I cant find a replacement air filter locally, can i use the foam one instead?

There is no question, but sounds like it doesn't run with air box closed. It isn't getting enough air and that is usually caused by someone saturating the foam that covers the filter by too much oil. Remove filter thoroughly clean it, wash the foam with gas, let dry and then lightly coat foam with spray WD-40 instead of pouring motor oil on it.

Have you replaced the air filter? Without the air filter the engine is getting enough oxygen to support combustion. With the air filter it does not get enough oxygen. If you have not replaced the filter it sounds like it is time to replace it. If you have been "cleaning" the old filter and reusing it, small particles may be lodged in the filter, reducing the airflow. Is it a pleated paper filter or a oil dipped foam filter. If it is a foam filter, have you cleaned it? If you replaced it was it a store replacement or a piece of foam you obtained and made it fit? Foam can be closed cell or open cell , both look similar but closed does not allow airflow and open does .

the foam filter for a Conti Aquarobot 1800w vacuum cleaner is unavailable. Can I use ordinary foam instead? Is the foam filter permanent or replaceable ?

If the motor stopped running, it most likely did because it was not being cooled, becuause something was stopping up the air flow. If you have cleaned the secondary filter out and it still stops while running, then remove the exhaust filter cartridge and try it then. If it does not stop running now, then the problem is the previous models like this had a foam filter as the exhaust filter, not a heppa cartridge filter. You will not hurt the machine by running it without the heppa cartridge. Cut a foam filter to fit to replace it. I do not think Hoover knows this yet.

I have a customer working on this problem today (Sept 2010) I have repaired machines for 30 years. This filter is meant to just catch the tiny particles from getting back into the air. However, it worked fine on the previous machines with the foam filter here instead of this Heppa cartridge.

Dave in Texas

Remove seat, two bolts at rear...
unscrew wing bolt located in the middle of the air filter element
remove filter, and clean the mating area in the air box (WD-40 works well) Make sure not to get any dirt in here!!!
Remove the inner air filter cage, Clean the air filter (Stock ones are foam) I use kerosene or gas, but be careful, wash it then with soapy water and let it dry completely.
Use a foam filter oil, either spray on or in a bottle, and make sure it's covered completely, and tacky, not soaked. (I use Bel-Ray foam filter oil) Replace the air filter cage
Put a small bead of grease on the filter where it mates to the air box, this helps it seal.
Re-install air filter and seat, I use a little bit of lock-tite on the seat bolts, as they vibrate out.

I'm not sure what your question is, but you need to replace the air filter and in the future, don't clean the foam filter in gasoline. They will deteriorate with time, but gas will speed up that process. Wash it in dishwashing detergent next time.

Yes you can buy it in hardware, just cut it to fit in the frame.

http://www.totalvac.com/HAPF600.html?zmam=6843742&zmas=1&zmac=6&zmap=HAPF600

You can actually use the size B....made by Holmes...Also C for the carbon filters