user-1: There are many parables and stories from the Buddha's life suggesting he valued above all what is practical and conducive to virtue. He discouraged metaphysical speculation and musings. 
Given without knowledge one can only speculate, does Buddhism suggest one should delay certain questions until one can empirically get a definite answer?
For example, the jhanas are not immediately accessible, and thinking about them might lead to speculation. What is speculation in Buddhism? Should one always restrict their attention and cognition to what is immediately accessible?
I guess I could also add: How does one know when one is speculating?

user295: Kesa,puttiya Sutta (Kālāma Sutta) encourages free inquiry but this may also lead to misinterpretations due to erroneous reasoning. Even experiences can be miss identified and miss interpreted. [Brahma,jala Sutta]
In the context of Buddhism, it is best one's questions and speculations are best geared towards understanding the 4 Noble Truths which gives rise to the right view.

"And what is right view? Knowledge with regard to stress, knowledge with regard to the origination of stress, knowledge with regard to the cessation of stress, knowledge with regard to the way of practice leading to the cessation of stress: This is called right view."
— DN 22

Right View
Ultimately one's understanding would be based on:

learning of the dhamma (pariyatti)
put it into practice (patipatti)
verification of what was learned at the experiential level (pativedha)

If one tries to interpret empirical experiences by oneself one may be miss led. What is wise or praiseworthy if to look for the theory that explains the experience.
Ultimately only the fully enlightened Buddha has the know-how to correctly infer every experience. One speculation may be right or wrong. To find what is right one might need to consult the dhamma.
For part of the dhamma, ones hs not experienced the best cause of action is to learn the theory and practice. Put the practice into action and verify both theory and practice.

As a side note, the 1st few Jhana's are accessible if one practice the technique properly. It is a matter of putting in the time and effort to do it

user-1: Speculation is the rumination on any topic that can only be truly apprehended through direct insight.  Unfortunately, about the only way you'll ever figure out if you're guilty of that is by asking your teacher.  I know this sounds like a flip answer, but it's the only safe way (at least until you're enlightened to some degree) to ensure that what you're coming up with isn't just flabby philosophizing.  Let me give you an example.  There's a koan that goes like this:

Case 29 from the Mumonkan: The Sixth Patriarch's "Your Mind Moves"
  The wind was flapping a temple flag, and two monks started an argument.
  One said the flag flapped, the other said the wind flapped; they
  argued back and forth but could not reach a conclusion. The Sixth
  Patriarch said, "It is not the wind that moves, it is not the flag
  that moves; it is your mind that moves." The two monks were
  awe-struck.
  Mumon's Comment 
  It is not the wind that moves; it is not
  the flag that moves; it is not the mind that moves. How do you see the
  patriarch? If you come to understand this matter deeply, you will see
  that the two monks got gold when buying iron. The patriarch could not
  withhold his compassion and courted disgrace.
  Mumon's Verse 
       Wind, flag, mind, moving,
       All equally to blame.
       Only knowing how to open his mouth,
       Unaware of his fault in talking.

When I was working with this koan, I was convinced that Master Mumon was talking about something along the lines of how the wind is karma, we're flags, and that our mind moves as a result of that.  If only we could suspend karma and escape that, we'd be liberated.  My teacher told me I was full of shit.  Next I told him that everything exists in the mind; that experience isn't real and that only our minds are producing it.  He proceeded to throw the meditation bell at me and asked if it was all in my head, why did I flinch when he threw the bell!  
I eventually answered the koan (and just an FYI - it has ABSOLUTELY nothing to do with anything I wrote above), but without my teacher there to reject my ridiculous ideas, I'd still be working on it today.  So to answer your question - you never know if you're speculating or not.  Only your teacher can tell you when you've gone off the rails.   

user471: 
OP: Given without knowledge one can only speculate, does Buddhism suggest one should delay certain questions until one can empirically
  get a definite answer?
OP: For example, the jhanas are not immediately accessible, and thinking about them might lead to speculation. What is speculation in
  Buddhism? Should one always restrict their attention and cognition to
  what is immediately accessible?

The Buddha certainly valued pragmatism and discouraged metaphysical speculation.
Does this mean that ALL speculation is discouraged?
The answer to this is NO. Only speculation that is not useful towards the ending of suffering is discouraged.
For e.g. when did samsara first begin? How did the universe originate? What's the range of the Buddha's abilities? How do the consequences of karma work precisely? You can find some of these in Acintita Sutta and also the Parable of the Poisoned Arrow. Speculation on topics like these were strongly discouraged.
As for topics that are useful towards the ending of suffering, speculation is not discouraged. Without investigation, contemplation, reflection and speculation, it's impossible for a learner to learn and understand new things. Correcting your mistaken speculations is a great learning method.
Over time, as you learn and understand more aspects of the Dhamma and gain more insight, your speculations would be replaced by wisdom. It's like putting together a jigsaw puzzle of which you do not know how the final picture looks like. But it gets clearer over time.
Also, in the suttas, asking difficult questions based on speculative thinking was not just allowed by the Buddha but also used by him as a tool to convey teachings.
For example, from SN 12.46:

As he was sitting there he said to the Blessed One: "What now, Master
  Gotama: Is the one who acts the same one who experiences [the results
  of the act]?"
[The Buddha:] "[To say,] 'The one who acts is the same one who
  experiences,' is one extreme."
[The brahman:] "Then, Master Gotama, is the one who acts someone other
  than the one who experiences?"
[The Buddha:] "[To say,] 'The one who acts is someone other than the
  one who experiences,' is the second extreme. Avoiding both of these
  extremes, the Tathagata teaches the Dhamma by means of the middle:
  (explains dependent origination)
When this was said, the brahman said to the Blessed One: "Magnificent,
  Master Gotama! Magnificent! Just as if he were to place upright what
  was overturned, to reveal what was hidden, to point out the way to one
  who was lost, or to carry a lamp into the dark so that those with eyes
  could see forms, in the same way has Master Gotama — through many
  lines of reasoning — made the Dhamma clear. I go to Master Gotama for
  refuge, to the Dhamma, & to the community of monks. May Master Gotama
  remember me as a lay follower who has gone for refuge from this day
  forward, for life."

And there are many more such examples in the suttas. In the Attakari Sutta, we find someone speculating that there is no self-doer and he was corrected by the Buddha.
Of course, one also needs to practise the teachings (through the Noble Eightfold Path) and not just contemplate it. But, investigation, contemplation, reflection and speculation forms part of the journey of learning the Dhamma, in order to foster Right View, which is the forerunner of the path.

OP: I guess I could also add: How does one know when one is speculating?

If you have doubt (vicikiccha) in your mind, then you're most likely speculating about the Dhamma. The explanation of doubt in Buddhism from this Wikipedia page is:

The reality of vicikicchā is not the same as what we mean by doubt in
  conventional language. Vicikicchā is not doubt about someone's name or
  about the weather. Vicikicchā is doubt about realities, about nāma and
  rūpa, about cause and result, about the four noble Truths, about the
  “Dependent Origination”.

Another sign of mistaken speculation is that it may lead you towards the three poisons of lust/ greed, aversion and delusion. A good example of this is the Vesali Sutta - in which some monks who misunderstood the Dhamma committed suicide.

-1: <p>There are many parables and stories from the Buddha's life suggesting he valued above all what is practical and conducive to virtue. He discouraged metaphysical speculation and musings. </p>

<p><strong>Given without knowledge one can only speculate, does Buddhism suggest one should <em>delay</em> certain questions until one can empirically get a definite answer?</strong></p>

<p>For example, the jhanas are not immediately accessible, and thinking about them might lead to speculation. <strong>What is speculation in Buddhism? Should one always restrict their attention and cognition to what is immediately accessible?</strong></p>

<p>I guess I could also add: <strong><em>How</em> does one know when one is speculating?</strong></p>
 295: <p><a href="http://www.themindingcentre.org/dharmafarer/wp-content/uploads/2010/02/35.4a-Kesaputtiya-S-a3.65-piya.pdf" rel="nofollow noreferrer">Kesa,puttiya Sutta (Kālāma Sutta)</a> encourages free inquiry but this may also lead to misinterpretations due to erroneous reasoning. Even experiences can be miss identified and miss interpreted. [<a href="http://www.themindingcentre.org/dharmafarer/wp-content/uploads/2009/12/25.2-Brahmajala-S-d1-Sutta-piya.pdf" rel="nofollow noreferrer">Brahma,jala Sutta</a>]</p>
<p>In the context of Buddhism, it is best one's questions and speculations are best geared towards understanding the 4 Noble Truths which gives rise to the right view.</p>
<blockquote>
<p>&quot;And what is right view? Knowledge with regard to stress, knowledge with regard to the origination of stress, knowledge with regard to the cessation of stress, knowledge with regard to the way of practice leading to the cessation of stress: This is called right view.&quot;</p>
<p>— DN 22</p>
</blockquote>
<p><a href="https://www.accesstoinsight.org/ptf/dhamma/sacca/sacca4/samma-ditthi/index.html" rel="nofollow noreferrer">Right View</a></p>
<p>Ultimately one's understanding would be based on:</p>
<ol>
<li>learning of the dhamma (pariyatti)</li>
<li>put it into practice (patipatti)</li>
<li>verification of what was learned at the experiential level (pativedha)</li>
</ol>
<p>If one tries to interpret empirical experiences by oneself one may be miss led. What is wise or praiseworthy if to look for the theory that explains the experience.</p>
<p>Ultimately only the fully enlightened Buddha has the know-how to correctly infer every experience. One speculation may be right or wrong. To find what is right one might need to consult the dhamma.</p>
<p>For part of the dhamma, ones hs not experienced the best cause of action is to learn the theory and practice. Put the practice into action and verify both theory and practice.</p>
<hr />
<p>As a side note, the 1st few Jhana's are accessible if one practice the technique properly. It is a matter of putting in the time and effort to do it</p>
 -1: <p>Speculation is the rumination on any topic that can only be truly apprehended through direct insight.  Unfortunately, about the only way you'll ever figure out if you're guilty of that is by asking your teacher.  I know this sounds like a flip answer, but it's the only safe way (at least until you're enlightened to some degree) to ensure that what you're coming up with isn't just flabby philosophizing.  Let me give you an example.  There's a koan that goes like this:</p>

<blockquote>
  <p><strong>Case 29 from the Mumonkan: The Sixth Patriarch's "Your Mind Moves"</strong><br>
  The wind was flapping a temple flag, and two monks started an argument.
  One said the flag flapped, the other said the wind flapped; they
  argued back and forth but could not reach a conclusion. The Sixth
  Patriarch said, "It is not the wind that moves, it is not the flag
  that moves; it is your mind that moves." The two monks were
  awe-struck.<br><br>
  Mumon's Comment<br> 
  It is not the wind that moves; it is not
  the flag that moves; it is not the mind that moves. How do you see the
  patriarch? If you come to understand this matter deeply, you will see
  that the two monks got gold when buying iron. The patriarch could not
  withhold his compassion and courted disgrace.<br><br>
  Mumon's Verse<br> 
       Wind, flag, mind, moving,<br>
       All equally to blame.<br>
       Only knowing how to open his mouth,<br>
       Unaware of his fault in talking.<br></p>
</blockquote>

<p>When I was working with this koan, I was convinced that Master Mumon was talking about something along the lines of how the wind is karma, we're flags, and that our mind moves as a result of that.  If only we could suspend karma and escape that, we'd be liberated.  My teacher told me I was full of shit.  Next I told him that everything exists in the mind; that experience isn't real and that only our minds are producing it.  He proceeded to throw the meditation bell at me and asked if it was all in my head, why did I flinch when he threw the bell!  </p>

<p>I eventually answered the koan (and just an FYI - it has ABSOLUTELY nothing to do with anything I wrote above), but without my teacher there to reject my ridiculous ideas, I'd still be working on it today.  So to answer your question - you never know if you're speculating or not.  Only your teacher can tell you when you've gone off the rails.   </p>
 471: <blockquote>
  <p><strong>OP:</strong> Given without knowledge one can only speculate, does Buddhism suggest one should delay certain questions until one can empirically
  get a definite answer?</p>
  
  <p><strong>OP:</strong> For example, the jhanas are not immediately accessible, and thinking about them might lead to speculation. What is speculation in
  Buddhism? Should one always restrict their attention and cognition to
  what is immediately accessible?</p>
</blockquote>

<p>The Buddha certainly valued pragmatism and discouraged metaphysical speculation.</p>

<p>Does this mean that ALL speculation is discouraged?</p>

<p>The answer to this is NO. Only speculation that is not useful towards the ending of suffering is discouraged.</p>

<p>For e.g. when did samsara first begin? How did the universe originate? What's the range of the Buddha's abilities? How do the consequences of karma work precisely? You can find some of these in <a href="https://www.accesstoinsight.org/tipitaka/an/an04/an04.077.than.html" rel="nofollow noreferrer">Acintita Sutta</a> and also the <a href="https://en.wikipedia.org/wiki/Parable_of_the_Poisoned_Arrow" rel="nofollow noreferrer">Parable of the Poisoned Arrow</a>. Speculation on topics like these were strongly discouraged.</p>

<p>As for topics that are useful towards the ending of suffering, speculation is not discouraged. Without investigation, contemplation, reflection and speculation, it's impossible for a learner to learn and understand new things. <strong>Correcting your mistaken speculations is a great learning method.</strong></p>

<p>Over time, as you learn and understand more aspects of the Dhamma and gain more insight, your speculations would be replaced by wisdom. It's like putting together a jigsaw puzzle of which you do not know how the final picture looks like. But it gets clearer over time.</p>

<p>Also, in the suttas, asking difficult questions based on speculative thinking was not just allowed by the Buddha but also used by him as a tool to convey teachings.</p>

<p>For example, from <a href="https://www.accesstoinsight.org/tipitaka/sn/sn12/sn12.046.than.html" rel="nofollow noreferrer">SN 12.46</a>:</p>

<blockquote>
  <p>As he was sitting there he said to the Blessed One: "What now, Master
  Gotama: Is the one who acts the same one who experiences [the results
  of the act]?"</p>
  
  <p>[The Buddha:] "[To say,] 'The one who acts is the same one who
  experiences,' is one extreme."</p>
  
  <p>[The brahman:] "Then, Master Gotama, is the one who acts someone other
  than the one who experiences?"</p>
  
  <p>[The Buddha:] "[To say,] 'The one who acts is someone other than the
  one who experiences,' is the second extreme. Avoiding both of these
  extremes, the Tathagata teaches the Dhamma by means of the middle:
  <em>(explains dependent origination)</em></p>
  
  <p>When this was said, the brahman said to the Blessed One: "Magnificent,
  Master Gotama! Magnificent! Just as if he were to place upright what
  was overturned, to reveal what was hidden, to point out the way to one
  who was lost, or to carry a lamp into the dark so that those with eyes
  could see forms, in the same way has Master Gotama — through many
  lines of reasoning — made the Dhamma clear. I go to Master Gotama for
  refuge, to the Dhamma, &amp; to the community of monks. May Master Gotama
  remember me as a lay follower who has gone for refuge from this day
  forward, for life."</p>
</blockquote>

<p>And there are many more such examples in the suttas. In the <a href="https://www.accesstoinsight.org/tipitaka/an/an06/an06.038.niza.html" rel="nofollow noreferrer">Attakari Sutta</a>, we find someone speculating that there is no self-doer and he was corrected by the Buddha.</p>

<p>Of course, one also needs to practise the teachings (through the <a href="https://www.accesstoinsight.org/lib/authors/bodhi/waytoend.html" rel="nofollow noreferrer">Noble Eightfold Path</a>) and not just contemplate it. But, investigation, contemplation, reflection and speculation forms part of the journey of learning the Dhamma, in order to foster <a href="https://www.accesstoinsight.org/ptf/dhamma/sacca/sacca4/samma-ditthi/index.html" rel="nofollow noreferrer">Right View</a>, which is the forerunner of the path.</p>

<blockquote>
  <p><strong>OP:</strong> I guess I could also add: How does one know when one is speculating?</p>
</blockquote>

<p>If you have <a href="https://en.wikipedia.org/wiki/Vicikitsa" rel="nofollow noreferrer">doubt (<em>vicikiccha</em>)</a> in your mind, then you're most likely speculating about the Dhamma. The explanation of doubt in Buddhism from this Wikipedia page is:</p>

<blockquote>
  <p>The reality of vicikicchā is not the same as what we mean by doubt in
  conventional language. Vicikicchā is not doubt about someone's name or
  about the weather. Vicikicchā is doubt about realities, about nāma and
  rūpa, about cause and result, about the four noble Truths, about the
  “Dependent Origination”.</p>
</blockquote>

<p>Another sign of mistaken speculation is that it may lead you towards the <a href="https://buddhism.stackexchange.com/q/26544/471">three poisons</a> of lust/ greed, aversion and delusion. A good example of this is the <a href="https://www.accesstoinsight.org/tipitaka/sn/sn54/sn54.009.than.html" rel="nofollow noreferrer">Vesali Sutta</a> - in which some monks who misunderstood the Dhamma committed suicide.</p>
