The throttle springs back to the middle position no matter what position i put it in. Can be manually held in place. The throttle seems to be moving in an out fine Husqvarna model mz6128 28hp brigg

Does not have a fuel pressure regulator . Fuel pressure is controlled electronically by the fuel pump control module an a fuel rail pressure sensor . Your best bet , take it to a qualified repair shop. You have no clue what's up an won't learn here . IF you want to fix this ,suggest you go to http://alldatadiy.com/?affiliate_id=93770&click_id=2526329174 an pay for service repair info. Don't guess . Engine power reduced and stabilitrac message ??? this is a problem caused by the electronic throttle control system . Gas pedal has position sensors that are a direct link to the engine computer , throttle position sensors in the throttle body also are hooked to the PCM / ECM - engine computer . Throttle body also has a D/C motor that controls the position of the throttle plate .
Throttle Actuator Control (TAC) System Description
The throttle actuator control (TAC) system is used to improve emissions, fuel economy, and driveability. The TAC system eliminates the mechanical link between the accelerator pedal and the throttle plate. The TAC system eliminates the need for a cruise control module and idle air control motor. The following is a list of TAC system components:
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The accelerator pedal assembly includes the following components:

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The accelerator pedal

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The accelerator pedal position (APP) sensor 1

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The APP sensor 2

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The throttle body assembly includes the following components:

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The throttle position (TP) sensor 1

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The TP sensor 2

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The throttle actuator motor

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The throttle plate

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The engine control module (ECM)

The ECM monitors the driver demand for acceleration with 2 APP sensors. The APP sensor 1 signal voltage range is from about 0.98-4.16 volts as the accelerator pedal is moved from the rest pedal position to the full pedal travel position. The APP sensor 2 range is from about 0.49-2.08 volts as the accelerator pedal is moved from the rest pedal position to the full pedal travel position. The ECM processes this information along with other sensor inputs to command the throttle plate to a certain position.
The throttle plate is controlled with a direct current motor called a throttle actuator control motor. The ECM can move this motor in the forward or reverse direction by controlling battery voltage and/or ground to 2 internal drivers. The throttle plate is held at a 7 percent rest position using a constant force return spring. This spring holds the throttle plate to the rest position when there is no current flowing to the actuator motor.
The ECM monitors the throttle plate angle with 2 TP sensors. The TP sensor 1 signal voltage range is from about 4.86-0.86 volts as the throttle plate is moved from 0 percent to wide open throttle (WOT). The TP sensor 2 voltage range is from about 0.82-4.14 volts as the throttle plate is moved from 0 percent to WOT.
The ECM performs diagnostics that monitor the voltage levels of both APP sensors, both TP sensors, and the throttle actuator control motor circuit. It also monitors the spring return rate of both return springs that are housed internal to the throttle body assembly. These diagnostics are performed at different times based on whether the engine is running, not running, or whether the ECM is currently in a throttle body relearn procedure.
Every ignition cycle, the ECM performs a quick throttle return spring test to make sure the throttle plate can return to the 7 percent rest position from the 0 percent position. This is to ensure that the throttle plate can be brought to the rest position in case of an actuator motor circuit failure. Observe, under cold conditions, the ECM commands the throttle plate to 7 percent with the ignition ON and the engine OFF to release any ice that may have formed on the throttle plate.

washer on first as the hole will be the same size as the guide
then the seal then the spring then the spring retainer then the Colettes
if the hole is big enough to fit over the seal , then the seal first as that will hold the washer in position

You don't say what make an model vehicle ! But with most vehicle makes it has to do with the Throttle Actuator Control (TAC) System , electronic throttle control ! are you having reduced engine power ? There are probably DTC'S - diagnostic trouble codes stored in the PCM - engine computer .
The throttle actuator control (TAC) system is used to improve emissions, fuel economy, and driveability. The TAC system eliminates the mechanical link between the accelerator pedal and the throttle plate. The TAC system eliminates the need for a cruise control module and idle air control motor. The following is a list of TAC system components:
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The accelerator pedal assembly includes the following components:

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The accelerator pedal

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The accelerator pedal position (APP) sensor 1

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The APP sensor 2

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The throttle body assembly includes the following components:

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The throttle position (TP) sensor 1

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The TP sensor 2

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The throttle actuator motor

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The throttle plate

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The engine control module (ECM)

The ECM monitors the driver demand for acceleration with 2 APP sensors. The APP sensor 1 signal voltage range is from about 0.98-4.16 volts as the accelerator pedal is moved from the rest pedal position to the full pedal travel position. The APP sensor 2 range is from about 0.49-2.08 volts as the accelerator pedal is moved from the rest pedal position to the full pedal travel position. The ECM processes this information along with other sensor inputs to command the throttle plate to a certain position.
The throttle plate is controlled with a direct current motor called a throttle actuator control motor. The ECM can move this motor in the forward or reverse direction by controlling battery voltage and/or ground to 2 internal drivers. The throttle plate is held at a 7 percent rest position using a constant force return spring. This spring holds the throttle plate to the rest position when there is no current flowing to the actuator motor.
The ECM monitors the throttle plate angle with 2 TP sensors. The TP sensor 1 signal voltage range is from about 0.5-4.25 volts as the throttle plate is moved from 0 percent to wide open throttle (WOT). The TP sensor 2 voltage range is from about 4.45-0.7 volts as the throttle plate is moved from 0 percent to WOT.
The ECM performs diagnostics that monitor the voltage levels of both APP sensors, both TP sensors, and the throttle actuator control motor circuit. It also monitors the spring return rate of both return springs that are housed internal to the throttle body assembly. These diagnostics are performed at different times based on whether the engine is running, not running, or whether the ECM is currently in a throttle body relearn procedure.
Every ignition cycle, the ECM performs a quick throttle return spring test to make sure the throttle plate can return to the 7 percent rest position from the 0 percent position. This is to ensure that the throttle plate can be brought to the rest position in case of an actuator motor circuit failure. Observe, under cold conditions, the ECM commands the throttle plate to 7 percent with the ignition ON and the engine OFF to release any ice that may have formed on the throttle plate.

THROAT PLATES ARE HELD ON EACH SIDE WITH A SMALL PIECE OF METAL (SPRING). 1. PLACE THE PLATE AT AN ANGLE OF THE THROAT. 2. AT THE SAME TIME CAREFULLY PUSH EACH SPRING UNDER THE PLATE. 3. SLIDE THE PLATE TOWARDS THE AREA WHERE THE PLATE SHOULD FALL INTO THE LOWER POSITION.

I'll assume you have the pivoting arm with the rubber washer stopper apart. The rubber stopper pulls out of its housing. If you have that out you need to stuff it back in its hole with a small screwdriver. If not: the pivot for the arm rests in two grooves; the arm is held UP with a spring under the arm. (The top of the washer seals the dispensing hole from inside the pot.) The pot has an indentation the spring fits in, the arm also has a depression to hold the spring. It's tricky, but u can get everything lined up in order to put the holding plate (with the 2 screws) in place. (Hold the assembly in place with one finger and with the other hand put a finger thru the holding plate. Move the holding plate toward its final position and switch fingers holding the assembly in place. You're now holding the assembly with the finger that has the holding plate.) Position the holding plate and tighten the screws.

The spring or wire that connects your pedal to the throttle body may broken or damaged. Check under your hood and look at the throttle body (it's the carburetor looking thing). There should be a steel wire connected to it coming from the back of the engine bay. It should connect on the side where you'll see a few springs. If nothing looks broken there, turn the part of the throttle body connected to the spring and wire. If it revs up when you move it, then the cable could be broken somewhere between there and the gas pedal.

Alternatively, the linkage on the pedal could have been disconnected some how. Check behind and above the gas pedal to see if eveything is connected properly.

This machine has a metal throttle rod from the trigger to the carb, you need to look at the side of the carb where the rod connects to the carb, with your finger ( with the engine stopped ) work the carb throttle spindle manually to see if it is moving freely and it springs back to the small throttle idle screw easily, if it does unscrew the idle screw with a cross head screw driver until the machine idles ok, if not you need to take the l/h side cover off to investigate what has happened .

throttle spring could be broken stretched or missing

Hi again Ed,
Starting at the carb throttle shaft, your linkage rod (with tiny spring wrapped around it) should be connected directly to the governor's arm. If there's more than one hole in the arm, connect the rod in the upper-most hole.
Now, at the base of the governor's arm is another hole (possibly on a tab jutting from the arm). The main governor spring should be hooked in this hole and then back to the sliding throttle control that moves when the lever on the dash is moved. (This one definitely has a tab and hole.)

The choke cable end hooks directly on the choke shaft and it has intuitive movement. Verify that the choke butterfly closes and opens relative to the cable control on the tractor's dash.

To insure that we only do this procedure ONCE, and we get it right the first time, let's set the internal governor to base-line at this time.

With everything bolted back up, move the throttle control lever on the tractor's dash to full throttle. Then get a 3/8 wrench and a slotted screwdriver and locate the round governor shaft coming out of the engine block. The governor arm attaches to this with a clamping-bolt. You will notice that the round shaft is slotted for a screwdriver as well.
Loosen the clamping bolt slightly with your 3/8 wrench, insert your screwdriver and rotate the round shaft clockwise until it stops. (It has VERY little travel, so turn it both ways to verify that your are at FULL clockwise position.)
Hold screwdriver there and move the governor's arm in the same (clockwise) direction until the carb's throttle is set to WIDE OPEN throttle. Still holding the screwdriver clockwise, retighten the clamping bolt with the wrench.
Next, move the throttle lever on the dash of the tractor to idle and back to full several times and observe the carb's throttle shaft, it should come down from full throttle to idle and back again freely.

Start the engine and be prepared to shut it off if it over-revs.
If it doesn't over-rev, your base-line settings are correct.

By bending the tab on the sliding throttle control (the one that has the main spring attached) you can now increase or decrease spring tension and change top speed of the engine. Do this in small increments and if you have a tachometer for single cylinder's set top speed to 3400-3600 RPM.

Sorry this is lengthy, but as mentioned above, we only want to do this once.

Hope this helps, and feel free to ask away if you need more info!
Dolf-

if the laser is moved to the middle of the area and when you first turn it on it doesn't go back to starting possession's then some thing is wrong with the motor drive more than not it's likely you have a crack in the board.