HaRav Moshe Mordechai Karp

To quote from Ben’s Tallit Shop:

“Why don’t more Ashkenazim tie techelet according to the Tosafos? One reason might be found in a responsa written by HaRav Moshe Mordechai Carp in 5772, where he notes that you cannot use a techelet string in place of white, therefore if halacha is not like the Tosafot, you would have too many techelet strings in your tzitzit.”