need help. my 1990 mercruiser 3.0L runs fine for the first 1/2 hour
or so. then after it sits while we swim or change skiers it starts
and revs fine, but runs very rough and even stalls when put into gear
and given gas. Not a fuel or fuel system problem since it always runs
good for the 1/2 hour or so. this problem is driving me crazy. any
suggestions welcome.

You didn't mention ... does the problem sometimes/eventually clear itself?
Last season I had a similar (intermittent) experience caused by a sticking
carburator float. When the float stuck, excess fuel would overflow into the
engine causing rough running and stalling when put into gear. I would have
to increase the idle setting by adjusting the idle stop screw just to get
underway. And for whatever reason, the problem never occurred when I first
started out for the day ... only after running for as you say a 1/2 hour or
so. If when you stop, and you're carburated, pop the hood at that first stop
.... if there's a strong fuel smell (in my case fuel could be seen leaking
out of the carb onto the manifold due to remaining fuel pressure and stuck
float) a stuck float, or dirt/worn needle/seat is likely the problem ... my
2 cents.

"sudsy" wrote in message
om...
need help. my 1990 mercruiser 3.0L runs fine for the first 1/2 hour
or so. then after it sits while we swim or change skiers it starts
and revs fine, but runs very rough and even stalls when put into gear
and given gas. Not a fuel or fuel system problem since it always runs
good for the 1/2 hour or so. this problem is driving me crazy. any
suggestions welcome.

Yep, had exactly the same problem with a 4.3. Not being any sort of expert
I spent days trying all sorts of things. I was about to check the timing
advance with a old cheap neon strobe light I had knocking around, the type
that plugs between the spark lead cap and the spark plug. I noticed the
flashes from the light were very erratic on all cylinders after the engine
had warmed up. Cleaning and de-crudding the distributor and rotor arm
contacts resulted in strong regular flashes and a smooth running engine. It
worked for me, but I don't quite know why.

Steve

"sudsy" wrote in message
om...
need help. my 1990 mercruiser 3.0L runs fine for the first 1/2 hour
or so. then after it sits while we swim or change skiers it starts
and revs fine, but runs very rough and even stalls when put into gear
and given gas. Not a fuel or fuel system problem since it always runs
good for the 1/2 hour or so. this problem is driving me crazy. any
suggestions welcome.

"Steve L" wrote in message ...
Yep, had exactly the same problem with a 4.3. Not being any sort of expert
I spent days trying all sorts of things. I was about to check the timing
advance with a old cheap neon strobe light I had knocking around, the type
that plugs between the spark lead cap and the spark plug. I noticed the
flashes from the light were very erratic on all cylinders after the engine
had warmed up. Cleaning and de-crudding the distributor and rotor arm
contacts resulted in strong regular flashes and a smooth running engine. It
worked for me, but I don't quite know why.


typically, just putting a timing light on a v6 mercruiser wont give
you the correct timing. depending on the ignition system a base needs
to be set before the distributor can be set


Steve

"sudsy" wrote in message
om...
need help. my 1990 mercruiser 3.0L runs fine for the first 1/2 hour
or so. then after it sits while we swim or change skiers it starts
and revs fine, but runs very rough and even stalls when put into gear
and given gas. Not a fuel or fuel system problem since it always runs
good for the 1/2 hour or so. this problem is driving me crazy. any
suggestions welcome.

do you have a water separator? if not get one. it could be water in
your gas.
also get your ignition system checked, including your coil.

Thanks for that. I had no intention of trying to reset the timing, the
engine was intermittently idling rough (hunting), I was hoping to just
confirm there was HT going to the plug.- it couldn't be far off as the
engine would start OK from cold.
Steve

"BenC" wrote in message
om...
"Steve L" wrote in message
...
Yep, had exactly the same problem with a 4.3. Not being any sort of
expert
I spent days trying all sorts of things. I was about to check the
timing
advance with a old cheap neon strobe light I had knocking around, the
type
that plugs between the spark lead cap and the spark plug. I noticed the
flashes from the light were very erratic on all cylinders after the
engine
had warmed up. Cleaning and de-crudding the distributor and rotor arm
contacts resulted in strong regular flashes and a smooth running engine.
It
worked for me, but I don't quite know why.


typically, just putting a timing light on a v6 mercruiser wont give
you the correct timing. depending on the ignition system a base needs
to be set before the distributor can be set


Steve

"sudsy" wrote in message
om...
need help. my 1990 mercruiser 3.0L runs fine for the first 1/2 hour
or so. then after it sits while we swim or change skiers it starts
and revs fine, but runs very rough and even stalls when put into gear
and given gas. Not a fuel or fuel system problem since it always runs
good for the 1/2 hour or so. this problem is driving me crazy. any
suggestions welcome.

do you have a water separator? if not get one. it could be water in
your gas.
also get your ignition system checked, including your coil.

(sudsy) wrote in message . com...
need help. my 1990 mercruiser 3.0L runs fine for the first 1/2 hour
or so. then after it sits while we swim or change skiers it starts
and revs fine, but runs very rough and even stalls when put into gear
and given gas. Not a fuel or fuel system problem since it always runs
good for the 1/2 hour or so. this problem is driving me crazy. any
suggestions welcome.

tank vent clogged?

Could be the coil and/or plug wires breaking down once it gets hot.
Could also be the carb needs a rebuild.

--
Tony
my boats at http://t.thomas.home.mchsi.com

"Matt Lang" wrote in message
om...
(sudsy) wrote in message
. com...
need help. my 1990 mercruiser 3.0L runs fine for the first 1/2 hour
or so. then after it sits while we swim or change skiers it starts
and revs fine, but runs very rough and even stalls when put into gear
and given gas. Not a fuel or fuel system problem since it always runs
good for the 1/2 hour or so. this problem is driving me crazy. any
suggestions welcome.

tank vent clogged?