Metering rod adjustment

[Carls_jeep]

On the CJ2A site we are having an interesting discussion about adjusting the metering rod. They discussed buying the tool or making your own. I went through all that when I rebuilt my carb but didn't explore how it works, I just found out how to do it without the tool and did it and it worked. With all the discussion about it I made the tool as suggested and then modified it to a shorter length as suggested by Sean. I had to modify the top of the tool to get it to work at all. If your interested in reading the discussion go here. Otherwise, below the link is what I found by doing some experimenting.

http://www.thecj2apage.com/forums/metering-rod-gauge_topic21789.html


Metering rod

The tool won't go in far enough to make the difference. I just did some measuring and looking. The tip barely enters the .072" jet hole since the tip of the tool is .060". There was no more than .010" to .020" sticking in the hole, not the .043" difference.

I measured my old metering rod and from the shoulder that rests on the jet entrance to the bottom of the loop is very close to 2.718". Manufacturing tolerances from the loop to the shoulder is probably in the area of +/-.005" as a SWAG on my part.

I took the cover off two spare used carbs I have and checked the setup. The length of the metering rods were very close to the 2.718� as best I could measure with calipers and a magnifying glass.

The metering rod must set on the top surface at the entry of the .072" jet hole to close off the high speed circuit. The oblong loop at the top of the metering rod allows for throttle travel before lifting the rod. Here�s where it gets interesting. According to the Willys shop manual when the pin is set using the tool the top of the pin should have no more than 1/64� between the pin and the loop in the metering rod when the throttle plate is set for the correct idle speed.

I experimented with one of the used carbs and set the pin with my gage with a length of 2.718�. Then I put the metering rod in and adjusted the throttle screw as it was originally. There was a gap at the top between the pin and metering rod of about 1/32� which is double what the manual said it should be. Since the low speed circuit is separate from the high speed circuit and is supposed to be effective up to about 20 mph before the high speed circuit starts to open I assume that setting would be ok.

EDIT: If you used a tool with a length of 2.761" to set the pin then when you installed the metering rod the pin would not rest on the bottom of the loop and when you adjusted the throttle screw to set the idle it would lift the metering rod while at idle. I do not think the 2.761" is a valid measurement.

I also found out that when the pin is set using the tool the pin will rest on the bottom of the loop of the metering rod. It should do so because the measurement of the tool and the mettering rod are the same. Also, if the pin didn�t hold the metering rod down in the jet the high speed circuit would siphon fuel out and interfere with the idle circuit. With that discovery one could install the metering rod and adjust the pin to gently rest on the bottom of the loop and have the same setting as using the tool.

After having considered all this it's my contention the tool is NOT needed and in fact the metering rod will better set the distance due to tolerances in manufacturing. Knowing the jet or metering rod was not made to NASA specs. and variations exist in manufacturing it's a no brainer there will be variations in the parts. Also, the factory gage looks to be a stamped tool and variations will occur there as well.

I also found out the 1/8� round rod of the home made tool will not clear the backing washer on the pin and I had to grind half the diameter off for about �� to be able to use the tool.

So there you have it. Whether you scrounge up or make a tool to set the pin is your choice as it seems to make little difference in setting the pin. In fact using the metering rod to set the pin eliminates any manufacturing tolerances of the rod and jet.

The accelerator pump and idle circuit are supposed to handle anything up to about 20 mph and then the high speed circuit should have begun to open up with the lifting of the metering needle.    

[philshevlin]

I'll say it again...

I don't understand why it's so precise.  The entire assembly is based on springs, levers, pins and plungers.  There is no way tolerances up to a 10,000th of an inch are even relavent.

Cut a piece of 1/8" tig filler rod (or anything 1/8" in diameter) to about 2.7 inches long and set the metering rod.  Fine tune it with small movements.

BTW, 1/8" fit in my backing washer.  More "exact" tolerances, I guess.

Worked for me.

[Carls_jeep]

You don't need to make a gage, just lower the pin to the bottom of the loop and snug the nut and your done. The only reason I can see for the factory gage is convenience or to sell a tool or to make you think it's critical.

Today I did some checking on my Jeep. Read below.

I got curious and checked my Jeep's metering rod setting. The Jeep carb is running just fine so it should be close but I don't remember how I went about setting it when I rebuilt it.

With the throttle screw backed off the metering rod has .007" free travel and the spring holds it down as it should do. Then I screwed the throttle screw back in to where it was and the travel is .030" with the spring still holding the rod down. The lift off of the rod from the jet occurs with very little throttle opening so it would be about 20 mph I suppose.

When I get a factory tool I will do a comparison of the factory tool to the home made tool in my Jeep carb and the used carbs. If the home made and factory tool setting allows about .005" of free travel then you could set the metering rod without a tool by just allowing a little movement in the rod.

It's not really rocket science setting these carbs and I suppose they would tolerate a little variation in the setting. In fact I don't remember things being this complicated setting the metering rods in Chevy carbs.

We'll see how it comes out.