Rephrasing the Question
It seems you are asking us, "How do I get my current company to give me an extended time off so that I have a job to come back to while looking for another position?" What you give us as background is contradictory:
It's possible that I might want to leave this company after a few months later. So, I do not wish to sour any relationships by asking for this favor and then leaving.
I have considered the possibility of leaving this job in 2 months and then travelling without time restriction and then returning to find a job. But I think I should be concerned about finding work, because I have been doing a few interviews (for coding/systems engineering positions) and haven't had any luck yet.
If you don't want to sour relationships, it obviously isn't a brilliant idea to ask a huge favor and then plan to leave. If you don't want to leave without having a job lined up, then you probably shouldn't fixate on traveling in two months. If you don't want to stay in this job long, you're probably better off focusing on finding a new position rather than focusing on traveling. Asking for a long vacation is not a small thing, and the casualness with which this question reads doesn't sit well with me.
I hope it was a brief lapse in judgment, and that thinking it through will set things straight.
Put Yourself in your Boss' Shoes
So your boss just promoted you. He said to his bosses, "CareerQuestions is responsible and hard-working. I want to give him a promotion and a raise." He has put his neck out for you.
And how do you repay your boss? "Thanks for the promotion, by the way, could you convince your bosses to give me another 6 weeks of unpaid leave before I get that promotion?"
So let's say he's a really nice boss. And he says, "Sure CareerQuestions, if that's what it takes for you to be happy, I am more than willing to help you out." He expends political capital to convince his management you're worth giving the special treatment to, and covers for the position you're absent from for the 6 weeks.
Upon coming back, you find yourself another job in a couple weeks/months. You come in to your boss and say, "Hey boss, thanks for everything, here is my two weeks notice."
How are you going to feel about that?
You have fought for this employee not once, but twice. You put your personal reputation on the line on the assumption that the employee would not abuse your trust. Instead, the employee was being 100% selfish and gave absolutely no consideration to what impact their actions would have on you or the company.
Think you'll get a nice recommendation letter?
You have three choices:
- Ask and live with the answer
- Ask and quit if the answer is no
- Don't ask and find a job that will give you time before starting
Abide by your Boss' Decision
Go in and talk to your boss:
Hey boss, I was wondering if before starting at my new position in August, I could take 6 weeks off (unpaid of course). I know it is a big favor to ask, but I've always wanted to travel, and since I will be handing my responsibilities off before switching to the new role, this seems like the best time to do that. I know there is no ideal time to ask for a long break, but you can't get what you don't ask for!
If your boss says "no" then you ask if there's the possibility in the future, and live with whatever he says. Because he is your boss. And he's already given you a promotion. And this is a mighty big favor to ask.
Be Willing to Lose the Promotion/Job
The second option will give you a better chance at getting the job, but at a bigger personal risk. You ask the same as the above, but are willing to say the following depending on the situation:
I know I was just promoted, and if the promotion is the only issue with me taking an extended unpaid leave, I am more than happy to pass on the promotion because this is important to me.
Or even in a more extreme case (where the boss says it is flat out impossible).
I understand it's a big request and I'm sorry to have put you in such a tough situation. This is incredibly important to me, much more so than this job, and if that's the choice I have to make I will hand in my resignation letter and give notice.
In the best case your boss decides that keeping you is more important than whatever barriers there are to agreeing to your request, and you get what you want without giving up anything. In the worst case, you quit the job and get to travel.
Realize that if he gives permission, there is an expectation that this will prevent you from quitting. If you return and quit you will burn bridges. Very quickly. And for good reason.
If you aren't willing to give up your job or your travel, then your only other choice is to negotiate with other companies. Don't tell your boss about your plans to travel, and keep looking for other jobs. When you discuss your plans with other potential employers, be clear that you will be ready to start from date X, where X is your notice period plus six weeks for traveling. This way you have a job waiting for you when you come back, and the future employer is agreeing to let you do that up front.