It was backfire'n thru carb i removed valve cover and saw that #4 exhaust valve rocker was REal loose and did'nt seem to be moving as much as the others i adjusted that rocker arm no more backfire and it seems to be running ok but that rocker does'nt seem to be moving as much as the others did a comp check 1 3 5 7 2 6 8 were 135 t0 140 #4 WAS 130 I think all the valves need to be adjusted motor has 72,000 on it it is in a motorhome i wonder if cam is bad how can i tellif it is a collapsed lifter i watched it while running and oil is flowing up pushrod on #4 exh just as much as all the others if the lifter was collapsed would it still be getting oil up the pushrod?

The 24-valve overhead system is a "low-maintenance"
design. Routine adjustments are no longer
necessary, however, measurement should still take
place when trouble-shooting performance problems,
or upon completion of a repair that includes removal
and installation of the valve train components or
injectors.
(1) Disconnect battery negative cables.
(2) Remove cylinder head cover (Refer to 9 -
ENGINE/CYLINDER HEAD/CYLINDER HEAD
COVER(S) - REMOVAL).
(3) Using the crankshaft barring tool #7471-B,
rotate crankshaft to align damper TDC mark to
12:00 o'clock position.
(a) If both number one cylinder rocker levers are
loose, continue to next step.
(b) If both number one clylinder rocker levers
are not loose, rotate crankshaft 360 degrees.
(4) With the engine in this position, valve lash can
be measured at the following rocker arms: INTAKE
1-2-4 / EXHAUST 1-3-5. Measure the valve lash by
inserting a feeler gauge between the rocker arm
socket and crosshead (Fig. 32). Refer to VALVE
LASH LIMIT CHART for the correct specifications. If
the measurement falls within the limits, adjustment/
resetting is not necessary. If measurement
finds the lash outside of the limits, adjustment/resetting
is required.
VALVE LASH LIMIT CHART
0.152 mm ( 0.006 in.)
MIN.
0.381 mm (0.015 in.)
MIN.
0.381 mm (0.015 in.)
MAX.
0.762 mm (0.030 in.)
MAX.
note:
If measured valve lash falls within these
specifications, no adjustment/reset is necessary.
Engine operation within these ranges has no adverse
affect on performance, emissions, fuel economy or
level of engine noise.
(5) If adjustment/resetting is required, loosen the
lock nut on rocker arms and turn the adjusting screw
until the desired lash is obtained:
² INTAKE 0.254 mm (0.010 in.)
² EXHAUST 0.508 mm (0.020 in.) Tighten the
lock nut to 24 Nm (18 ft. lbs.) and re-check the valve
lash.
(6) Using the crankshaft barring tool, rotate the
crankshaft one revolution (360°) to align the
damper TDC mark to the 12 o'clock position.
(7) With the engine in this position, valve lash can
be measured at the remaining rocker arms: INTAKE
3-5-6 / EXHAUST 2-4-6. Use the same method as
above for determining whether adjustment is necessary,
and adjust those that are found to be outside of
the limits.
(8) Install the cylinder head cover (Refer to 9 -
ENGINE/CYLINDER HEAD/CYLINDER HEAD
COVER(S) - INSTALLATION).
(9) Connect the battery negative cables.

The tappet adjustment is 16thou or .4mm for both inlet and exhaust.
Remove the rocker cover.
Turn the engine over till the timing mark on the pulley lines up on the pin in the cover.
Try to move the rockers on no1...if they are loose and four are tight you are on TDC no1 other wise you are on TDC 4
If on TDC one,

From here you can adjust inlet exhaust on one, inlet two, exhaust three.

Turn the engine 360 degrees one will be tight and 4 will be loose...so now you can adjust both inlet and exhaust on 4, inlet on three and exhaust on two.

Very carefully my friend. Kidding. Ok, remove the valve cover and anything in your way of removing it. By hand, with a socket and ratchet, on the crankshaft bolt head, turn the engine over to get #1 piston to top dead center on compression stroke. You'll need to find out what the valve lash should be in order to adjust properly, I run the 22R engines and only have specs for that, and have a feeler gage to place between the valve head and rocker arm. Check the gap on the intake valve for it is in position to adjust now because the cam lobe is at the farthest away from the valve. If it needs adjusted closer, turn the nut loose and screw the center in to tighten and out to give more gap, then tighten the jam nut after adjusted. Now turn the crank to bring #3 to top dead center and adjust that intake. Once done go back to #1 exhaust and adjust it. You'll know when the #3 is TDC when the rocker is loose on intake valve along with #1 exhaust. Now bring #4 piston up to TDC and adjust intake valve. Like I said, you'll know when it's @TDC when that rocker is loose, but with the spark plugs out you can stick a screwdriver in the plug hole to be sure it's up. Firing order on 4 cylinders is 1-3-4-2 so do your adjustment in that order for each intake valve and go back to the last one adjusted and do the exhaust valve.

If it starts then I would leave that possibility to last. I would remove the rocker cover and check the operation of the rockers as one may have slipped sideways and is not directly over a valve stem. t is also possible to check the valve timing with the rocker cover off. Turn the engine clockwise until the rockers on no 1 cylinder start to move watch these rockers and when you see no1 exhaust rocker start to come up then it will be getting close . Keep turning until you see the exhaust rocker stop moving up and the inlet no1 cylinder rocker start to go down. Move the crank back and forth until you get the position of movement either way moves either rocker. This is called valve overlap and if you look at the timing mark on the harmonic balancer pulley then the mark will be almost /on TDC
I would then run the fault codes as there may be other problems that caused the stall situation.

I have that same motor in my '71 SS El Camino!
I hate to tell you this, but you might have a bad cam. Remove your valve covers, start it up, and make sure all your rocker arms are rockin' the same.

Backfiring is caused by either the firing order being incorrect or the engine timing is incorrect. generally backfiring through the carburetor is caused by incorrect firing order. confirm the distributor is inserted properly and double check firing order beginning with cylinder number 1.

I think you have a valve problem. Need to remove valve covers and look for loose or broken rocker arms. If an exhaust valve does not open because the rocker is broken the trapped combustion gasses will revert through the intake when the intake valve opens. This can interupt the intake flow to other cylinders and prevent starting.

from autozone.com

ADJUSTMENT

22R and 22R-E
See Figures 3, 4, 5 and 6

1. Start the engine and warm it up to normal operating temperature.
2. Turn the engine OFF . Remove the air cleaner and housing, along with the hot air and cold air intake ducts.

CAUTION Components will be hot. The engine head, block and radiator will be very hot.

1. Remove any other hoses, cables, or wires attached to the valve cover. The valve cover (or cylinder head cover) is the domed steel item with the oil filler in it.
2. Remove the small nuts holding the valve cover, then lift the cover off. Retrieve the rubber gasket and put it aside; it can be reused if not damaged or crushed out of shape. Beware of hot oil dripping from the inside of the cover.
3. Use a large wrench on the crankshaft pulley bolt to turn the engine clockwise until the timing mark on the pulley to 0 on the scale. Turning the engine will be easier if the spark plugs are removed, but this is not required.

CAUTION Do not attempt to align the engine by using the ignition switch to turn the engine. Doing will splash hot oil onto everything in the area, including you.

1. Check that the rockers on No.1 cylinder are loose and the rockers on No.4 are under tension. (No.1 is closest to the radiator; No. 4 is closest to the firewall.) If this is true, the engine is aligned with No.1 piston at top dead center. If it is not true, turn the engine one full revolution clockwise and realign the timing mark at zero; recheck the rockers.
2. Adjust the clearance 0.008 in. (0.20mm) intake and 0.012 in. (0.30mm) exhaust. Insert a feeler gauge and check for proper clearance between the top of the valve stem and the bottom of the rocker arm on the No. 1 intake valve. To adjust, loosen the locknut on the end of the rocker arm and turn the adjusting screw until the clearance is correct. Tighten the locknut and recheck the clearance; there should be a slight drag felt when the feeler gauge is pulled through the gap. Repeat the procedure for No 1 exhaust, No 2 intake and No. 3 exhaust.

Fig. Fig. 3: Adjust the clearance of half the valves, do the arrowed ones first

Fig. Fig. 4: Use a gauge to measure the distance between the stem and the rocker arm

Fig. Fig. 5: Hold the adjusting screw in position, then tighten the locknut

1. Turn the crankshaft pulley one full rotation clockwise until the marks align at 0 and for the remaining valves.

Fig. Fig. 6: Adjust the second set of valves the same as the first

1. Clean the valve cover thoroughly with a lint-free rag. Wipe any oil off the cylinder head edges in the area of the valve cover gasket.
2. Fit the gasket into the valve cover, making sure it is not crimped or twisted. If the half-moon rubber plugs came out of the valve cover, clean them and apply sealant to the part of the plug contacting the valve cover; install the half-moon plug.

The use of sealants on the valve cover gasket is not recommended.

1. Install the valve cover onto the head. Make certain is squarely seated and not pinching any adjacent wires or cables.
2. Install the valve cover retaining nuts. Tighten them to 43-60 inch lbs. (5-7 Nm) This is little more than finger-tight; overtightening will deform the cover and cause leaks.
3. Connect the lines, hoses and cables which were removed for access. Make certain electrical and ignition wires are firmly held by their clips or brackets.
4. Install the air cleaner with the hoses and duct work.
5. If still in place, remove the wrench and socket from the crankshaft pulley.

Double check all your vacuum lines and make sure none have come loose or were damaged when the engine was rebuilt. Check any egr, pcv, mass sensors and make sure they are clean etc...Not as familiar with Nissan trucks but some Nissan cars were equipped with an anti-backfire valve.

check your fuel pump pressure and volume.