I have the same problem too. I searched and found that Q2A 1.7.0 has already add a space between 2 Kanji for Japanese sentence. And it make search result to be awfully very bad.
So I add some code to qa_string_to_words() function in qa_include/util/string.php
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/* remove this because japanese word can contain 1 or more kanji. Replace this code with these below
if ($splitideographs) // put spaces around CJK ideographs so they're treated as separate words
$string=preg_replace('/'.QA_PREG_CJK_IDEOGRAPHS_UTF8.'/', ' \0 ', $string);
*/
//split を = (\p{3092})
$string=preg_replace("/(\x{3092})/u", ' \1 ', $string);
//split れ[る|た] = (\p{308c}[\p{308b}|\p{305f}])
$string=preg_replace("/(\x{308c}[\x{308b}|\x{305f}])/u", '\1 ', $string);
//split の = (\p{306e})
$string=preg_replace("/(\p{Han})(\x{306e})(\p{Han})/u", '\1 \2 \3', $string);
//split は = (\p{306f})
$string=preg_replace("/(\p{Han})(\x{306f})(\p{Han})/u", '\1 \2 \3', $string);
//split が = (\p{304c})
$string=preg_replace("/(\p{Han})(\x{304c})(\p{Han})/u", '\1 \2 \3', $string);
//split not kanji and kanji
$string=preg_replace("/(\P{Han})(\p{Han})/u", '\1 \2', $string);
$string=preg_replace("/(\p{Han})(?=\P{Han})(\P{Hiragana})/u", '\1 \2', $string);
//split katakana and not katakana
$string=preg_replace("/(\p{Katakana})(?=\P{Katakana})(?=[^\x{30fc}])/u", '\1 \2', $string);
$string=preg_replace("/(?=[^\x{30fc}])(\P{Katakana})(\p{Katakana})/u", '\1 \2', $string);
//split Hiragana and katakana, romaji
$string=preg_replace("/(\p{Hiragana})(\P{Hiragana})/u", '\1 \2', $string);
//$string=preg_replace("/([a-zA-z0-9])(\p{Hiragana})/u", '\1 \2', $string);
$string=preg_replace("/(?=\P{Han})(\P{Hiragana})(\p{Hiragana})/u", '\1 \2', $string);
// removing Japanese punctions
$string=preg_replace("/[\x{3000}-\x{3004}]|[\x{3008}-\x{303f}]|\x{30fb}|[\x{ff00}-\x{ff0f}]|[\x{ff1a}-\x{ff20}]|[\x{ff3b}-\x{ff40}]|[\x{ff5b}-\x{ff65}]|[\x{ffa0}-\x{ffef}]/u", ' ', $string); // remove apostrophes in words
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Here is my page: Kiwidic.com Japanese - Vietnamese dictionary
I think it is not the best but the result is improved alot.
P.S remember to reindex DB after modify this function
