The hydro drive doesn't have much power

Might be that you need to bleed the tranny. Or perhaps the tranny has an internal problem. Following are some general notes. Go to the websites and review the tranny instructions.

The Huskvarna YTH150 uses Hydrogear transmission 319-0650, drive model 150071. One place to find parts and diagrams is here:http://www.m-and-d.com/Hydro-Gear_parts....

Hydrostatic Transmission Fluid is actually 10W-50 synthetic motor oil (like mobil). Do not use hydraulic fluid or transmission fluid.

Hydro static transmissions on most lawn tractors are very similar. Most of them are made by one or two manufacturers. Tuff Torq makes a lot of them. They definitely make all the John Deere lawn tractor hydro drives. The lawn tractor manufacturers (like JD) often say they are sealed and not serviceable. That is flat out, a lie. All of the JD hydros, and most all others, are serviceable. A complete line of rebuild/replacement parts can be found at Tuff Torq. Info and instructions can be found there. It's quite likely they made your tranny too.

Additionally, I have found that the mower manufacturers fill them (or spec them) with motor oil, 10W-30W. In the service manual, and at the hydro manufacturer, you are told if your tractor is used in extreme or heavy duty situations it's best to drain the oil and replace it with 10W-50 synthetic motor oil (like mobil). Truth is the tranny should have this oil from the get go. It will give you more power and extend the life of the tranny. The mower manufacturers are all just being cheap. They figure why not, you're the one who has to pay for the early failure. Oh yeah, one other minor detail, the tranny manufacturer also recommends regular oil changes, like every 100 hours.

Virtually all of these trannys have a black plug on the top. You have to remove the tranny from the mower (not a difficult things to do, takes about 30 min). Pull that black plug and turn the tranny upside down for about 30 min to completely drain. Most hold about 2 quarts. Flip it back, right side up, and refill. Install plug and reinstall tranny. Note: under the plug there is often a magnet which collects metal shavings. Lift this magnet out while you have the plug out, clean it and replace it. It may fall out while the tranny is upside down. When you refill the fluid just keep filling till it comes up within a quarter inch of the plug. You may have to go through a bleed process after reinstalling the tranny. The tuff torq site explains the bleed process.

Was it working correctly earlier or is this your initial operation?

According to the product manual for "loss of drive":
1. Bypass linkages in the bypass position
2. Hydro belt worn, damaged, or broken
3. Idler spring loose or broken

1. Make sure that the bypass linkages are in
the drive or engaged position. Located at
the rear of the machine. Refer to decal on
the rear of the unit.
2. Replace Hydro belt.
3. Replace Idler spring

first check the air pressure in the tires make sure they are the same then move the linkage rods on the slower side to balance the speeds out if it doesn't balance out you might have a pump or drive motor failing each side has its own pump and drive motor assuming you already check the fluid level

it's a safety feature to keep the mower from getting going to fast downhill and out of control

Check the drive belts first. and make sure the hydro release lever is in the correct position. If it is not- you can push the lawn tractor and have the rear wheels turn. It sometimes doubles as a "Forward" "Neutral" "Reverse" lever on other brands of lawn tractor.

The belts should be in their guides and pulley grooves- check to make sure it does not have a hidden break by having the engine off, and pulling on the belt.

Lastly, check to make sure the hydro itself is not out of oil or low on oil. It uses low viscosity "HyGard" oil. Capacity if you drain the old oil out and refill is 4.6 quarts. Look for a leak if the level is low.

If it still does not move, make sure the levers are set so you are in forward motion positions, and push- if the rear wheels move there is a problem in the hydro unit.

check the belt tention for the drive pump

You will have air in the system and will need purged. To do that you jack up the wheels to where they will turn freely then disengage the hydro and at an idle alternate forward and reverse for a for about 4 or 5 minutes. Recheck the fluid level and top off. Now engage the hydro and do the same forward and reverse for 3 or 4 minutes. You should have axle movement at this point. Hope this will help.

You have to do one more thing to move this tractor without the motor running.
It takes two people to move the Revolution. One needs to sit on the seat and hold the forward pedal down. It will then freewheel.
So, brake off, both hydro release levers pulled (do NOT pull hard or you will break the springs) and the forward (right hand side, big pedal) pedal pushed down.
Any other question? I know this unit pretty well.

Have you checked the fluid level in the hydro trans, there is a drive belt from the engine to the hydro trans check and make sure that this is not worn or loose. Make sure that when you check the tension of this belt that you do not have the engine running or the parking brake set, since tension will be released from the belt when parking brake is applied. good Luck

My lawnmower done the same thing right after we purchased it. They had to replace the whole motor