem (though the battery is over 12 years old).
Anyone think of anything else which might be causing this issue? Any possibility the shift interrupter switch might be intermittent and might adjust if the engine is reversed?
Thanks for any feedback!