I have had an issue lately with my boat stalling at idle after a more than 15 minute run at cruising speed. Last night I had a hard time just getting to the beach at the boat launch. I start in neutral - requires a bit of extra 'gas' on the throttle to get it going and then dies immediately when I put it in gear. Sometimes, I could idle in forward, but anything above that would result in a stall.
Any thoughts?

You say it only does this after it has been run for awhile? Temps OK? I assume it has breaker point ignition, how long since it was tuned up. If within a year or so then it might be a bad coil or a carb problem. Ignition coils will do funky things if they are cracked. work fine cold but act up when hot. If it has a Holley carb it could be a blown power valve but you would likely notice the black soot on the transom.

It could also be a vacuum leak. The already low vacuum would drop further when you give it gas.

Just a few of the possibilities.

When I've had such a problem it turned out to be poor lubrication. Which, after endlessly fussing with tuneups, led to a lunched motor. Most curiously, I experienced this with both 4-stroke and 2-stroke motors.

Kelly

It does have a Holley carb. No black soot though. I'm going to clean the flame arrester tomorrow. I have no idea when the last tune-up was. The marina does spring start-up and oil changes. Must call them to see when the plugs were replaced last. Never had this sort of trouble with this boat before. Temps were OK, but I did see some smoke from the engine bay after a quick run down the river. I haven't even checked the engine oil level (how bad is that??), but the oil pressure gauge read normal.

If you can't say when the last tune-up was, I would do it all. Change out the points for a pertronix breakerless kit, cap, rotor, wires, oil/filter, PCV valve. That will get you a good start and it sounds like you are due for them all anyway, so no wasted money if it doesn't fix your problem.

Any idea how many hours on the motor? could have jumped a tooth on the timing belt if high hours. Also could have an exhaust shutter plugging the exhaust.

Start with a tune up and then see what the symptoms are.

Oh, and get a new coil. They often act that way and they are pretty cheap.

Thanks prober - good solid advice. I've never asked anyone to "do a tune-up", so it's been at least 5 years ! (since I've owned the boat)
I'm useless around engines, so I'll take it in to the OMC repair shop and have them look at it and check out my battery charging problem as well. Going out of town in a few days without the boat. Would be good timing...

I had a similiar issue on my old boat and after taking it to a mechanic and working on it myself for years I could never solve the problem. One day when the boat finally died, it ended up being the coil wire. Once that was changed the boat ran like new.

I would change out the electrical components for around $100 and go from there.

Good luck!

Time for a new throttle cable.

Coil
Exhaust
Plugs
Points
Throttle cable?
I'll have to replace the boat!

What a great boat!
I certainly would feel you owe it a tune-up after 5 or so years. Throttle cables will give you stalling issues when shifting, but this does not sound like that to me.
I am an old-school guy who happens to like an ignition system with points and condensor. With the low usage most boats get as opposed to automobiles, they are fine. I also like the idea of being able to see what is going on. I also have to admit it is the last ignition system I understood! (I guess being a diesel mechanic for 30 years gives me a pass on ignition)
Do the cheap and easy stuff first.

I couldn't agree more.
It has a large cockpit. Room enough for a kids Pack-N-Play. Everything works (except for alternator and now stalling problem). I mean every gauge including the steering column 'outdrive position meter' It is in very good condition - much of it was redone about 7 years ago. It runs exceptionally quiet and sips gas. It has features that are making a comeback; darker coloured seats (Doral BR) and modified tri-hull for extended beam width in the bow (Regal). Excellent stability, including while anchored. I had it out to the fireworks on the river a few days ago and every light (dash, running, and floor courtesy lights too) was working. I love this boat!

I agree, nice boat. And points are fine but can be troublesome to get adjusted just right. The electronic units are more plug and play. But hey, if you happen to still know where your feeler guage is thenstick with the points. Seems to me you could just use a matchbook cover in a pinch. Now I'm showing my age.

It went to the repair shop yesterday. We'll see what they say...

Well... It just needed $800 - I wish there was a slot on the boat to slide in twenties, but it had to go in to the shop for a week (my first week of a 2-week cottage rental!).
After a carberator cleaning/servicing and a previously enjoyed alternator, my boat runs better than it ever has for me. I've never seen the volt meter so high and every accessory, including my fish finder works better. I thought my tilt motor was getting weak, but it just wasn't getting enough juice.
For the past week we were fishing after dark using all running/mooring lights, playing the stereo, lighted fish finder, and charging my iPod all at the same time!
What a boost.
It is clear to me now that my alternator was poor from the start of my ownership of this boat.