I bought a used Simonelli Mythos with tamper. When grinding, the grounds "spray out" after half way during grind cycle and misses the portafilter. I am assuming due to burrs being used or old. Has anyone come across this issue?

I also have a "Plus" model with the tamper attachment. With it in place, it's nearly impossible to see & access your chute to diagnose what may be going on. I removed the fold-down metal face panel from the machine on day 1, so I can't recall if it too would obstruct your view. With the PF cradle mounted, you're fairly limited in what size or shape of a tool you might use to stick into the chute and clear it out. I used a disposable chopstick or bamboo skewer and also compressed air with a long narrow nozzle.

Did you do any degree of cleaning to the machine? If not, perhaps knock that out before continuing use for coffee. I removed the hopper & opened the burrs up to large/coarse setting so I could thoroughly blow out any residuals with a can of compressed air. Then all I did was pick up some cheap freshly roasted beans from our neighborhood market (they roast in house) and some instant rice. Adjusted grind setting to very fine, ran half a box of rice through, then ~4oz coffee beans, the second half of the rice, and another ~4oz of coffee beans. I used the remaining beans to do small single doses of grinds just to view & adjust the burrs to an approximate starting place for the coffee I'd actually be pulling shots with, and never had a problem.

My PF is a measly 49mm and I use an OE funnel ring to ensure I don't have stray grounds scatter, and the grinder never makes a mess until I've overdosed the basket by at least 2g+. I've also experimented with grinding into a standard ramekin before manually dosing the basket, and with the ramekin I've never had grinds spray outside it's diameter. If yours continue wildly spraying, you may need to replace the plastic flap/curtain that covers the chute exit. Alternatively, if your unit is older than 2014 (I think?) you could purchase and install the clump-crusher, but that's not necessary. YMMV, but my average grind setting is at the 5 mark on the dial and I get a 16.5-17.5g dose in about 2.5 seconds (2 pulses at 1.25 sec each; taping/shaking the mound down between pulses so it actually stays within my basket).

*edit: if this continues, perhaps you could contact a local cafe maintenance company who's familiar with the Mythos. Describe your issue to them and ask if they'd think the burrs are to blame - I'd hate to shell out for and need to break in a new set of those burrs!

If its an oder variant then installing the new clump crusher will benefit you
It may be cheaper to oder in the Uk from Bella Barista than the USA suplier
its easy ish to do
http://coffeeforums.co.uk/showthread.ph ... thos+clump