multi decks beatable wit ko count?

when i count throught the 8 decks of cards it takes a long time until it gets to the key count of -6. when u start out wit the irc of -28. a lotta times it gets there at the last 2 decks or so. should i start raising my bets when its like -18 or sumthing?

Yes, it could be beaten, but definitely not the way you're playing it right now, as BJ expert Don S. wrote in his book, BJA3. Consult chapter 1 of that book for a full description of backcounting. This gives u better results than playing the whole shoe with the same bet spread.

The bet spread as mentioned in BJA3 assumes you're using the Hi-Lo or other balanced counts. For KO, refer to the KO book for the betting strategy, but of course, you can let the bets ride as the count goes up, no matter which system you're using. A bet spread of 1-16 or more should be used to provide better results in the 8-deck game, while 1-12 for a 6-decker, as recommended in the article, is fine.

I play KO and of necessity play mostly 8 decks, so I understand what you are saying. The game can be beaten, but not for much money unless back-counting is a reasonable option and you have the bankroll and table limits to get big bets out.
In answer to your question, do not raise your bet at -18. That's only 10 points above the IRC, and even if you were to get there after only one deck you are still at a disadvantage. Wait for the Key Count,or use entry points of -14 after 1 deck, -11 after 2 decks, -8 after 3 decks, and the KC from there on.
It takes patience and it can be boring.