Carburator Zenith 12848 vs 13660B

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Carburator Zenith 12848 vs 13660B

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Carburator Zenith 12848 vs 13660B

Postby on-to-berlin » December 19th, 2017, 11:58 am

O.k. Gentlemen, here we go again with the carburetors. I am fighting a little bit with my installed 12848 Zenith on my A2. I had a 13660 with emission control first, did a complete overhaul but could not get a proper idle and had no power. Since I hade and overhauled 12848 from the Army installed this one. Was running fine first but after 20 miles it stuttered and stopped for some reason. I took the carb off, cleaned it and made more cuts in the line so it gets more fuel in the bowl. I will do a test run tomorrow and see about the performance.

If it does not run I still have a NOS 13660B from 1996.

So which one is better? And why are the carbs so difficult to get it properly running? Did anyone tune up their used carbs so they run fine?

On my A1 which I had for 7 years I also installed a NOS Carb once and since then it even runs 70 miles.

Generally the carbs do not seem too diffictult but for some reason if they are used performance/idle etc seems to drop.

But why?
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Re: Carburator Zenith 12848 vs 13660B

Postby muttguru » December 19th, 2017, 1:28 pm

The 13660B is probably the best carb for your A2. It was introduced as a replacement for the pre-emission 12848 carb and the pre-emission A2 carb.

The 13360 (no-suffix carb) was introduced on the emission-controlled engine but was a poor performer. The 13660A version was introduced as a better alternative to the basic 13660 but it didn't work too well on the M151, A1 and pre-emission A2 engines. That's the reason the 13660B was introduced. Feedback seems to indicate that it works well on all versions of engines.

As you probably know, the 12848 carb does not have a return line back to the tank. It also has a different inlet setup which seems to restrict the flow of fuel when used on A2s.

The best carb of the lot was, surprisingly, not the Holley. It was the 13841 but parts for this carb are very hard to find.

Ken
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Re: Carburator Zenith 12848 vs 13660B

Postby Horst » December 19th, 2017, 5:09 pm

as said above, the 12848 is not designed for use in an A2 with a return line. Other than that, I tried different carburetors on my A2, including the Holley. I found differences, but not significant, so at the end of the day I left the carb on which was on the truck in the first place. With proper setting of the ignition, a small pre-setting of the choke, it runs very nice.
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Re: Carburator Zenith 12848 vs 13660B

Postby rickf » December 19th, 2017, 6:06 pm

You are using a 13660 fuel inlet fitting on a 12848 carb. the cuts in the threads one way to get around using the proper fitting but I can assure you that you are getting plenty of fuel past all of those cuts. You would be much better of with the filter fitting that belongs in there and would have been there if it were a military rebuild. The 13660 carb uses the solid fitting with the small calibrated hole in it for vapor return to the tank. The 12848 has smaller primary passages so it will be down a bit on power. I have to disagree with Ken on the different versions of the 13660 series. They were mostly just changes in contracts more than anything else. The 13660A had a different flange casting that holds the idle mixture needle and also used a #28 main jet.
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Re: Carburator Zenith 12848 vs 13660B

Postby paracord » December 20th, 2017, 12:08 am

Got to start listening to Rick on these carbs, he has done bucket fulls of them. Someone correct me on these numbers if I missed wrote these #'s down in my pad. My jets weren't marked so I found a # 30 jet is .055 inch ,# 54 wire gauge drill bit and a jet # 28 is .052 which is a # 55 wire gauge drill bit. My friend gave me four take off carbs and I soldered up jets and re drilled down to # 56 and # 57 wire gauge drill bit experimenting like the days of old on the SMC Holley and Rochester Quadajet carbs changing the jets out to beat the guy to the next street light and week end 1/4 mile every Sat night at our favorite blacktop. For some reason it's not in my paperwork ,what I ended up using but I believe it's a # 28 jet which I'm happy with. Plugs are running chalk white even around town. My trouble and being able to pull the carb off in the dark ,had it off so many times, was a coil working good one day and getting hot and breaking down. Acting just like a carb problem. Changed several coils since ,different story, and runs good ever since. Has been since 1979 since I worked on one of these ole girls and just forgot the symptoms. Rick is the carb man on this site .Listen to what he says. Trust me. He's been there done that. He knows these carbs
Last edited by paracord on December 20th, 2017, 3:00 am, edited 2 times in total.
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Re: Carburator Zenith 12848 vs 13660B

Postby on-to-berlin » December 20th, 2017, 2:01 am

Rick, I was using the inlet because on the engine was the 13660A originally. The 12848 was a spare carb and I was playing around to get the A2 running. Then I got the 13660B which I am tempted to install right away.
So my question would be if all the carbs have the same body how difficult it would be to modifiy them with the proper jets etc so the performance is best?
Do not get me wrong I do not want to build a race car out of my A2 but my A1 runs so smooth that I thought it must be possible to do this with every other engine, too.
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Re: Carburator Zenith 12848 vs 13660B

Postby Horst » December 20th, 2017, 2:35 am

Well, you know by now that the bodies of the different carbs are NOT identical. A 12848 carb is designed to be used with the internal fuel filter and not the return fitting. So it is not forward compatible with an A2, any later carb is however backward compatible with an A1 set-up. The emissions carbs have a very different design of the main mixture tube, as such this is not interchangeable, the threads are totally different. Furthermore the size of the main jet is different in different types. Less obvious and not known to many folks is that the spring of the accelerator pump is available with 2 different spring rates and that can give bad acceleration / flat spots if the wrong spring is used with the set of main jet and main mixture tube.
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Re: Carburator Zenith 12848 vs 13660B

Postby on-to-berlin » December 20th, 2017, 3:05 am

Good Morning Horst, thank you for pointing out again the differnce in the bodys. For me the differences are minimal and could be modified easily. Or am I too naive here? Main Mixture tube could be changed?
I know this topic has been talked over and over again and it might get boring. For me it is fine just to install my 13660B and leave it like that.
Howver, as a wanna be mechanic like myself there is always the curiosity and urge to work on these things and open up bolts that should be better stay in place. :D
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Re: Carburator Zenith 12848 vs 13660B

Postby Horst » December 20th, 2017, 3:46 am

I will take a picture of the different tubes, you will be able to see that they are fundamental different. That part you cannot modify, how to make the 12848 on an A2 is a modification I did not know about until I saw your post
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Re: Carburator Zenith 12848 vs 13660B

Postby SturmTyger380 » December 20th, 2017, 7:53 am

The spring of the accelerator pump is available with 2 different spring rates and that can give bad acceleration / flat spots if the wrong spring is used with the set of main jet and main mixture tube


I find this interesting as I used to tune Holley 4 barrels in the 70's. Back then we did just that changed out jets and secondary springs to get the right balance to get a smooth transition from two barrel to four barrel operation per engine setup.

Does anyone have any notes on the M151 spring and Main jet / mix tube combos?
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Re: Carburator Zenith 12848 vs 13660B

Postby rickf » December 20th, 2017, 9:17 am

A couple of things here. You are not going to modify one or the other between the 12848 and the 13660 to fit the metering tubes, totally different. Can it be done? Yes, with more machining than is practical. Just put the 13660 on and see how it runs right out of the box to start with. Pop out the plug over the idle mixture screw and back out the idle mixture until it just starts to stumble a little and then go back in about a half turn to where it JUST smooths out. This will give a little rich idle but it will completely eliminate the usual flat spot at mid range.
Main jets, Everybody talks about the main jets. There are two sizes of jets that come in these carbs, #28 and #30. #28 was only supposed to come in the 13660A but I have found them in all three of the 13660 series carbs. I out one of those carbs on a vehicle that went on the first MVPA cross country convoy across the US. That convoy covered sea level to quite high altitude and he never had any issues. Another thing to note is I disconnect the return line and even crossing the desert he had no issues with vapor lock. So there is something for you to think about. Another thing to think about in relation to the main jet is the fact that you will only ever hit the full capacity of the main jet at wide open throttle at high rpm. I would be pretty sure that you would find on a dyno that you never move enough air through that engine to get a 12:1 ratio to lean out at high rpm before the rpm just stops climbing due to exhaust restriction in the exhaust manifold.
Basically what I am saying is this, changing the main jet will make no difference because you are not moving enough air to make use of the extra fuel. The flat spot that everyone complains about is in the mid range and not controlled so much by the main as it is by the transition circuits. There is a lot more to a carburetor than just the idle jet, accelerator pump and main jet.
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Re: Carburator Zenith 12848 vs 13660B

Postby on-to-berlin » December 20th, 2017, 10:22 am

Rick, point well made and understood. There are lots of rumors and questions about those carbs. Lots of people I usually talk to on military vehicle meetings could´t give me answers to my questions. So I thought I couldn`t be that difficult really. I also have a Harley Davidson WLA and there altough it is a very basic carb there can be done quite a lot with some minor adjustments. I though on the Mutts it can be the same. So I then underestimated the complexity of the Zeniths for the M151.
However, this afternoon I installed the 13660B. After some warming up, timing adjustment as well as carb adjustment the A2 runs just fine. I will probably leave it like that and put the 12848 carb on the shelf as a spare. Tried also my "new" timing light which I bought of German Ebay.
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