still overheating, what next? 1990 70hp

1990 70hp rude on a 24' pontoon. I've already replaced the water pump (complete kit with new thermostat spring and telltale outlet,) and the temp sender. I can cruise all day at low speeds with no issues and have run short distances wide open with no alarm. But trying to run long distances around 4000rpm, it's still alarming. I have two thoughts. First would be that there's a lean spot in the midrange and maybe I should try bigger jets (orifices.) But I'm also wondering if I'm overpropped just a little and it's loading the motor too hard. It's spec is 5-6k wot. With two people on board I get 5400rpm wot. I've got an omc 14x13 prop. I tried a 14x11 but it slipped like crazy and rpm went over 6200rpm and top speed dropped. I usually get the overheat when we have several people and running between 3500-4500rpm. That's what makes me think maybe it's loaded too much with the current prop.

Any thoughts?

Re: still overheating, what next?

My knowledge of the engine's required installation height on a pontoon boat is zilch BUT that problem comes across to me as the engine's water intake being too high/too close to the surface of the water whereas the water pump could draw air at the higher speed/rpms.

If the top rated full throttle full spark advance setting is indeed 6000 rpm, a ideal top rpm would be in the area of 5800 rpm. Hopefully someone with knowledge of the pontoon boat scenario will jump in here with helpfull advice.

Re: still overheating, what next?

I have had overheating issues after water pump C/O's before and I have found 2 different problems.
I have found that a piece of waterpump impeller can become lodged in the water tube or block. With thermostat removed I use an air hose to purge out any debris.
Another problem is the water tube groumets, of course its going to be the upper one if thats the problem. I had a torn flap that would only cause problems at higer speeds.

just my $0.02

Re: still overheating, what next?

Well, here's how it went today. We had 8 people and two dogs aboard. On the river running upstream and into the wind at 3550rpm for half an hour and no alarm. Then on the way back downstream running at 3700rpm, alarm went off about 2/3 of the way back. What a giant pita!

Re: still overheating, what next?

Is this pontoon boat and listed engine new to you or have you had it for some time?

If you've had it for some time and it operated okay previously, I'd suspect that the water pump is failing but you've already installed a new one.

If the complete rig is new to you, the water pump installation could have encountered a problem and could still be suspect. I would also suspect the installed height of the engine due to the fact that as the rpms increase, the higher that engine comes to the surface of the water.

I'm somewhat out of my league here with a pontoon boat so I'll say no more on the subject and will just sit back and read to see what I can learn.

Re: still overheating, what next?

This is the start of the third season with this motor. The first year we had it on the boat, we had the overheat alarm maybe twice. Last year it started happening more often. At first I thought the temp switch was bad as it was peeing a strong stream and I could still put my hand on the side of motor so it didn't physically feel too hot to me. So I replaced the temp switch and that didn't help. Before this season began, I had the water pump replaced. The old impeller didn't look as if it had left any pieces behind that might plug up the water passages. The new pump is genuine OMC and also included the new pressure relief spring for the t-stat and the unrestricted outlet for the telltale. (I assume the reason for these new parts is that this pump is higher pressure than the original)

I'm thinking the next time I get the overheat alarm, while I'm sitting there waiting for it to cool off, I should pull the plugs and see if it looks lean at that rpm. Or I could just put bigger midrange jets in and see if that makes a difference. Or I could put in a new t-stat and see if that helps.

I think the motor actually sits pretty low in the water - especially with the load we were carrying - but maybe I should look at it when we're underway and see if the pickup looks like it's sucking air.