i installed new big valves for my lancer 97 from a 98 model galant, after assembling the engine the engine misfire.... my friend adviced me to change my cam gear to an adjustable cam gear. because the problem he told me was the valves closes a little bit late than the usual. could this be true?

 

more than likely the rod of the valve is slightly longer causing the open durration to be slightly longer than the stock ones.... Did you just replace the valve or the entire head assembly? Either way you need to advance the timing a little and would benefit from a tuned cam shaft.

 

Did you match the valve seats with your bigger valves ?

I hope you did. For it won't have a proper seal when the valve closes. Which will cause a missfire. Not getting good compression.