carburetor zenith 12848 problem after reconstruction

1959 - 1978, M151, M151A1, M151A2, Technical questions and discussions, regarding anything related to the M151.
lolo France
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carburetor zenith 12848 problem after reconstruction

Post by lolo France » Fri Jan 14, 2022 4:33 am

Hello everyone,
My Zenith 12848 carburetor is causing me a lot of worries!
it worked well before disassembly,I cleaned it with ultrasound and rebuilt with new kit

since my M151 A1 has never started, the carburetor flows through the outlet on the air filter side and don't spray,

if I spray a pilot start bomb from the air filter, it starts up instantly

I hope you understood my explanation and I thank you for your help because I do not know what to do, I disassembled and reassembled almost 4 times without result....


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Re: carburetor zenith 12848 problem after reconstruction

Post by Rickf » Fri Jan 14, 2022 7:00 am

It sounds like maybe you are saying it does not spray when you push the gas pedal before you start it? If that is what you are saying then this is perfectly normal for this carburetor. It has a vacuum operated accelerator pump so the pump will not work unless the engine is running. To start these engines the choke has to be fully on/ pulled out. Once it fires once or twice you push the choke in a little and it will fire right up and run as normal. On the accelerator pump, when you installed it you have to be sure you put the rubber diaphragm in first and then the spring and the the cover in that order. This is reverse from most carburetors and a lot of people get it wrong.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

lolo France
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Re: carburetor zenith 12848 problem after reconstruction

Post by lolo France » Fri Jan 14, 2022 8:24 am

do you agree with this direction of assembly Rickf?

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Re: carburetor zenith 12848 problem after reconstruction

Post by lolo France » Fri Jan 14, 2022 8:47 am

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Re: carburetor zenith 12848 problem after reconstruction

Post by lolo France » Fri Jan 14, 2022 8:51 am

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owensexport
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Re: carburetor zenith 12848 problem after reconstruction

Post by owensexport » Fri Jan 14, 2022 10:01 am

The photo in the early manuals is incorrect.
The spring goes on the outside of the diaphragm.
The flat part of the Diaphragm pushes on the power valve, when
vacuum drops by stepping on the accelerator pedal, it pulls
back on the spring allowing the power valve to release
a shot of fuel into the carburetor passages.
See the attached photo from the later manual.
Attachments
Capture.JPG

lolo France
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Re: carburetor zenith 12848 problem after reconstruction

Post by lolo France » Fri Jan 14, 2022 11:03 am

super detailed thank you, so it's good for a zenith 12848 your plan?

Rickf
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Re: carburetor zenith 12848 problem after reconstruction

Post by Rickf » Fri Jan 14, 2022 7:17 pm

Yes, those original picture are wrong, The spring has to be out against the outer cover, and the diaphragm will be flipped around so it is facing the spring just as it is in the picture. The spring fits in the metal cup side of the diaphragm. That will make sure your accelerator pump works while the engine is running. BUT, it still will not work when the engine is NOT running.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

lolo France
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Re: carburetor zenith 12848 problem after reconstruction

Post by lolo France » Sat Jan 15, 2022 3:16 am

I also found an old topic here, thank you very much for your answer too,

I have an external electric pump and when I try to start my carburetor overflows with gasoline from the inside and does not spray gasoline

Rickf
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Re: carburetor zenith 12848 problem after reconstruction

Post by Rickf » Sat Jan 15, 2022 5:39 am

If you have the intake hose of this is also normal. As long as it is only doing it while you are cranking. If it is overflowing as soon as the switch is turned on without cranking then the pump probably has too much pressure and is pushing past the needle and seat. You need to have the intake hose attached while starting and running for everything to work correctly on these. The has overflowing during cranking is the vacuum accelerator pump injection little bits of gas as te\he engine vacuum pulses with cranking. If you have that hose off while it is running and watch as you shut it off there will be one shot of gas as it shuts down. This shot of gas usually lays in the hose and will help with the next start up.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

lolo France
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Re: carburetor zenith 12848 problem after reconstruction

Post by lolo France » Sat Jan 15, 2022 7:57 am

my electric pump works with the ignition, the carburettor does not leak at this time, as soon as I start the engine it leaks

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Re: carburetor zenith 12848 problem after reconstruction

Post by Rickf » Sat Jan 15, 2022 1:14 pm

Is it leaking while the engine is running? That would be caused by one of two things, either the float setting is too high or the fuel pressure is too high. the settings should be 9/32" on the float setting and 3-5lbs on the fuel pressure leaning towards the lower side of the pressure setting. Sorry, I don't have the conversions in front of me but I think they are listed as both in most of the manuals. I am thinking though if you used the manual that showed the wrong spring location that may have been the same manual that incorrectly listed the float setting at 3/32" I think it was. The correct setting is 9/32" for the float height setting and 3/8" for the float drop.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

lolo France
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Posts: 42
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Re: carburetor zenith 12848 problem after reconstruction

Post by lolo France » Sat Jan 15, 2022 3:05 pm

tank you for your help, I installed the diaphragm spring correctly today, it turns better, I have adjusted the float correctly with your measurements

currently if I do not prime it with a "pilot start" bomb it does not start, when I try to start it the carburetor leaks on the filter side but when it turns the carburetor no longer flows

and it turns very well once on the road!

I specify I installed the intake duct and the gasoline flowed in it no difference, I also specify I have a paper filter and not oil in the air filter

Rickf
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Re: carburetor zenith 12848 problem after reconstruction

Post by Rickf » Sat Jan 15, 2022 3:25 pm

Is your choke cable adjusted so that when you pull it out the choke plate fully closes? Yu need to do this to get it to start when cold. Even if it is not fully cold sometimes you will need to use just a little choke to get it to start. You have to remember this is into a computer controlled engine so no gas is automatically injected into the engine. And constantly using ether (starting bomb) is not good for the engine. When it is cranking it will leak some gas into the intake hose, this is normal and helps it start. The paper filter will not make any difference at all. If it runs well after starting then the fuel pressure is good and the float level is good and it is not leaking into the hose at that point. If it were you would see a lot of black smoke from the exhaust.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

lolo France
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Posts: 42
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Re: carburetor zenith 12848 problem after reconstruction

Post by lolo France » Mon Jan 24, 2022 2:07 pm

today it starts all by itself like a big one! without adding "start pilot in bomb" I adjusted my ignition because it was weak, to finish my adjustments can you tell me how to adjust the richness screw, how many turns? and spacing of ignition points


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