There are times when it is best to get out of the way and let someone else say something. To that end I have borrowed some information from the CJ3B Forum. I have edited some irrelevant information out. My thanks to Jon Hardgrove.
YF Carburetor Tips by Jon Hardgrove
The Carter YF carburetor first appeared in 1950, and enjoyed one of the longest runs (if not the longest) of any carburetor designed for original equipment use. The YF design was versatile enough to be calibrated for smog emission, and was used by some original equipment vehicle manufacturers through the 1986 model year. The YF was, and is, an excellent carburetor, if calibrated to the manufacturer's specifications. There are however, several issues of interest to CJ-3B owners concerning these carburetors.
Engineering Changes The most common carburetor appearing on the CJ-3B is the number 938 (4 variations - 938S, 938SA, 938SC, and 938SD). In Carter's terminology, any consumer product consisting of 2 or more individual pieces was considered an 'assembly'. Each assembly would have the letter 'S' appended to the part number. As a carburetor is obviously more than 2 individual pieces (see the CJ-3B Parts List illustration, 60K GIF) the carburetor would have the letter 'S' appended, hence 938S. If a carburetor was updated with a 'major engineering change', it was Carter's normal practice to append another letter after the 'S'. Hence the first major engineering change would be SA, the second SB (the system was not infallible, as I have no record of a 938SB), the third SC, etc. Some of the items which were changed in the 938 series are: idle jets, main jets, metering rod, and pump delayer weights; all items which will alter the calibration of the carburetor. Also, not all of the changes were compatible (example - the SD metering rod would not work with an SA jet).
Often, someone will attempt to make the best carburetor out of 2, or 3, and may end up with a carburetor with incompatible parts. This also happened with some of the less expensive commercial rebuilders. The better commercial rebuilders would of course use the correct specifications in their rebuilds.
Generic repair kits can be a problem. As the CJ-3B has one of the smallest engines to use the YF (the YF was used on the Ford 300 CID 6-cylinder,) some kits which may be offered may not have correct gaskets, and especially the correct fuel valve. The fuel valve used for the 938 series carburetors was part number 25-190S with an orifice of 0.076 inch. The more common YF (used on Chevrolet and Ford) used orifices from 0.093 to 0.101 inch. Do the math with the equation for area on these and you can understand why many experience flooding problems. The YF as used on the Jeep will take 5-1/2 pounds pressure if the proper fuel valve is used. Many repair kits of today have too large an orifice. The larger orifice requires more buoyancy from the float (at idle), resulting in flooding and the need for a fuel pressure regulator.
Also, the correct fuel valve for the 938 series was spring-loaded. This fuel valve was developed by Carter for use on 'rough service' carburetors (off-road and marine). Its function was to smooth the jolting of the float, maintaining a more constant fuel level in the bowl, and minimizing damage to the float. If the Jeep is used only on the streets and highways, the replacement solid valve will suffice; however off-road Jeeps should use the spring-loaded fuel valve.
At The Carburetor Shop we make a superior repair kit which does contain the spring-loaded fuel valve (for carburetors where it was specified by Carter), as well as many other parts not in the parts store kits. We also make kits for the W-O, and YS Jeep carburetors. Check out our website. Should other carburetor questions arise (1974 and earlier only), please telephone and we will try to answer them."
Throttle Bushings and Idle Quality While the YF is pretty much indestructible, it will wear over time. Throttle shaft to throttle body would have originally been 0.004 to 0.006 inch. Normal wear up to about 0.009 is acceptable. Wear of 0.010 or more is going to start showing up as poor idle quality. As the clearance of throttle body to throttle shaft increases, unmetered air will enter the intake manifold, causing a lean idle. Tuners will compensate for this lean condition by increasing the idle mixture. This in turn, can cause a hesitation or bog in transition from the idle to the main metering circuit. We are often asked about bushing kits, but we are hesitant to sell them. If it is determined (use a dial indicator) that throttle bushings are to be installed they MUST be correctly installed. One may NOT use a reamer (or drill) and ream completely through the throttle body and then install bushings, as this will create an internal vacuum leak resulting in a higher than normal idle (not adjustable). Visualize looking down the air intake of the carburetor. You see a round throttle bore. If bushings are installed with the use of a reamer, the square edge of the end of the bushing will leave two small triangles on each side of the throttle plate that allows a small amount of air to slide by the throttle plate. This is unacceptable.
Last Edit: Jan 28, 2015 5:15:38 GMT -5 by Scoutpilot
Scoutpilot: Rest easy my friends...Maintenance turned up some issues I need to address tomorrow.
May 25, 2020 18:07:49 GMT -5
Scoutpilot: Coffee's ready. Good morning!
May 26, 2020 4:16:59 GMT -5
leemn: Morning!.... 66* and humid here 😮
May 26, 2020 5:52:38 GMT -5
grumpy: 😷 Morning, we had a low of 26* here last night NOT humid 🤓
May 26, 2020 7:38:00 GMT -5
64cj5: Did not make the trip to six mile yesterday. Got it done to day. A beautiful day for a Jeep ride in the hills. Elk, deer, blue birds, meadow larks, squirrels and chip monks got our attention. Had a nice lunch and then down a road less traveled to home.
May 26, 2020 17:07:40 GMT -5
Scoutpilot: Sleep well my friends.
May 26, 2020 18:23:19 GMT -5
brucew: You too Boss.
May 26, 2020 18:37:58 GMT -5
rickg: That's nice Tom, You took the M38 or the "5"?, Huck and the "B" as well?
May 26, 2020 19:59:33 GMT -5
Scoutpilot: Good morning! Coffee's ready.
May 27, 2020 5:23:58 GMT -5
64cj5: We took Eeyore. I have been using Popeye most every day.
May 27, 2020 9:43:52 GMT -5
Scoutpilot: Good night Y'all.
May 27, 2020 17:47:55 GMT -5
Scoutpilot: C'mon in! Coffee's on.
May 28, 2020 4:14:51 GMT -5
cline48: Thanks Scout!
May 28, 2020 4:25:41 GMT -5
leemn: Morning friends 👍👍🏿
May 28, 2020 6:58:22 GMT -5
Muley: Hi Guys!
May 28, 2020 7:12:07 GMT -5
Scoutpilot: Doggone, where has this week gone to? Night Y'all.
May 28, 2020 18:17:34 GMT -5