Hi Peter, and thanks for the update. My name is Jeff, "Zaphod" is just a screen name; when I signed on as an expert here I wasn't aware I'd be seen as Zaphod. Anyway - 0.016" is proper clearance cold for the intake valves, 0.018" is the exhaust setting. The clearance settings are necessary to ensure there's still some freeplay at operating temperature, when things expand due to heat. Exhaust valves run hotter so they expand a bit more, hence the larger gap. Having no clearance can prevent the valve from fully seating, and during combustion the flame can blast through the gap between the valve face and the valve seat eating away at the surfaces exactly like a torch. Light damage can be eradicated by seating the valves with valve grinding compound [silicon carbide granules mixed with grease]. Clover is the most popular brand and should be available at any auto parts store. Heavy damage usually requires replacing the valve; seats are incredibly hard and rarely damaged. My first car was a 1972 Toyota Carina, with their shrunken copy of half of Chrysler's Hemi in it. Shortly after my uneducated tuneup it burned two valves; when cranking it made ERRR ERRR TSSS TSSS noises and was nearly impossible to start. When the head came off, two exhaust valves were visibly burned allowing the compression to leak out and make the hissing noises. One had a very small crevice but another looked like a pizza with two slices missing. The 'crack' went all the way to the middle of the valve. Both seats cleaned up and the engine ran another 40,000 miles. I digress: this is about your
vehicle. The increase in compression you obtained is a very good sign that your valve is now seating. Most if not all of the 'stuff' that had accumulated on the sealing surfaces will shortly be pounded away and your compression should improve unless there's more serious damage. Valve clearances can increase or decrease over time, even on the same engine. As the valve face wears out, the valve will be moving further into the head, raising the height of the valve stem under the rocker arm and thus reducing the clearance. This condition can be checked for by measuring the height of the installed valve, from the surface where the spring seats to the top of the stem. On the flip side, valve train clearances will increase as part surfaces wear: the cam lobe, tappets or lifters, pushrod ends, rocker arms at the ends and where it pivots, and the valve stem. Tiny bits of wear all over can add up to excessive clearance, loss of power and annoying tapping noises.
If the diaphragms are stiff they won't react properly and will eventually develop holes or tears. As the rpm increases, so does the airflow through the carb. If the diaphragm is stiff, it can't move the plunger and the metering rod can't be lifted out of the main jet which would allow it to add fuel for high speed operation. According to this page of YS data your float setting is spot-on:
"Carter YS Carbureter No. 950S
For Willys 4 Cylinder Engine: 3 1/8 Inch Bore, 4 3/8 Inch Stroke
Dimensions: Flange size, 1 1/4 inch S. A. E.
Primary venturi, 11/32 inch I. D.
Secondary venturi, 19/32 inch I. D.
Main venturi, 1 1/4 inch I. D.
Float Setting: Distance from seam of float to float chamber
cover, with weight of float on needle and spring: 9/64 inch.
Vents: Balanced thru air horn to air cleaner via tubing external
Vent: Bowl chamber to carbureter bore, size No. 68 (.031
Gasoline Intake: Square vertical (spring loaded) needle No.
53 (.0595 inch) drill size in needle seat. Bleeder tube, as-
sembled over idle tube, with 2 side holes--No. 65 (.035 inch)
Low Speed Jet Tube: Jet size, No. 70 (.028 inch) drill (in end
of tube). Auxiliary jet (on side of tube) size: No. 75 (.021
inch) drill. By-pass (in air horn) size: No. 52 (.0635 inch)
drill. Economizer (in body) size: No. 54 (.055 inch) drill.
Idle Port: Upper port: slot type, length, .165 inch. Width,
Idle Port Opening: Top of port--.125 to .129 inch above top
edge of valve with valve tightly closed.
Lower Port (For Idle Adjustment Screw): Size, .053 to .057
Set Idle Adjustment Screw: 1/2 to 1 1/2 turns open. For richer
mixture, turn screw out. Idle engine at 700 R.P.M. with deep
fording vents open.
Main Nozzle: In primary venturi, angle 20°. Discharge jet size,
.0935 inch inside diameter. Main nozzle bleed (tube in air
horn) Size: No. 70 drill.
Metering Rod (Diaphragm operated): Economy step, .070 inch.
Power Step .054 inch diameter.
Vacuum passage restriction, size: No. 50 (.070 inch) drill
Vacuum passage bleed to bore, size: No. 65 (.035 inch) drill.
Metering Rod Jet: Size: .096 inch diameter.
Accelerating Pump: Diaphragm type, vacuum operated. Dis-
charge jet size, No. 72 drill. Intake ball check size: No. 40
drill. Discharge ball check under needle, size No. 50 drill.
Vacuum passage restriction, in flange size: No. 52 (.0635 inch)
drill. Vacuum passage air bleed to bore, size: No. 62 (.038
Pump Stroke: No adjustment.
Choke: Manual--Offset, butterfly type with poppet valve.
Vacuum Spark Port: None.
Specifications come from a one page bulletin from Carter Carburetor Form 4960B released March 1953, revised March 1956."
One of the more annoying things about carbs is that the fuel in the bowl evaporates every time the engine is shut off hot. Heat dissipates [called heat soak] from the exhaust manifold and gradually boils off the more volatile components of gasoline in the carb. This wastes fuel and also pollutes the air, that's why newer vehicles had an evaporative emissions system.
If I was standing in front of your Jeep, I'd want to do two simple things. Pull off a plug wire and stick a spark plug [or spark tester] in the end, leave the plug sitting on the engine, watch the spark when it starts missing-it should always be there. Spark should be bluish-white, it should never be reddish or yellow. MYTH: the spark jumping the gap ignites the fuel. FACT: of the total amount of energy produced in the coil, only a portion is needed to jump the gap. The voltage needed is quite high, between 7,000 and 15,000 volts. The remaining energy that continues to flow across the gap for about 0.004 seconds is what ignites the fuel. This voltage is much lower, usually between 1,000 and 5,000 volts. K-D tools makes a spark tester that would be very useful here, part number 2757 for standard
or points ignition systems [n the left] has
a center electrode, the 2756 is for GM HEI and other electronic ignition systems [on the right] and does not
have a center electrode:
Clipped to an engine ground and a plug wire, these will force the ignition system to use all the energy in jumping the gap so it's capabilities can be verified. Spark must be bluish-white, steady and consistent under all operating conditions Weak spark can't ignite lean mixtures very well - the fuel acts as a conductor between the spark plug electrodes. You can get effective results with points ignition by breaking the ground electrode off of a spark plug, though they're a bit difficult to clamp to ground. The testers are around $10 each. If spark is weak or intermittent, focus on the ignition system. There are other problems that might exist in the ignition system besides points, condenser, cap, rotor and plugs which are typical 'tune-up' parts. There is the coil, the associated wiring and ground connections; specifically the distributor ground for the points. Think about this: if
the distributor ground is poor the coil won't be able to develop enough energy-this will become more evident as spark frequency increases. Changing unrelated parts repeatedly cannot cure the problem. The only way to be able to completely rule out an ignition system problem would be to change it entirely.
#2 on my quick test list is adding propane. This is probably the quickest and easiest way to find out if it is or is not a fuel related issue. You can look at the spark plugs to see what color the ceramic is for an overall mixture check: light reddish or tan is ok, dark brown or black indicates rich [or oil if it's slimy], white indicates lean [or new plugs that haven't been in long enough]. If you can set the throttle at a position where it's breaking up, add propane right at the entrance to the air filter housing. A plain old Bernz-o-matic plumber's torch will do fine. If you don't want to unscrew the nozzle and take out the orifice/filter [it looks like a small bolt in the end of the neck] we might be able to get enough propane by turning the torch upside down. If the engine smooths out and speeds up, focus on the carburetor as the cause. If there's no reaction and spark was ok, the next places I'd go are running compression, leakage/leakdown testing and checking the valve springs to see if they're weak. I really don't think you have a mechanical problem but I'm not known for guessing correctly 100% of the time: this is why I do testing. I can't emphasize enough the need to do testing in order to figure out what the cause of the breakup is. The 'shotgun approach', or 'well it might be this so let's change it' is only a proven method for wasting time and money. Please try the tests I've described, it can only help solve your problem. I'll be waiting for your reply, Jeff