I'm at my wits end....My 1998 Mercuiser 3.0L has been very reliable
until the end of last year and the beginning of this year. My engine
will start fine, idle fine and run great until I get up beyond 3000rpm
or 30+ mph. At this point the engine acts like it backfires (best
description), lunges, and loses power, but never stalls. The tachometer
goes to 7000+ but the engine is not running 7000rpm. After the engine
warms up running at 3500 rpm the tach returns to normal and the noise
(backfiring) goes away. If I stop the boat and sit fro 30 minutes the
process repeats. I've done the following:
1. Changed plugs
2. Changed rotor/ditributor
3. Changed plug wires
4. Replaced fuel filter
5. Cleaned carburetor

Any help would be appreciated. Thanks, Dave

Maybe the choke is taking it's time opening up ... or jamming or ... maybe
*tie* the choke open after a brief warmup, see if that helps, if so ...

After the engine
warms up running at 3500 rpm the tach returns to normal and the noise
(backfiring) goes away. If I stop the boat and sit fro 30 minutes the
process repeats. I've done the following:
1. Changed plugs
2. Changed rotor/ditributor
3. Changed plug wires
4. Replaced fuel filter
5. Cleaned carburetor

Any help would be appreciated. Thanks, Dave

Make sure the points are set properly. Check em with a dwell meter. The coil
could be breaking down or the tach could be acting up. If you have
electronic ignition instead of points, you can add the sensor in the
distributor and control module to the list of things to check.
JIMinFL
wrote in message
oups.com...
I'm at my wits end....My 1998 Mercuiser 3.0L has been very reliable
until the end of last year and the beginning of this year. My engine
will start fine, idle fine and run great until I get up beyond 3000rpm
or 30+ mph. At this point the engine acts like it backfires (best
description), lunges, and loses power, but never stalls. The tachometer
goes to 7000+ but the engine is not running 7000rpm. After the engine
warms up running at 3500 rpm the tach returns to normal and the noise
(backfiring) goes away. If I stop the boat and sit fro 30 minutes the
process repeats. I've done the following:
1. Changed plugs
2. Changed rotor/ditributor
3. Changed plug wires
4. Replaced fuel filter
5. Cleaned carburetor

Any help would be appreciated. Thanks, Dave

I think it's safe to say that in 1998 no boats were made with breaker point
ignition.

"JIMinFL" wrote in message
k.net...
Make sure the points are set properly. Check em with a dwell meter. The
coil could be breaking down or the tach could be acting up. If you have
electronic ignition instead of points, you can add the sensor in the
distributor and control module to the list of things to check.
JIMinFL
wrote in message
oups.com...
I'm at my wits end....My 1998 Mercuiser 3.0L has been very reliable
until the end of last year and the beginning of this year. My engine
will start fine, idle fine and run great until I get up beyond 3000rpm
or 30+ mph. At this point the engine acts like it backfires (best
description), lunges, and loses power, but never stalls. The tachometer
goes to 7000+ but the engine is not running 7000rpm. After the engine
warms up running at 3500 rpm the tach returns to normal and the noise
(backfiring) goes away. If I stop the boat and sit fro 30 minutes the
process repeats. I've done the following:
1. Changed plugs
2. Changed rotor/ditributor
3. Changed plug wires
4. Replaced fuel filter
5. Cleaned carburetor

Any help would be appreciated. Thanks, Dave

Sounds like an ignition problem. It could be the coil, wires, or the
module under the cap.

Capt John wrote:
Sounds like an ignition problem. It could be the coil, wires, or the
module under the cap.

Distributor bushings worn a bit. Unlikley but possible, causeing the
rotor to flop around a bit and misfiring on the cap contacts,

but thats a long shot.

Here's another:

Could be the valves floating out. at 3500+ rpm those valves have to
open and seat extremely quickly. it's ppossible that a weak valve
spring could cause an intake to stay open momentarily too long causeing
the engine to lose compression on that one (or more) cylinders and
huffing back through the carb. OR a hydrolic lifter is defective and
not holding the pushrod to the valve tight, thus causing the valve to
not open correctly.

just some thoughts.

Tim

Thanks for all the suggestions...Resolved the issue by rebuilding the
carb.



JIMinFL wrote:
Make sure the points are set properly. Check em with a dwell meter. The coil
could be breaking down or the tach could be acting up. If you have
electronic ignition instead of points, you can add the sensor in the
distributor and control module to the list of things to check.
JIMinFL
wrote in message
oups.com...
I'm at my wits end....My 1998 Mercuiser 3.0L has been very reliable
until the end of last year and the beginning of this year. My engine
will start fine, idle fine and run great until I get up beyond 3000rpm
or 30+ mph. At this point the engine acts like it backfires (best
description), lunges, and loses power, but never stalls. The tachometer
goes to 7000+ but the engine is not running 7000rpm. After the engine
warms up running at 3500 rpm the tach returns to normal and the noise
(backfiring) goes away. If I stop the boat and sit fro 30 minutes the
process repeats. I've done the following:
1. Changed plugs
2. Changed rotor/ditributor
3. Changed plug wires
4. Replaced fuel filter
5. Cleaned carburetor

Any help would be appreciated. Thanks, Dave