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willysmjeeps.com :: View topic - Need m38a1 carb advice
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Need m38a1 carb advice

 
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RichJohnson
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Joined: Apr 15, 2005
Posts: 162
Location: San Diego CA

PostPosted: Fri Mar 29, 2019 9:29 pm    Post subject: Need m38a1 carb advice Reply with quote

Ok you carb experts, I rebuilt my ys950 carb the other day, (probably the 4th time in the 20 years Ive owned it) but this time the jeep will not start.
My M38a1 is all stock set up. It used to start with choking etc and got a little more difficult over the last 6 months or more until it started driving poorly, the diaphragm just got stiff. But it still would start.
So I rebuilt it with a Midwest military kit as I always do and put it all on and I cannot get the #@$%@ thing to start to save my life.
One little shot of starting fluid and it fires up instantly. after its hot it starts fine. An hour later it starts fine. Two hours later it starts fine. 4hours later its cold enough it wont start. I can choke it, hold the pedal down on and on it will just kill the batteries.
I don't think its getting fuel.
How does the carb put fuel into the engine while starting?
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mdainsd
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PostPosted: Fri Mar 29, 2019 9:53 pm    Post subject: Reply with quote

Richard, it's Mark.

Since I know your Jeep somewhat, I ask the following?

Last time it had a full tune up, points, condenser, plugs and wires?

How old are the batteries, and when was the last compression test performed?

How old is the gas?

Cold starting to me has never included choke with the throttle wide open, although every internal combustion engine on the face of the planet has its own little idiosyncracies.

J is coming over in the morning, if you guys are still stumped call me, you have my number.
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'52 Dodge M37, '42 GPW, '48 FrankenJeep CJ2A/M38, '50 CJV-35(U), '51 M38, '42 WC-57 Command Car, '44 WC-51 Weapons , (2) M1941 Sperry 60" Anti-Aircraft Searchlights, John Deere M-gator, '44 White M3A1 Scout Car
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wesk
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Posts: 16227
Location: Wisconsin

PostPosted: Sat Mar 30, 2019 12:16 am    Post subject: Reply with quote

Quote:
How does the carb put fuel into the engine while starting?


Good question under the circumstances!

Most carbs have an accelerator pump that can squirt gas into the throat of the carb with the engine not even turning.

The Carter YS carbs can't. Their accelerator pump is vacuum operated. With a very healthy engine they can provide a wee bit of squirt while cranking. In your case you may have a leaking gasket, poor Compression/Suction. You mentioned a stiff diaphragm above, which one?

Did you blow all passageways clear? Especially the low speed feed tube and port. It's the normal source of fuel during cranking.


Did you get the metering rod diaphragm and accelerator pump diaphragm springs on the correct side of the diaphragms? Remember, that illustration in TM 9-1826A is incorrect.


[mg]http://willysmjeeps.com/v2/modules/gallery/albums/album115/Pg_114_Parts_List.sized.jpg[/img]
This is my corrected drawing with the metering spring "BB" in it's proper location.

Don't overlook the discharge check ball and weight HH & JJ. They are often dropped during disassembly and forgotten.

Don't overlook a sticking float.
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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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RichJohnson
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Joined: Apr 15, 2005
Posts: 162
Location: San Diego CA

PostPosted: Sat Mar 30, 2019 12:51 am    Post subject: Reply with quote

The old diaphrams were stiff, the new ones are soft and floppy. That is what I meant.

I did blow all the passages out good enough I think.

There Is no check weight and ball on the 950 for the m38a1

I took each set of diaphragm and spring out and laid them all on the bench in place as they came out and assembled new in same manner. I believe they are correct.

The jeep drives fine once I get it started.

It seems my issue may be a vacuum leak on a gasket or an issue with the low speed feed tube. Im going to pull it back off tomorrow and go back through it again and also make sure the longer spring is on the metering rod side. I will spray cleaner back through the low speed feed tube ports, where is it fed fuel from? I replaced the brass tube and shroud this time, whereas I had not the last 3 times I rebuilt this carb....
Is it possible to install the tube too far in or improperly to where it wont feed fuel?
_________________
"Those who enjoy freedom must endeavor to preserve it"

KI6EZA
1954 M38A1 (first civilain owner)
1965 M37B1 Commo shelter truck
MBT
GRC-122 TTY rig
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wesk
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PostPosted: Sat Mar 30, 2019 1:04 am    Post subject: Reply with quote

Quote:
There Is no check weight and ball on the 950 for the m38a1


You sure. The M38A1 ORD 9 SNL G-758 shows them.




Items "T" & "U".
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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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RichJohnson
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Location: San Diego CA

PostPosted: Sat Mar 30, 2019 8:44 am    Post subject: Reply with quote

yeah, I rebuilt a 650 on an m38 years ago for a friend and I found them on his.
Never saw them in my m38a1 carb. Maybe they disappeared before I got the jeep from the CAP.
I know they have never been in my carb, I will look for a spot for them to fit to see if they will.
This is interesting....
_________________
"Those who enjoy freedom must endeavor to preserve it"

KI6EZA
1954 M38A1 (first civilain owner)
1965 M37B1 Commo shelter truck
MBT
GRC-122 TTY rig
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mdainsd
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Joined: Oct 20, 2017
Posts: 444

PostPosted: Sat Mar 30, 2019 9:15 am    Post subject: Reply with quote

That core carburetor I offered you has that weight in it, I just looked.
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'52 Dodge M37, '42 GPW, '48 FrankenJeep CJ2A/M38, '50 CJV-35(U), '51 M38, '42 WC-57 Command Car, '44 WC-51 Weapons , (2) M1941 Sperry 60" Anti-Aircraft Searchlights, John Deere M-gator, '44 White M3A1 Scout Car
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RichJohnson
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Location: San Diego CA

PostPosted: Sat Mar 30, 2019 11:00 am    Post subject: Reply with quote

Awesome! I was going to call you when Jerry got here and ask if you could open it up and look.

In the 20 years I have owned this jeep being the first civilian owner, That is the original carb on it and it never had that ball or weight and I have driven it for so long. Wonder why they are needed....
I found where they will fit so I will install them. Just going back through the whole thing on my bench right now. Hopefully second time is a charm.
_________________
"Those who enjoy freedom must endeavor to preserve it"

KI6EZA
1954 M38A1 (first civilain owner)
1965 M37B1 Commo shelter truck
MBT
GRC-122 TTY rig
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wesk
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PostPosted: Sat Mar 30, 2019 11:41 am    Post subject: Reply with quote

Quote:
That is the original carb on it and it never had that ball or weight and I have driven it for so long.


Most likely they fell on the floor on an early rebuild and never went back in and were obviously never missed on that or any followup rebuilds.

Quote:
Wonder why they are needed....

Take a moment and read Par 93 on page 111 of TM 9-1826A dated Dec 1952.

Pages 107 thru 112 explain the operation of the entire carb in great detail. Granted the Manual predates the YS950S but the YS950S is nothing more than a larger bore, greater flow volume carb with the base oriented 90 degrees different from the YS637S.

If you don't have a copy of this manual lying on your work bench right now during your rebuild then download a free copy of the YS pages from our downloads page:

http://www.willysmjeeps.com/v2/modules.php?name=Downloads&d_op=getit&lid=142
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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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RichJohnson
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PostPosted: Sat Mar 30, 2019 1:14 pm    Post subject: Reply with quote

Ok. with all of that, I pulled the carb, went back through everything and added the check ball and weight.

Reinstalled everything and it started right up. I refuse to believe the ball and weight are responsible for the results since they were non existant for the last 20 years of operation but Im sure it will probably perform better.
It starts fairly well and drives great now. Not sure what the problem was but did pay extra attention to the long brass low speed fuel rod and its circuit when cleaning everything back up.
_________________
"Those who enjoy freedom must endeavor to preserve it"

KI6EZA
1954 M38A1 (first civilain owner)
1965 M37B1 Commo shelter truck
MBT
GRC-122 TTY rig
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mdainsd
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Joined: Oct 20, 2017
Posts: 444

PostPosted: Sat Mar 30, 2019 2:05 pm    Post subject: Reply with quote

Surprised
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wesk
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PostPosted: Sat Mar 30, 2019 2:32 pm    Post subject: Reply with quote

Quote:
I refuse to believe the ball and weight are responsible for the results since they were non existant for the last 20 years of operation


You mentioned in your first post that this rebuild makes about the 4th rebuild during the 20 years you had the jeep. Are you 100% sure the ball & weight were not there on ALL 4 rebuilds. Is there a chance it laid there undisturbed during the first couple of rebuilds then fell out and got overlooked the last time but your engine had such great vacuum and compression that you still got some fuel squirt on startup and finally this time the engine lacked adequate vacuum to pull in the gas with the missing check ball & weight.

Did you read the page I listed?




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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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RichJohnson
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Location: San Diego CA

PostPosted: Sat Mar 30, 2019 10:28 pm    Post subject: Reply with quote

Wes, you propose a good scenario, and all I can say is I know for sure I have not had the check ball and weight for at least the last two maybe 3 times I have rebuilt this carb. I do know, that I developed the assumption that the 950 did not have the ball and weight a very long time ago after having rebuilt my friends 650 and found them in his.
I am quite meticulous when it comes to things like this and take them apart slowly and place all the parts on the bench top as I go so I know how they were oriented and where about they go.
If I had the check ball and weight I lost them the first time opened it up but I still doubt that. When I first got the jeep from Uncle Sam it was a pile of junk. I got it back on springs and running and drove it till a rod bearing went. Then I did a full restoration. But to get it running that first time, all that was available were crappy Chinese kits, Midwest Military had not started making his wonderful American made nos replica kits. The tolerances of the Chinese kits sucked and no matter what I did, mixing and matching parts from about 4 different brand new kits, I could not get a step or hesitation out of the carb until I got his kit when it was released about a year after I had my jeep running. Its possible I lost them through that saga, but considering its two parts, and knowing how I am at taking stuff apart, I still kinda doubt it, but I know I have not had them since at least the early 2000s. All this time have thought they were a 650 only part.

I have not had the time to go read those pages yet. I worked on the jeep most of the day and played with my son while the wife napped a bit.

Is that ball and weight part of the actual starting circuit? Is this where to look when having issues starting if its a fuel issue?

One thing that confuses me right now, in the first diagram you posted, the long brass tube with the brass shroud on the horn that goes down into the metering rod jet, is labeled "Low Speed Jet Tube"
In the last diagram posted, its labeled with a blue pointer line as "High Speed Passage" These appear to both be that long brass tube with the brass shroud. So is it the high speed circuit since its fed by the metering rod jet?
_________________
"Those who enjoy freedom must endeavor to preserve it"

KI6EZA
1954 M38A1 (first civilain owner)
1965 M37B1 Commo shelter truck
MBT
GRC-122 TTY rig
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View user's profile Send private message Visit poster's website MSN Messenger
wesk
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Joined: Apr 04, 2005
Posts: 16227
Location: Wisconsin

PostPosted: Sun Mar 31, 2019 12:00 am    Post subject: Reply with quote

Quote:
Is that ball and weight part of the actual starting circuit? Is this where to look when having issues starting if its a fuel issue?

Yes, that is why I suggested you read Par 93 on page 111:



What is described in that paragraph is how a wee bit of gas gets injected into the venturi of the carb by the accelerator pump vacuum chamber. That check ball gets closed as the fuel is squirted so it can go into the venturi and not back into the bowl.


Quote:
One thing that confuses me right now, in the first diagram you posted, the long brass tube with the brass shroud on the horn that goes down into the metering rod jet, is labeled "Low Speed Jet Tube"
In the last diagram posted, its labeled with a blue pointer line as "High Speed Passage" These appear to both be that long brass tube with the brass shroud. So is it the high speed circuit since its fed by the metering rod jet?


You are referring to two different parts. here, I will put the illustrations side by side:


_________________
Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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