USX 37 Carb... 1850 Oliver / 4020 JD ... pictures

[Mortimer D]

I'm working on an Oliver 1850 with the Marvel Schebler USX 37 carburetor. It runs fine at idle and will take full throttle slowly while sitting in nuetral. If you try to crack the throttle open while in sixth gear it wants to breakup and stumble. I replaced the rubber diaphragm and cleaned everything to the best of my abilities. In my second photo, I have removed the back of the carb and am pointing to a passage with a screw driver. Is this a removable jet or a threaded plug? It appeared to have a straight slot with a screwdriver but it did not seem real interested in being removed. I filled the passage with brake clean and it doesn't seem to drain out of the passage. I applied shop compressed air with a rubber tipped blow nozzle but it didn't seem to take air. Is this passage #20 in the diagram below?

Any input would be appreciated.

This post was edited by Mortimer D on 11/11/2022 at 10:07 am.

 

[wore out]

I think you are playing with the passage to the accelerator pump discharge nozzle/check valve, which takes a pressure of three inches of mercury to unseat.

http://www.oldcarmanualproject.com/manuals/Carbs/Marvel-Schebler/MS-USX.pdf

Download the manual at the link above and look at pages 12 and 13.

 

[Mike M]

Take it from a JD 4020 gas owner. Those carb. are best used for a door stop. Best thing I ever did to
ours was get a Zeinth carb. for it.

 

[sms]

Are you sure it is the carburator? The
symptoms also fit distributor advance not
working properly.

 

[JIM MN]

Did you adjust the load screw on side of carb; in picture no one, at bottom right, the
small flat blade screwdriver slot, screw out little by little, normal about three turns out??
Is the gas strainer clean; behind large flat-blade screwdriver slot top right, picture one??
Is the gas bowl vent open?? You put in a new diaphragm in the accelerator pump; were the
passage-way for the pump open to give extra gas when accelerating??

 

[Mortimer D]

Could you go into a little further explanation of how to test / check distributor advance?

The service manual said to check the ignition timing by plugging off the vacuum advance and then use a timing light. I adjusted it accordingly to get it to 24 degrees. I did not come across the procedure for the advance. I will admit that when using the timing light, the 24 degree mark seemed to float around a little more than I expected.

 

[sms]

Besides the vacuum advance there may be a mechanical advance, spring loaded weights that expand as the rpms increase. You may have seen them in cars under the rotor, they can also be below the plates the points are mounted on. If the weights freeze or are sticking an engine will run okay at one rpm but stumble when rpms are changed. Often just free then up and lubricate will fix the problem. I am not familiar with your application so do not know if it has mechanical advance or where the weights would be.

 

[550Doug]

Can you give the Zenith carb number that you used?

 

[roy prins]

they are not even good for a doorstop -- when new they worked kinda sortof when they are 30 years or more old they are junk! Get
e zeneth and be done with it! I have worked on a few of them with very little success. they seem to corrode internally !

 

[used red MN]

To add some additional comments on what sms is talking about in regards to the centrifugal advance I will add. An initial check is to pop the distributor cap off and give a twist to the rotor in the direction the distributor turns using your finger and thumb. It should feel resistance as the rotor turns slightly and it should spring back. If it seems like it does not readily rotate this tells you that the advance is likely stuck. If it moves some that is good but it should be opened up and lubed and check that it moves freely within its stop limits with the springs off. You show pages of a repair manual in you pictures it should show you how to check out the advance. I am linking the online parts diagram of the distributor. The parts below the plate the points mount on are the advance. But you can see the cam that opens and closes the points is separate from the main shaft of the distributor. The two pieces moving independently by the movement of the centrifugal weights is what creates the advance.
CNHI Case 400 distributor diagram

 

[Mike M]

I don't have number handy but it was actually from a 3020. Was lucky and found it used. That MS alum carb. is the only one I ever gave up on. Everyone told me to get rid of it. I did not believe them. After a few years I finally got a zenith and didn't look back. Turns out they were correct.