What type of oil do I need to mix with the gas for the Echo PB 2100?  Also, what ratio?

I am guessing that this is a 2 stroke. If so, you must make sure you are mixing the oil and gas ratio correctly. Two much oil and it will smoke alot and run out the exhaust in the form of watered down oil. If the mix is correct, you may have a a bad ring.

Sometimes, the fuel ratio is stamped on the fuel cap. Your ratio is 50:1. Echo specifies using 89 (mid-grade ) octane Here is a chart that tells you how much oil you need in a gallon of gas...Just 1/2 it for a half-gallon mix. A copy of your manual is also here.

http://support.mtdproducts.com/app/answers/detail/a_id/347/~/two-cycle-engine-fuel%3Aoil-mixture-chart

http://www.echo-usa.com/getattachment/624aee9f-103c-416c-ae9b-55fb42dd94af/PB250es1112_010410_1.pdf

per http://www.echo-usa.com/faqs/ it 50:1, there is a chart at the site that shows how to mix it.

50:1 means 50 parts gas to 1 part oil
you can mix a quart of gas with 1.5 oz of 2 sroke oil

Since every engine is different the manufacturer almost always has it printed in the gas cap. It should look like 50:1 or 50/1 (50 parts gas to 1 part oil).

40:1 is the standard. You can achieve this by mixing a 3.2 ounce bottle of two-cycle oil with a gallon of gasoline. The best way to do that is to buy a one gallon gas can, add a little bit of gas then pour the entire 3.2 ounce bottle of oil into the tank, swish that around for about 5 seconds then fill the can the rest of the way.

I didn’t see a manual for your blower but page 14 for a PAS 260 says:
“IMPORTANT
Echo Premium 2-Stroke Oil may be mixed at 50:1 ratio for application
in all Echo engines sold in the past regardless of ratio
specified in those manuals”
http://www.echo-usa.com/documentation.asp?Model=PAS-260&lang=E
HTH
Lou

50 to 1

Your manual can be found here:
http://www.echo-usa.com/documentation.asp?Model=PB-251&lang=E
Please download a copy, IPL too.
Page 21 - Fuel Oil Ratio- 50:1 (2.6 oz/ 1 gal)
A rich mixture often causes hard starting.

My echo uses 50-1 ration. Home Depot sells the small containers for Echo Leaf Blowers that you mix with one gallon of gas.