4-screw roundtop piston sticking

I have that one Phillip linked above.  It does more than I can even figure out how to use, initial timing, full advance and the RPM functions are all I've needed so far.   

I bought the red one from amazon.com on that internet holiday sale, Cyber Monday? it was about $100.  You might ask local chain parts stores if their loaners have the RPM function and save some money.

Here it is, still $100. https://www.amazon.com/dp/B000EVU8J8/ref=sxr_zg_dy_1?pf_rd_m=ATVPDKIKX0DER&pf_rd_p=3525596382&pd_rd_wg=0Swi0&pf_rd_r=SW120JF40SSVWMTZN8GZ&pf_rd_s=desktop-rhs-carousels&pf_rd_t=301&pd_rd_i=B000EVYH72&pd_rd_w=eEJpa&pf_rd_i=innova%2Btiming%2Blight&pd_rd_r=8d4828d6-c1ff-41d9-a5d3-90c5452b20df&ie=UTF8&qid=1523201371&sr=1&th=1