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My M38 with the MB engine has an air tube from the carb horn to the filter that does not have the provision for the air tube that I see in pictures running from the air tube to the oil fill tube. Nor does the oil tube have the smaller tube for the hose to attach to.

So my question is, what purpose did this hose (air tube to oil fill tube) have in the first place, and is this something I need to have on my engine when I put it all together. Thanks as always.

That tube lets air into the crankcase through the oil filler. The PCV valve is sucking air and oil vapors out and the air must be replaced. The tube should hook to the air filter -by way of the crossover tube- so the air going into the crankcase is filtered.

Older engines didn't have a PCV system, so they didn't need any replacement air into the crankcase. Those engines had a vent tube that just let crankcase vapors just vent out under the vehicle.

You say you have an MB engine in your jeep. I'm not familiar with those older engines. Does it have a PCV system? If not, then the crossover wouldn't need a connection for the air tube and niether would the oil fill tube.

Oldsalt, thanks for the reply. Yes, I do plan to install a PCV valve. So that means I don't need the crossover tube, correct?

With the PCV valve installed you must use a vent hose to the filler pipe to keep the PCV valve from pulling a full suction on your crankcase which will suck oil laden air into the intake.

For a correct M38 setup you must have the PCV valve, the filler pipe with a small nipple for the vent hose and a crossover pipe with the small nipple for the vent hose with the fording valve.


This is the early WWII Willys MA production engine. It has the road draft pipe, no PCV system, no nipple on the crossover tube and no nipple on the filler pipe.


This is the later WWII production MB/GPW engine installtion which has a PCV valve.


This is the correct M38 engine layout

Thanks Wes, that clears it up for me.

Thanks Wes, that clears it up for me.

What happens if the air cleaner doesn't have oil in the bottom reservoir? How would that impact engine performance?

The filter wouldn't be catching dirt particles in the incoming air.

The other side of that coin is, if the air cleaner has too much oil or too light a weight oil you will find it going thru the carb.

Right. Manual calls for crankcase oil (30 wt) and 1.25 qts. I filled the reservoir to about the fill line (3/4 qt). Don't know why I didn't have oil in there from previously but obviously must have failed to check that. However, it's only really been driven this year so no time on the road, and none on dirt or dusty roads.
I was also fiddling with the plate that the throttle/gas pedal spring attaches to below the carb. Everything there was loose so I tightened it up last night and it seemed to have less surging. I still need to play with the idle screw but I think that helped the high idle I was dealing with a couple weeks ago..
Thanks all!

The loose carb mount nut will result in an air leak at the base of the carb and give you the symptoms of lean running as you have seen.