After new clutch sprocket &good bar &chain chain r bar moves prematuely and chain comes off bar nuts r tight

Check that the chain brake is in disengaged position by
moving the front hand guard towards the front handle.
Remove the bar nuts and remove the clutch cover (chain
brake). Take off the transportation ring.
Fit the bar over the bar bolts. Place the bar in its rearmost
position. Place the chain over the drive sprocket locate it in
the groove on the bar. Begin on the top edge of the bar.

Make sure that the edges of the cutting links are facing
forward on the top edge of the bar.
Fit the clutch cover (chain brake) and locate the chain
adjuster pin in the cut-out in the bar. Check that the drive
links of the chain fit correctly on the drive sprocket and that
the chain is in the groove on the bar. Tighten the bar nuts
finger-tight.
Tension the chain by turning the chain tensioning screw
clockwise using the combination spanner. The chain
should be tensioned until it does not sag from the
underside of the bar.
The chain is correctly tensioned when it does not sag from
the underside of the bar, but can still be turned easily by
hand. Hold up the bar tip and tighten the bar nuts with the
combination spanner.
When fitting a new chain, the chain tension has to be
checked frequently until the chain is run-in. Check the
chain tension regularly. A correctly tensioned chain
ensures good cutting performance and long life.

The chain may have come off the sprocket.
There are 2 nuts about an inch apart down just past the bar which hold it in place.
Remove the 2 nuts, take off the cover which they hold in place and the bar and chain will loosen.
Check that the teeth on the inside of the chain are on the sprocket. Pull the chain tight against the sprocket and move it. Check that the sprocket turns. With the chain set in the sprocket replace the bar, chain and cover and try turning it again by hand. If it doesn't turn don't try to start the saw as you'll do damage.

If the chain was on properly and it still won't turn the clutch may be burned out. However the fact that you couldn't turn it by hand suggests that the chain was off the sprocket.

Remove the cover at the drive end and back off the chain adjustment several turns. Make sure the chain brake is released. Check the condition of the sprocket on the outer clutch drum looking for tooth damage--also check the outer clutch drum bearing for looseness. Lay the chain out on a table or board--look for lateral bending, broken or bent links/teeth, or other obvious damage. If ok, clean the bar oil passages including the two small holes that feed oil out to the chain groove from the larger holes on each side. Also, clean the entire chain groove and lube the end sprocket--check to see that it turns freely. If all ok, fit the bar in place, then thread the chain over the bar and sprockets. Pull the bar outwards and fit the cover back over the bar. Turn the clamp nuts finger tight, then adjust the chain tension until the chain pulls up to the lower edge of the bar so that it doesn't droop, but not so tight that you can't move the chain by hand. Make sure the teeth cut toward you on the bottom run and that the teeth are properly sharpened. Tighten the clamp nuts fully and recheck tension. If ok, start the engine and run the bar tip near a piece of cardboard for a few moments--it should throw off a thin line of oil onto the cardboard. When properly sharpened, the chain will readily cut quickly without undue effort and will make straight cuts. Forcing the cut will overheat the chain and cause it to come loose. Hope this helps!

Make sure the chain brake is off (pull it toward you), then remove the bar mounting nut(s)/bolt(s) and the remove the side cover, bar, and chain. Clean the oil passages in the rear of the bar including two small passages that run from the larger holes out to the chain groove on each side. Clean the entire chain groove and make sure the end sprocket turns freely and is well oiled. Clean the engine face where the bar fits, then start the engine. Oil should ooze out of a small port in the engine case. If ok, then lay out the chain so that it can be inspected for bent links, dry/sticking joints, or missing/badly worn drive teeth. Inspect the sprocket on the clutch outer drum component for wear or damaged teeth. Make sure the pitch of the chain teeth match those of the drive sprocket (is this a new chain?). Check the hub bearing of the clutch drum for obvious wear. If all looks good, fit the chain around the clutch sprocket, then slip the bar into the the engine mounting space and work the chain over the entire bar. When setting the outer cover, make sure the adjuster tang fits into the related bar hole, then attach the clamp nut(s)/bolt(s), run up finger tight. Use the adjuster to set the chain tension--it should just pull the lower part of the chain up to the bar plus a little more. Check that you can move the chain by hand without any binding, then tighten the clamp bolts/nuts firmly. Recheck that the chain is properly tensioned. If not, re-loosen the clamp fasteners and re-tension the chain. When the clamp fasteners are tight, the tension must be rechecked. Hope this long-winded response helps!

ok i assume it is an electric chain saw? chain usually gets loose and can jump off clutch sprocket . to remedy this there should be atleast one nut or bolt sometimes two nuts or bolts depending on brand and model on the side cover where bar and chain inter the clutch area remove those and cover will lift off second there is a screw that faces front next to bar should be a regular slotted screw head loosenit all the way up till bar gets loose and remove bar by pulling away from saw this will come off veryeasy and chain shoouls fit on teeth of clutch sprocket and fit back onto bar tighten screw till chain tightens up on bar then replace cover and nuts on side cover but do not tighten the with one hand put tension on chain by pulling down on it while tightening screw till chain feels elastic snapping back to bar then toghten up nuts . chain should have no more then 1/4 inch slack around bar this is the correct tension youshould maintain.

Your chain should not be tight ... only snug. If it keeps throwing the chain, your bar is not on correctly and/or the clutch needs replaced. Do not use old chain with a new clutch.

On the chain saw there should be a panel held on by two screws or nuts where the bar and chain is attached. Remove the two screws or nuts and the panel and you will expose the chain and sprocket. Loop the chain around the sprocket and bar. Put the bar with the chain around the sprocket back on the saw, reinstall the cover with the two screws/nuts, but do not tighten too tight.(You will want to be able to move the bar with the chain.) Then pull the bar away from the saw to tighten the chain, once the chain it tight, then you will retighten the screws or nuts to lock the bar and chain in place. Be sure you lube the chain and bar with "Bar Oil" before starting to put the saw into service. Good Luck...I hope this helped Please rate me, Thanks

Did the chain come off while at speed? The chain catcher causes burrs to form on the drive links when occurs. Without the catcher the spinning sharp chain contacts your leg. Were you wearing chaps? We don't need no stinking PPE!!!
Use a flat file on the drive link flats to remove the burrs.
Chain Adjustment

1. Make sure the chain brake is off (gloves will protest your fingers).
2. If necessary, loosen the bar nuts that are clamping the clutch cover in place (or whatever holds the clutch cover and bar on). They don't need these really loose - finger tight will do.
3. Ease the tension adjustment until the chain is obviously loose.
4. Using one hand, hold the nose of the bar up.
5. While holding the nose up, adjust the tensioner so that the center of the chain just touches the underside of the bar, add a touch more.
6. Continue holding the nose of the bar up & tighten up the bar nuts / bolts / quick-release system. HTH

Lou

Put it over the sprocket first, then work your way right putting it into the bar grove as you go, the nose last. Grasp and pull the bar right the chain should rise into grove, you may need to help. Loosen the chain tensioner some put the clutch cover on and tighten the bar nuts finger tight.
Chain adjustment

1. Make sure the chain brake is off (gloves will protest your fingers).
2. If necessary, loosen the nuts that are clamping the guide bar in place (or whatever your saw uses to hold the side plate and bar on with). We don't need these really loose - just finger tight will do.
3. Ease the tension until the chain is obviously loose.
4. Using one hand, hold the nose of the bar up.
5. Now, while holding the nose up, adjust the tension so that the chain just touches the underside of the bar.
6. Continue holding the nose of the bar up & tighten up the side plate nuts / bolts / quick-release system.

If you've already tried to tighten the bar tensioning nuts and the bar keeps "creeping" and gets tighter, that is unusual, as most times the bar should get looser, not tighter.

I would suggest you try a new bar and set of tension nuts, or at the very least a new set of tension nuts first.

The problem might be the tip sprocket has worn out (it needs to be lubed every time you add bar & chain oil to the reservoir in the chainsaw). If the tip sprocket is worn badly, you should replace the bar and inspect the cutting chain for excessive wear on drive side of the chain.

Remember that the slightest bit of dirt can cause rapid wear of the bar and chain and quickly ruin it beyond repair.

I hope you find this helpful and best regards!