The height adjust mechanism on my Royal Classic 4000 doesn't work well. It slips so the roller housing isn't very close to the floor. Any ideas on hoe to fix this problem?

This vacuum cleaner has no height adjustment. You can run it across any type of carpet without making adjustment. You should turn off brush roller when using on hard floors.

http://www.fixya.com/support/t24326835-repair_front_left_wheel_height

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look for a blockage -something may have dislodged the lever mechanism, or be stuck preventing the roller or position leaver from moving.

No not quite. For a start, the stiff bristles on the blue roller can last a few years before requiring to be replaced.

The SEBO sensor wheel adjusts to the height required - unless of course the microchip board has been tampered with. If it hasn't been tampered with. If the central wheel is excessively dirty then it could be that the SEBO isn't lowering the height properly. However, the X series is great at cleaning hard floors in general - i it is not designed for the brush roll to fully hit the floor, thus as you suggest crushing the bristles. It is designed, as you recognise quite correctly to skid across a hard floor lightly as there is a squeegee rubber strip built in at the back of the roller bar that scoops all dirt into the central vacuum path. The front plate also lends this job. You are not being conned - however check that the rubber squeegee strip has not worn away - SEBO do sell the strips and they are easy to slide into via the press release door at the side of the brush roll.

If the SEBO has died then its a possibility that there is a microchip function that gone wrong it. Some more information would be good to know here as it could be a number of things such as the cord cables coming away at the handle.

You changed the belt that drives the brushroll, it is a flat stretch belt. The V belt also should be replaced every year or two max. It will not be stretched out like the flat belt but with normal use the side of the v will of wear off and the belt will be a little thinner which can cause it to slip on the pulley. The V belt drives the transmission. The flat belt goes from the transmission to drive the brushroll. The burning smell could come from the V belt if it's slipping real bad.

Go here for the bags > http://www.vacuum-depot.com/royal_vacuum_bags.asp

Change the belt on the unit, and adjust the height adjustment until the brushes are just touching the carpet. I know you mentioned the belt looks good, but when you get the new EXT-U belt, you will notice a size difference. Good luck!

Try this site.
https://www.2instructions.com/search.php?cmarque=ROYAL&modele=4000%20CLASSIC

This is going to sound funny, but check the wheels. Sounds like the pin or shaft that holds them is broke. Also check the tension on your belt. If the tension is to loose then the brush will have a hard time spinning.Good luck