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Carter YF 938SC; Going through the Carter carburetor
Topic Started: Aug 11 2011, 09:35 AM (3,881 Views)
oldtime
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So we can safely say that your Hurricanes basic mechanical condition is very good and its not the root cause of the backfiring.

Now it is time to thoroughly inspect the ingition system for possible faults.

It all begins with the batteries condition.
Check the battery for voltage.
Then check all the the primary connections for a clean tight fit. (cables, ignition switch, coil wires, etc.)

After all the wiring / switches / connections pass inspection then continue to the distributor...
Check CAP for arcing / cracks / corrosion / carbon runners
Check ROTOR for burning at the tip
Check CONDENSOR for 21 to 25 microfarads of capacitence
Check POINTS for pitting / poor mating / metal transfer
set to .020" with a wire type gauge
set breaker spring at 17 to 20 oz. pull
Check spark plug cables for ohms of resistance. (I'll fill in the specs a little later)
Finally check the distributor shaft for any sign of play at the bushing.

Spark Plugs....
I cannot see the color but they look to have a light coating of black soot on all 4 of them ?
I suppose they all look the same ? Yes / No ?
Currently building my final F-134 powered 3B .
Rock Crawler using factory parts from the Willys Motors era (1953-1963)

http://s4.zetaboards.com/CJ3B_Bulletin_Board/topic/30228766/1/#new
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skiprocks
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The battery is new and holds a load when load tested. The alternator is a Delco SI with a self energizing regulator that allows it to charge at low RPM. The alternator provides 14.2 volts at 800 rpm and 14.7 volts at 1000 RPM. I have the same system on my Willys pickup in Colorado and have been very happy with it. See the following link for details.

http://mightymo.org/Proj_OneWire.html

All ignition wiring and battery cables are new - replaced this past winter. All connections are clean. There is also a master switch installed right off the battery.

Distributor: The CJ came with an MSD electronic ignition when I purchased it. Unfortunately, during the two year transport from where the Jeep was purchased to my home in Florida it apparantly sat for some unknown amount of time with the hood open and the distributor cap off. When I finally got it home last fall the distributor was a rusted mess inside. After pulling, dismantling, cleaning, testing and replacing damaged components, I still couldn't get it to run right. It would run, then abruptly die. Not trusting my handiwork I purchased a solid state ignition distributor from Kaiser Willys earlier this summer.

Posted Image

I also decided to have someone that knows more about this than me look at it and I took the CJ to a local classic car specialty shop. They installed the new distributor and got the CJ running for me. They aren't cheap and I could only afford to have them address the electronic ignition. Note: it turned out that the MSD Street Fire electronic ignition unit had an intermittent internal ground and I'd repaired the original distributor correctly - it works great - so now I have a fully functional spare :lol:

The CJ now has a new MSD electronic ignition unit mounted to the firewall, existing MSD coil (checks out fine), new distributor, new cap, new rotor, new MSD plug wires & new plugs. If there is one thing I am personally confident about it is that the ignition system is in good shape and not the cause of the backfiring.

PLUGS. Yes, in the photo the plugs do look a little sooty. I believe this is most likely because the engine is running so rich. I have run the CJ down our local two lane hiway under a load, limiting idle time, no coasting etc. (also means minimal backfiring) and the plugs look fine after. If I run the CJ for a few days around the neighborhood, streets with stoplights, lots of idling and coasting (and backfires), they look sooty. Note: in the last couple of months or so I've had the plugs in and out multiple times.

Skip
Skip Owens
1961 Willys CJ5 - F134, older restoration
1961 Willys Pickup - L226, 2006-8 frame off restoration
1930 Ford Model A Coupe
NE Florida
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oldtime
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O.K. then I won't touch your ignitions coil wire with a ten foot pole.
Before we go any further I should explain the difference between a BACKFIRE and a MISSFIRE.
If you look up Wikapedia they seem to indicate that a BACKFIRE is an explosion of the charge which occurs in either the intake or in the exhaust chamber.
I'll disagree with Wikapedia !
According to the old school mechanics a BACFIRE can only occur in the intake manifold.
A MISSFIRE will occur inside the exhaust system.

So now I suppose you want some Carter YF 938 SC details ?

UNDERSTANDING THE CARTER YF 938

Before the fuel goes in.....

All carburetors are dependant upon correct fuel supply.
This infers the fuel type, the pressure and the volume.

The YF 938 inlet valve is designed to work with a fuel pump capable of 2-1/2 to 3-3/4 PSI at 1800 rpm.
Technically this specification is good at 16" above the pump.
The fuel pumps pressure can be tested via of a vacuum gauge.
If the fuel pump pressure is too high; it can cause fuel surging in the well.

The fuel pumps volume can be tested by disconnecting the supply tube at the carburetor.
Fasten a hose to the open supply tube and into an open container.
The pump should discharge a quart of fuel within one minute with engine operating at idle.
Does your fuel pump meet those specifications ?

The fuel goes in...

All Carter YF 938 inlet valve orfices should ideally measure .076" diameter.
Due to small cubic inch discplacement .076" is much smaller than generic inlet valve orfices.
A correct original inlet valve is a spring loaded brass needle without a rubber tip.
The inlet valves spring serves as a shock absorber to better control the fuel flow when offroad jolts occur.
The spring tension itself is calibrated to the fuel pumps rated pressure.

The best generic kit available is the Hygrade 518 C.
This kit comes with a .088 inlet orfice and a spring loaded needle having a Vitron rubber tip.
The larger than standard orfice size works out O.K. for highway use where the fuel volume required is at its greatest.
But the smaller orfice will be a must when severe off road jolting occurrs.
Do not install any inlet valve having an orfice beyond .088."

The inlet valve and the float are used to control the fuel depth within the fuel well.
The ideal fuel depth will be 1-1/8" measured from the bottom of the well.
Too much fuel depth contributes toward flooding and/or a fuel rich condition.

Inspect the inlet valve and the valve orfice via magnification in direct sunlight.
Set the float level at 5/16" for all model 938.

Most of the fuel exits the well via the main jet.
The main jet diameter is dependant upon exact model number.
Example: The YF 938 SC and SD main jet measures .09375" diameter.

The main jet bore is restricted by the metering rod.
The standard (for use up to 5000' elev.) 938 SC metering rod is #75-1436
The metering rods must match the main jet and these both must match the specified YF model number.
If the metering rod is not correct for you specific YF model the fuel metering will suffer.

Visually inspect the main jet and metering rod in direct sunlight under magnification.
The metering rod will have 3 "STEPS".
Each step measures a different size and the middle step is tapered.
The metering rod step specifications are diifferrent depending upon which metering rod you have.

More to follow ........
Currently building my final F-134 powered 3B .
Rock Crawler using factory parts from the Willys Motors era (1953-1963)

http://s4.zetaboards.com/CJ3B_Bulletin_Board/topic/30228766/1/#new
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skiprocks
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Can't wait to get back home and try this.
Skip Owens
1961 Willys CJ5 - F134, older restoration
1961 Willys Pickup - L226, 2006-8 frame off restoration
1930 Ford Model A Coupe
NE Florida
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