1995 Speedster cavitation and overheating

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Busted Grouse

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Great forum here, and rest assured that I did some extensive searching before this post. I'm just sorry I have to use it so soon.

Bought a 1995 Speedster last week. Ran like a top on a half-hour test. RPMs were good, got out of the hole quick, speedo read 55 MPH, all seemed well.

Last weekend, with a full load of fuel and 500 pounds of passengers, and with the engines warm I had trouble getting on plane. The starboard engine would start to cavitate at 4500 RMP, zipping up to 8,000. Finally got up on plane (slowly, with no cavitation) and headed down the lake at around 40 MPH. Five minutes later, the engine overheat alarm came on, so I shut down and I got a friendly tow back to the dock. I was able to restart each engine for a few seconds, just to make sure they weren't seized.

The ONLY change from the good test drive to the bad trip was that I pumped the old oil out of the reservoir, and refilled it with BRP Rotax 2-stroke mineral oil, as I had no idea how old or what type the previous oil was and had read all the warnings about mixing. And I topped it off with 87 Octane fuel. Didn't, however, drain all of the old oil out the lines -- just did the reservoir.

Any thoughts? Could the cavitation and overheating be related? Coincidence?
 
Overheating goes hand and hand with cavitation. (you are pumping air, not water)

Sounds like to me, you are drawing air past the pumps. OR... the seals in your driveshaft packs are going bad. When you have more load... the air is easier to pull past a leaky seal.
 
Thanks Doc! Only starboard was cavitating (as far as I could tell), yet the overheat alarm still went on back at the dock when only port was briefly run. Do you know if the alarm would ordinarily still go off if starboard was still hot, and port was not?
 
Yep, all clear, at least as far as a visual inspection from underneath. I also recall that I went into a long left hand turn before the overheating alarm went off, and I may not have noticed if the starboard RPMs spiked during the turn.
 
OK, just got the scoop from the repair shop. It found no problems with the wear rings, impellor, or anything else. What it did find, however, was that it was missing a battery. Some previous owner/yahoo had rewired it to run on a single battery! After the shop fixed that, they said it ran just fine, at least on the hose. So, at this point I'm guessing that the overheat alarm was due to the crazy wiring job, and the cavitation was a coincidence, caused by carrying a full load of people and a full tank of gas. I'll try it in the water this weekend. Thanks!
 
OK, just got the scoop from the repair shop. It found no problems with the wear rings, impellor, or anything else. What it did find, however, was that it was missing a battery. Some previous owner/yahoo had rewired it to run on a single battery! After the shop fixed that, they said it ran just fine, at least on the hose. So, at this point I'm guessing that the overheat alarm was due to the crazy wiring job, and the cavitation was a coincidence, caused by carrying a full load of people and a full tank of gas. I'll try it in the water this weekend. Thanks!

some people like a single battery hookup right? ok, let us know what it was
 
Yeah the single battery conversion is actually preferred. I am guessing since you have both engines running, the previous owner did it correctly. The concept is to replace the 2 small PWC batteries with a single large marine battery.You just got hosed by the shop. Not only did they charge you to install a missing battery they diagnosed a non existent problem we would have gladly helped you with for free. The battery issue has zero to do with an overheat issue. These arent like cars, there is no fan or electric water pump. Stay away from shops. With a little bit of tools and reading here, most problems can be easily fixed.

I would have suggested taking it back on the water for a test to see what you get. More than likely you had a restriction that had cleared itself.
 
Thanks RF. It had never occurred to me that a single battery might be desirable, although in my case it was a single PWC battery, and not a full fledged marine battery. I'll report back after this weekend.
 
Try that again on the Blackberry.

Put the Speedster back in today, now with two batteries. Ran like a champ for 30 minutes. Got a better feel for the throttles and no cavitation problems. Turned it off at the dock, where it sat for two hours.

Started up later to go tubing. After idling out 5 minutes to our starting point, the overheat alarm went off, so back to the dock. I put my hand on the top of the engines and tuned pipes - warm, but not 200 degrees F. So, I idled 3 or 4 minutes back to the ramp. Right before we got there, the volume of the alarm diminished to almost nothing. Thoughts? Gremlin? Thanks all.
 
Could be a bad heat sensor, I had one on my port engine and it made me nuts!!! It was so bad one day I broke the buzzer to stop the noise.. Fixed the sensor and had to get a new buzzer...
 
Well, some progress made. I pulled the temp sensors (actually switches, as I now understand) and brought them home. I should have pulled the leads off while the engines were running, but I guess I forgot in my excitement to try a Mr. Wizard boiling sensor experiment.

Sensor 1 -- infinite resistance at room temp. At around 170 degrees, it dropped down to 2 or 3 ohms.
Sensor 2 -- showed about 60 ohms at room temp. Gradually fell to around 4 homes between 140 and 200 degrees.

I don't know much about electricity, but sensor 2 sure looks suspect. As for a low fuel warning, the tank is full, but the gauge isn't working. I didn't see anything in the shop manual about a low fuel alarm on a 1995 Speedster, but if it has one, I'll try pulling the lead next weekend if the new sensors (may as well get two) don't solve the problem. Anyone know a good source for a well-priced sensor?

Thanks guys -- great forum.
 
One little thing

Also noticed a lot of water splashed around inside the engine compartment at the dock. Lots of droplets on the top and bottom of the tuned pipe. Normal, or might a coolant hose be loose? One more reason to look in there while underway next weekend.
 
Sanded the carbon off both probes and regreased the spade connects. That worked great, for 24 hours. Pulled the connect off the suspect switch and killed the buzzer. Guess I'll send for a new one.
 
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