if i'm not mistaken that should be a pipe thread which is tapered, 1/4-18. taps are available from most home stores, an old school radiator petcock can be installed which makes future draining a lot easier.
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Last year the courses were late going in, I free skied the first week and a half, that seemed to help a bit, beyond that hot tub or really hot showers and stretching
this is a longshot but take a good long look at the pcv/breather parts, a modern well serviced motor burning oil is pretty rare nowadays, might be something else going on.
The other day I had to climb a snowbank and shovel the top off in order to snowblow my driveway.........again. I also expect ice in the pond into mid april for the second year in a row. Grrr....
I own an 04 rlxi and one of my ski partners has a 99 rlx, I don't think I could choose one over the other for skiing, they both drive pretty well too. I do like the interior space in mine a little better, seems to be a little less wind in the cockpit on the driver on cooler days and the extra space is nice to have. The rear locker in mine can be used from either the platform or inside the boat also.
Most likely you have 1 or 2 knock sensors, if it pings at all the ecm will back the ignition timing off slightly before you will ever hear it, the loss of power would be very slight.
imho the tantrum was a good looking boat as long as the original graphics were still on it. I'm somewhat biased as you have all guessed i'm sure. Mine had over 800 hours on it when i finally decided i deserved a windshield, it's cold in upstate ny in early and late season.
the best and easiest deal i've found is at walmart of all places, they sell mobil 1 15w50 in the 5 quart jugs, if you find it on sale it's around 25 bucks.
When i switched to hard shells i was amazed at how twitchy the ski felt, slow the boat down a little, run your easier lines and give it 6 or 8 sets and you should get used to it.
I've used a lift mate for a few years now. Motor with a rubber pulley that drives the wheel. Works ok but not awesome, found it works a little better with a dedicated battery mounted to the hoist instead of a long chord plugged into boat battery. I use a solar charger to keep the battery charged.