I have a toro 521 snowthower. The throttle slides from low to fast but I don't know how to turn the screw at the end on the throttle to adjust how far/fast the throttle goes. (clockwise/counter-clockwise etc)??????????????

the sequence to throttle adjustment is to adjust to high speed first ( screw closest to the air filter) turn in in or out until it fires evenly at rpm and picks up rpm easily
now hold the throttle at full throttle and slowly idle down while adjusting the idle screw ( closest to the butterfly) to maintain engine running
the screw should have been turned all the way in then back out 1 1/2 turns before starting the engine
keep slowing the engine while making very small adjustment until it idles at rpm slow enough that the chain is not driving
If you get it low but it stalls out , then adjust the throttle stop screw to maintain idle rpm
Correct adjustment is from this point the rpm increases rapidly with out falter ( fine tune high speed screw to achieve that)
and when left to idle will do so evenly without driving the chain
all of this working is depending on the carby having new diaphragms , needle height adjustment correct and the system cleaned out

Most chain saws have a simple adjusting screw that is tapered on the end to a point. The throttle plate actuator arm rests against the taper. A clockwise rotation of the adjustment causes the arm to slide slightly further up the inclined point, opening the throttle plate a tiny bit which results in a slightly higher idle.

Idle speed is entirely different than the low speed adjustment.

adjust fuel screw turn it one turn out see if that gets it running if dose turn it slowly in until throttle runs smooth the screw to adjust is the h screw on carb screw mark on top h or body will have a h on it turn it out one turn only but before that try to srart sqiuirt some starting fluid in carb sart engine leave choke off hold throttle wide open should run

I do that on people's mowers when they come in not running fast enough. If they are running too fast I stretch them a little and even cut the end off the spring and reshaped the end to get the right speed. Yes you can do this but be sure you do not over rev the engine or you will ruin it.

After seating the screws back them out 1 1/2 turns. This should be close enough to get it started. Let the motor warm up a little then, with it idling, turn the low screw in until it starts having a hard time running then back it out until it idles smoothly. Prop up the Mantis so it can run at full speed safely while you adjust the high side. With the throttle wide open, again turn the high screw in until it starts running roughly and back it out until the motor is running smooth. Some say to turn the screws out until the motor starts having a hard time running and then turn it back in half way to where it was running bad at the other end. This doesn't work as some engines don't care how far open the screws are.

The high speed mixture screw is initially set at 1 & 1/2 turns out. Then adjust it to give good acceleration and top speed, while not closing the screw more than you need to. The low speed mixture screw is set at 3/4 of a turn out. Adjust that, to give a smooth idle. The 3rd screw, the idle speed screw, is set to give a low idle speed without stalling.

First I would check the carb and make sure the throttle plate has not come loose from the throttle shaft. If it has there is a small screw missing which should be retrieved if at all possible. Also check the springs on the throttle linkage to make sure they are not in a bind.
If the carb checks out and nothing is hung up then you can adjust the running speed by bending the tang that holds the fixed end of the throttle spring. bending towards the carb and loosening the spring will slow the motor and away from the carb/ stretching the spring will increase speed.
If you have slacked the spring and the motor continues to run fast then it needs a govenor adjustment.

ok here goes. you are write to wind out the screws 1 1/4 turns +/-

you have 2 scews for the carb jets, the other is the idle speed adjuster. it has a conical shaft which simply open the throtle more as you screw it in. wind this in so that it will run a bit fast . if you can, start the saw. Without pulling the throttle, slowly turn the "low" jet in , or out untill the saw runs faster. if it starts running too fast, adjust the idle speed scew down a little. do this untill it is running best. adjust the idle speed down until the cahin just stops moving. the low speed is now set. now pull the trigger and run the saw full throttle. adjust the "high" jet in or out slightly until it is running fastest. Note..if you turn either screw in too far, the engine will stall.

I hope this helps,

Another thing that could cause problems is if there is an air gap between the carb and the cylinder. check that fixing screws are tight. check that the gasket is ok. some saws have a small rubber manifold between the carb and the cylinder. if this manifold is perferated or damaged,it will let air into the engine, and therefore will not run right.
good luck.

If the bike runs then Top Dead Center (TDC) is not an issue. But to check piston stroke position, Remove the valve cap cover on the intake of the front cylinder. Remove both the spark plugs. Rotate the crank shaft untill the rocker arm starts to move. The piston is now starting down on the intake stroke. Rotate the crank further and the rocker arm moves in the opposite direction. The piston is now going upward on the compression stroke. Look at the marks on the flywheel. Rotate the crank untill the TDC mark lines up with the timing index mark. The front piston is now at TDC.

If your model has carbs then the backfire fix requires you to syncronize and balance the carbs.
SYNCHRONIZE THE CARBS
Remove the air filter and rubber fittings to allow you to see into the throat of both carbs. You may need a mirror to see inside. You need to be able to see the slides go up and down when you twist the throttle. Now turn the fitting where the cable goes into each carb such that there is just a bit of slack in the cable, 1/16" is plenty. Lock ONE of the cable adjusters down tight. All further adjustments will be made on the other carb.

With the motor turned off twist the throttle very slowly while looking at the slides. Both slides need to begin lifting at the exact same moment. If the slides don't raise at the exact same time then slowly twist the throttle until the locked adjuster slide just barely starts to move. Hold the throttle still and turn the adjuster on the other carb so that the slide on that carb just barely starts to move also. Now recheck the slide movement timing. Do this process until the slides on both carbs begin to raise at the EXACT same moment. Lock down the loose adjuster and re-check the slides.

BALANCE THE CARBS
Turn the throttle screw on the LEFT carb one half turn inward. Now start the engine and get it warmed up. With the engine running, remove the RIGHT spark plug wire. Adjust the idle speed on the left carb to the point where the engine just can't quite stay running and dies. Now put the loose spark plug wire back into place.

Now turn the throttle screw on the RIGHT carb one half turn inward. Restart and rev the engine. Next, remove the LEFT spark plug wire. Adjust the idle speed on the right carb to the point where the engine just can't quite stay running and dies. Now put the loose spark plug wire back into place.

Turn the idle screw on each carb EXACTLY ONE QUARTER turn outward and restart the engine. The engine will be probably be idling very fast. Adjust both the idle screws equally from this point to get to the desired idle speed. Congratulations, you have just balanced and synchronized your carbs.

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Try balancing ans syncronising the carbs.
BALANCE THE CARBS
Turn the throttle screw on the LEFT carb one half turn inward. Now start the engine and get it warmed up. With the engine running, remove the RIGHT spark plug wire. Adjust the idle speed on the left carb to the point where the engine just can't quite stay running and dies. Now put the loose spark plug wire back into place.

Now turn the throttle screw on the RIGHT carb one half turn inward. Restart and rev the engine. Next, remove the LEFT spark plug wire. Adjust the idle speed on the right carb to the point where the engine just can't quite stay running and dies. Now put the loose spark plug wire back into place.

Turn the idle screw on each carb EXACTLY ONE QUARTER turn outward and restart the engine. The engine will be probably be idling very fast. Adjust both the idle screws equally from this point to get to the desired idle speed.

SYNCHRONIZE THE CARBS
Remove the air filter and rubber fittings to allow you to see into the throat of both carbs. You may need a mirror to see inside. You need to be able to see the slides go up and down when you twist the throttle. Now turn the fitting where the cable goes into each carb such that there is just a bit of slack in the cable, 1/16" is plenty. Lock ONE of the cable adjusters down tight. All further adjustments will be made on the other carb.

With the motor turned off twist the throttle very slowly while looking at the slides. Both slides need to begin lifting at the exact same moment. If the slides don't raise at the exact same time then slowly twist the throttle until the locked adjuster slide just barely starts to move. Hold the throttle still and turn the adjuster on the other carb so that the slide on that carb just barely starts to move also. Now recheck the slide movement timing. Do this process until the slides on both carbs begin to raise at the EXACT same moment. Lock down the loose adjuster and re-check the slides.

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