Is this Carter YS correct for my M38?

ArisL · Post by **ArisL** » Thu Dec 22, 2016 12:19 pm

Hi and Merry Christmas to all!

My M38 had a Delorto carb, but for originality I got from ebay a Carter YS stamped 0-548. From day one I had a starting problem, especially when the engine is warm. Even with the engine cold, sometimes I have to choke it, even in the summer. Once started all seem working OK, idle acceleration etc.

After many rebuilds, checking the float level etc, I'm now wondering if my YS 0-548 is for another military application, in which case I will have to find out if it has the correct parts inside.

Any help will be appreciated.

Post by **wesk** » Thu Dec 22, 2016 1:16 pm

No such Carter YS with number 0-548. Some have the O-548 casting # on their bowl. You should have posted a photo of the carb so we can be sure what you are describing.

This is a proper M38 YS637S

This is how it is installed

This is the proper mounting gasket

Below are descriptions of it's operating circuits:

Float circuit

Low speed circuit

High speed circuit. Note the metering rod is operated by engine vacuum!

Accelerator pump circuit. Note that the pump is operated by engine Vacuum)

You cannot start an engine with this carb using the accelerator pump for starting fuel. First the engine must be operating before the accelerator pump will receive engine vacuum to operate. Therefore the common start procedure is to use full choke as explained in the operator's handbook.

ArisL · Post by **ArisL** » Fri Dec 23, 2016 3:25 am

wesk thanks for the prompt reply, here are the pics. It does seem to be a YS, or is it a 'phony' one maybe?

(Sorry, cant put the pics upright.....)

southpw · Post by **southpw** » Fri Dec 23, 2016 4:49 am

Could you possibly have a YS950 carb instead of a YS637S?
The YS637S is correct for the M38, unsure if the 950 would work or not.
Heres a good post that has some comparisons of the 2
http://www.willysmjeeps.com/v2/modules. ... &view=next

scoutpilot · Post by **scoutpilot** » Fri Dec 23, 2016 5:14 am

Your pictures indicate a correct YS637S for the M38. L-head - 637S. F-head - 950S. 950S won’t mount on standard L-head intake.

Xamon · Post by **Xamon** » Fri Dec 23, 2016 7:09 am

Were there not some recasts ones a while back that needed some clean up of the passages to work properly? This thread here:
http://www.willysmjeeps.com/v2/modules. ... 776&highli

ArisL · Post by **ArisL** » Fri Dec 23, 2016 9:57 am

scoutpilot wrote:Your pictures indicate a correct YS637S for the M38. L-head - 637S. F-head - 950S. 950S won’t mount on standard L-head intake.

OK, I do hope it is a YS637S, but as you can see in the second picture it is clearly stamped 0-548, how can this be?

ArisL · Post by **ArisL** » Fri Dec 23, 2016 9:59 am

southpw wrote:Could you possibly have a YS950 carb instead of a YS637S?
The YS637S is correct for the M38, unsure if the 950 would work or not.
Heres a good post that has some comparisons of the 2
http://www.willysmjeeps.com/v2/modules. ... &view=next

That is a great link southpw, I'm now going to take mine apart and compare each picture to see what I come up with

Post by **wesk** » Fri Dec 23, 2016 10:21 am

Yes, that is a YS637S. The O (548) is the bowl casting #. The Carters came with the base prefix part of their number cast into the bowl, thus the YS by itself. Then an assembly tag attached to one of the air horn screws ID'd the specific model number.

You can see this triangle shaped tag on the YS950S top left of left carb.

The recasts mentioned were actually repops manufactured outside the US and sold by Brent Mullens who made it quite clear they had internal problems. They were very new looking. Yours is not. Also the bowl on the repops had no markings. You can see them in my photo album on this page:
http://www.willysmjeeps.com/v2/modules. ... php&page=5

Since it looks like you bought yours used I would strongly suggest you at least do a teardown inspection or an overhaul before you use it.

Modern Rebuild kits are available from Midwest military, AJP and Ron Fitzpatrick.

The manual for the rebuild is loaded in PDF form on this web site.
http://www.willysmjeeps.com/v2/modules. ... it&lid=142

Note that the parts illustration in the original Carb overhaul manual had an error that I have corrected.

Item BB had been shown on the wrong side of the diaphragm.

The parts blowup in the M38 ORD 9 SNL G-740 is correct.

Post by **wesk** » Fri Dec 23, 2016 10:37 am

ArisL,

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ArisL · Post by **ArisL** » Fri Dec 23, 2016 11:15 am

wesk wrote:ArisL,

I sent you an email from my personal account notifying you that your email system is bouncing our web sites automated emails to you. Please correct the issues in your mail system/server. To date I have not received a reply from you. The bounced emails add significantly to the workload time maintaining this web site.

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Post by **wesk** » Fri Dec 23, 2016 8:31 pm

ArisL · Post by **ArisL** » Sat Dec 24, 2016 2:32 am

Test no2 from Aris

oilleaker1 · Post by **oilleaker1** » Sat Dec 24, 2016 6:23 am

ArisL, I have 3 M38's, one is a A1. Your description on hard starting is quite normal. I always have to choke all three when cold. Normal. Once in a while when hot also. I've never understood why. Seems there is no fuel after sitting hot, and the vacuum operated accelerator pump doesn't supply enough fuel for re-start. I usually just step down on the gas petal and crank longer and it eventually will start. I do have to choke them if that fails. Possibly when sitting hot it may have flooded also. Hence the step down and crank for a while. The mechanical WO Cater carbs are superior to these waterproof carbs. I see why they made them, but they were not a improvement. I don't drive under water-------LOL. When you open the hood on a nice correct looking m38, I just can't bring myself to change to a WO. Doesn't look quite right to me. So, I just live with them. John

Post by **wesk** » Sat Dec 24, 2016 9:14 am

I've never understood why.

Because the accelerator pump is vacuum operated. To improve the engine's vacuum production at the carb during cranking you must apply full choke. This will increase the sucking (Vacuum production) efforts of the engine. This will in turn charge the two vacuum diaphragm pots on the carb. After a few seconds of cranking under full choke the accelerator pump will squirt just like the WO Carb's mechanical accelerator pump when you pump the gas pedal.