Chain tightens up under power to point engine stalls

First, RTFM- read the manual. Lost it? No problem, download it from Poulan PRO website page for PP4620avx. Pick a version you like, save and read it. Google is your friend.
Chain Adjustment Part cut and pasted as follows:
Adjusting the tension:
Chain tension is very important. Chain stretches during use. This is especially true during the first few times you use your saw. Always check chain tension each time you use and refuel your saw.
1. Lift lever on bar knob and turn counter-clockwise 1.5-2 turns to loosen clutch cover.
2. Turn adjusting wheel (downward direction) to tension chain until chain solidly contacts bottom of guide bar rail. Lift tip of guide bar while adjusting tension. Tighten chain Adjusting Wheel
3. Use a screwdriver to move chain around guide bar to ensure all links are in bar groove.
4. While lifting tip of guide bar, tighten bar knob securely (clockwise).
Hand tighten only.

Be careful not to destroy the adjustment mechanism. When it breaks, go for a clutch cover with the simple adjuster that needs a "scrench". Longer term, learn how to search this info. Saves time. Now you can work on sharpening chains- very important.

Manual page:
http://www.poulanpro.com/ddoc/PPOO/PPOO2006_USen/PPOO2006_USen_PP4620AVX_952802146.pdf

The main reason for a chain to tighten is because it is not getting enough oil to the bar, or too thin an oil is being used, you should see a good spray of oil off the end of the bar when held against a tree stump, if not try adjusting the flow if it has an adjuster.

Depends on what model, however there are two main locations for the adjuster screw, the most common is at the very front of the saw, running inline with the guide bar , you may well have to clean the dirt and saw dust from the front of the saw to see it,it will be close to the inside edge of the bar, it will be a normal screw driver head screw, turn clockwise to tention the chain, the other type will be in the side cover, at right angles to the saw, again it will be a normak screw driver head, turn clockwise to increase the chain tention.

Most Chain Saws use a 25 :1 Fuel : Oil Ratio.

You obviously have taken the chain and bar off the saw. Start by meticulaously cleaning the oil port/s. At the front of the bar on the engine side, you should find a screw that moves the blade back and forth. Turn this screw counter-clockwise to allow placing chain loop on the bar and drive gear. Make sure that the sharp portion of the tooth of the chain is facing toward the engine of the saw, then the cogs will fit correctly. At the back end of the screw that you turned, is a nib that fits into the blade of the saw. Once you start assembling it,you can tighten the screw. Tighten the two bolts holding the blade and chain just enough so that you can still adjust the screw. Now, tighten that screw to a point where the chain can still be moved freely, but does not have a droop on the under side of the blade. Tighten,run the saw a bit, readjust if necassary and you are ready to go.

Normal 0 This will be the basic instruction for most common saws. Check that the chain brake is in disengaged position by moving the front hand guard towards the front handle if the saw is equipped with a chain brake on the handle. Loosen the bar nuts on the clutch cover. Loosen tension screw or other adjuster (thumbscrew) to allow the bar to be moved toward the engine. Some saws have this tension adjustment built into the bar. This will provide enough slack in the chain to able to reinstall it. It may be necessary to remove the side cover to gain access to the clutch/gear assembly on the side of the engine if the chain also jumped the gear here. Remove the cover, inspect for any debris or other obstructions in this area. If none, replace the chain back over the drive gear and in the bar groove all the way around the bar. It may be necessary to turn the bar adjuster screw more to provide enough slack in the chain to do this. Once the chain is in place reinstall the cover and bar lock nuts. Snug up the nuts but do not tighten these. Turn the bar/chain adjuster screw to tighten the chain on the bar (take up slack). I usually tighten to a point where there is no slack between the chain and the bottom of the bar and I can still turn the chain easily around the bar by hand. With experience you will get the feel for this chain tension. Once the chain is properly tensioned, tighten the bar lock bolts/ nuts on the side cover. I hope this helps you out.

Normal 0 This will be the basic instruction for most common saws. Check that the chain brake is in disengaged position by moving the front hand guard towards the front handle if the saw is equipped with a chain brake on the handle. Loosen the bar nuts on the clutch cover. Loosen tension screw or other adjuster (thumbscrew) to allow the bar to be moved toward the engine. Some saws have this tension adjustment built into the bar. This will provide enough slack in the chain to able to reinstall it. It may be necessary to remove the side cover to gain access to the clutch/gear assembly on the side of the engine if the chain also jumped the gear here. Remove the cover, inspect for any debris or other obstructions in this area. If none, replace the chain back over the drive gear and in the bar groove all the way around the bar. It may be necessary to turn the bar adjuster screw more to provide enough slack in the chain to do this. Once the chain is in place reinstall the cover and bar lock nuts. Snug up the nuts but do not tighten these. Turn the bar/chain adjuster screw to tighten the chain on the bar (take up slack). I usually tighten to a point where there is no slack between the chain and the bottom of the bar and I can still turn the chain easily around the bar by hand. With experience you will get the feel for this chain tension. Once the chain is properly tensioned, tighten the bar lock bolts/ nuts on the side cover.


Some Poulan Pros have an adjuster screw on the front of the engine case right beside the bar, some have a tool less adjuster on the clutch cover, and some have a slot in the bar for adjusting the chain tension.


I hope this helps you out.

Check the muffler to make sure that the spark arrestor screen is not plugged. Make sure the air cleaner is clean and not plugged. Turn both L and H mixture screws CW to stops (lightly), then back out 1-1/2 turns each. Warm up the engine, then open the throttle and turn the H needle CW until the engine speeds up, but still '4-stroking' but should immediately start '2-stroking' with light cutting. Release the throttle and adjust the L needle CW for fairly smooth idle, yet rich enough to allow the engine to 'follow the throttle' immediately when accelerated. Adjust idle speed CW until the chain starts to turn, then back off until it quits turning. Make sure the chain is being properly oiled and is adjusted for no chain droop, yet turns freely by hand. Hope this helps!

Loosen Chain.
Remove Chain.
Install new chain.
Tighten new chain.
Done.

The McCulloch ProMac 600 series (610/650) doesn't recommend a saw chain replacement part because you can get differently-sized bars for it. This one came with a 20" bar.
According to an Amazon.com search tonight, the Oregon company makes a replacement part stock number D70. I have got an Oregon H78 replacement chain at my hardware store and it's a hair too small. Not all 20" saw chains are created equal!!