Rotax Max Coolant System (Newbie Need Help)

I recently was gifted a 2011 Top Kart Flash with a Rotax Max engine. I raced 250 shifter karts 30 years ago and I’m feeling the itch to get back on track. I will use the Rotax to get the feel of driving again and see if I want to start racing again. I know the Rotax isn’t used anymore. Anyway the kart has sat for about 6 years so I have cleaned out the carb and fuel system, new fuel filter and inlet manifold. Also new plug and battery. Flushed the radiator and filled up with coolant. Well today I fired the engine up, started very easy however I cannot see any coolant pumping around (I had the rad cap off). I’m assuming there is no thermostat regulating coolant flow so I would have expected the level to go down in the rad once the motor started running. Am I missing something, does it need to be primed some how?

Possibly you have air in the system. We used to purge the air out by running the on stand and lifting the left side of the kart so the cylinder head was not the highest point in the cooling system.
To monitor cylinder head temperature, there is a plug in the head that can be removed and a temperature sensor can be installed. An inline temp sensor will give a normal reading even if all the water drains out. A temp sensor in the head will rise as water leaves, possibly saving your engine.

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A Rotax DOES have a thermostat! It opens at about 50 deg C (120 deg F).

That should clear up most of the confusion. Make sure you’ve got the radiator all the way full of water, squeeze the upper and lower hoses until there’s no air in the system.

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Presence of thermostat will depend on year and/or previous owners preference. Initially Rotax shipped the FR125 with one, but stopped at a later point.

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As others have said, run a coolant temp sensor without it the motor could be damaged. Also, asked around, since I don’t recall, there isn’t any real turbulence evident when looking in the radiator.

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Thank you all for the replies. I saw some coolant dripping at the back of the engine when I fired it up again this morning. I do have the sensor in the head which is connected to the Mychron however something is not right as when the engine is running the temp numbers flash randomly anywhere from 0 to 1500! Did all of the hose squeezing tricks but still the coolant level stays the same in the rad. Maybe it is full but I doubt it after it sat for 6 years. When I disconnected the bottom hose after I got the kart nothing drained out from anywhere. Just realized the the coolant is dripping from the small hole in the back of the engine in the photo. Not sure what this means!

If water is coming out of the weep hole, you have failed water pump seals. The water pump system in a Rotax consists of an impeller that is driven off of the balance gears, which in turn means you have to have a well maintained balance gear case. Did you check to see if the gear case had oil in it? You can check the state of the balance gears by removing the gear case cover. The gears that drive the impeller are plastic, and can break if the gear case is neglected. The only way to replace the water pump seals is to split the case. It’s a racing engine…I wouldn’t expect it to function properly after sitting for 6 years. Also, when you say coolant, please make sure you’re using just straight distilled water.

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Ok thanks for the reply. I will check out what you have mentioned. I really didn’t expect it to run perfect but I also didn’t want to strip the engine before seeing where its at as far as getting it running. I know it’s old but the friend who gave it to me barely used it so at least I know it’s not worn out. I don’t know much about this engine yet but it seems like I will do pretty soon! I have had plenty of experience with racing engines so hopefully I can get this one sorted.

Good luck with it! Please check back with any other questions. For starters, I would at least drain the gear oil (if there is any in there) and put in 50cc of fresh oil.

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I think the plastic gears take 50cc of oil, the steel ones need 100cc.

If you change the oil every 2 hrs, you can use 50cc of oil with steel balance gears with zero problems at all and better performance.