On 1986 Suzuki Samurai for the timing to reach 10 degrees BTDC, why does the adjustment at the distributor have to be locked in at the extreme end of the adjustment spectrum.

the lines are probably 5 degrees and 10 degrees btdc
place it on 10 btdc and se the distributor so that the point of the cam is facing the pick up exactly

something wrong here
with 99 degrees btdc it will not run
have you got it on the correct plug lead
next the timing is regulated by centrifugal advance if electronic or points and by the crankshaft position sensor if ECM controlled ( some after market distributors that are used in place of points units have a small sensor in the distributor that can adjust the timing according to engine rpms, but it cannot make allowances for mixture and so they can advance the timing but not take into account anything else which sometimes results in detonation under certain throttle settings
so initial setting for most cars is 10-12 degrees btdc at idle and with engine rpms set up to 3000 rms it may advance to 25-30 degrees depending on the advance mechanisms
with ECM units the timing is controlled by the crank position sensor and is set to parameters according to engine rpms and air/fuel ratio settings but it still has a basic 10-12 degrees btdc on starting
to fail smog tests you probably do not have a working cat converter and other problems besides timing

You need to find the timing specification for the engine. Ex: 10 degrees Before Top Dead Center (or 10 Deg BTDC), 0 degrees Top Dead Center (or 0TDC), and so on. Then, locate and mark the indicator on the flywheel with white chalk. Connect the timing light to #1 spark plug wire and the + & - 12 battery terminals. Loosen the bolt that locks the distributor cap in place. Start the engine in park. Adjust the idle speed up or down to specification. Shine the timing light on the fly wheel. You should see the chalk mark "light up" close to the timing indicator / scale near the flywheel. Slowly twist the entire distributor slightly one way or the other to make the timing mark "light up" at the correct position on the timing scale. Shut the engine off and tighten the distributor lock down bolt. You have now locked the distributor and idle timing in place. Make sure all vacuum lines are connected as the vacuum advances the timing when accelerating, etc. Of course, this procedure only works on older engines with distributors. New engines with computers and coil packs, etc. have their timing adjusted as needed by the computer so there is nothing to adjust.

same as all non boxer 4 bangers.
1,3,4,2.
next is distributor rotor rotation, then timing, and well RTM yet>
read the manual.?
it's free...
Samurai. its called in usa, but jimny others places.

see the 3 manuals? its all there.
http://www.fixkick.com/sammi-stuff/index.html

15 degrees BTDC according to autozone.com

1. Start engine and allow it to warm up until the cooling fan comes on.
   
2. Connect a timing light to engine; while engine idles, point light toward pointer on timing belt cover.
3. Inspect ignition timing at idle. Timing should be 15plus or minus2 degrees BTDC, using the RED timing mark, at 680plus or minus50 rpm in N .

1. If timing adjustment is necessary, complete following:

1. Remove control box upper cover and ignition timing adjuster from control box.
2. Drill 2 rivets off with a 1 / 8 in. drill bit, then separate stay cover from adjuster.

make sure engine is warmed up. Put cylinder one at top dead center,remove the valve cover,loosen all adjustment lock nuts,adust intake and exhaust valves on cyl #1 and intake valve on #2 and exhaust valve on #3. now rotate crankshaft 360 degrees clockwise, now adjust intake valves on cyl#3 and cyl #4 and exhaust valves on cyl #2 and cyl #4.now tighten lock nuts and install valve cover. specs are intake valves 25mm clearance and exhaust 28mm clearance, use a feeler gauge,heres a picture to show you where to check clearance.

Ignition timing should be set at 10 degrees BTDC (before top dead center), with vacuum advance hose disconnected and plugged, engine idling at 800 rpm or less.

Fuel Pressure Regulator... they get old and sometimes the diaphragm wears out and you get fuel dripping into the vacuum line. This throws off the Oxygen Sensor. $133.00 from Checker; $233.00 from Car Quest. Changed mine out and voila! Perfect Idle....

unplug vacuum advance on the distributor. seal the hose and put a plug on the advance there should be a noticeably deeper groove in the harmonic balancer where the 2 marks should meet up when the light hits them.
in ex. (10 degrees btdc)

Automatic I presume. 8 degrees BTDC @ warm & 700 RPM.
distributor vacuum off & plugged. In drive, & accessories off.
Don't forget parking brake on.