Major fallacy in accepted manuscript I have submitted a manuscript around two months ago. The reviewers gave me generally good comments, and the editor suggested "minor revision".

Problem is, I think the main part of my manuscript have a big problem. I am currently making further readings into the subject and delving deeper into the math. All previous research suggest that I am giving an incomplete solution, resulting eventually in an inaccurate simulation.

Although I have a large mistake in the manuscript. This mistake if corrected will support my results even more (Lucky me).

I will tell the editor of course about the mistake, but I need time to correct my simulation, manuscript etc...

My questions are:

1- Can I ask the editor form more time?

2- Will the editor reject my paper because it have a major mistake?
The answers are yes and yes. But for most purposes more like yes and unlikely. 

You can ask for time. It may be possible to get time or not. For a conference it is harder, of course, especially when there is some uncertainty about when it will really be finished. For a journal asking for time might cause minor disruption, but it would likely only result in getting published in a later issue of the journal. 

But the editor can also ask that the paper be withdrawn and resubmitted when it is complete. That is up to them. If they thought the paper was originally good, then they are more likely to work with you. 

And be sure to indicate that the paper will be improved, not just corrected.