Lean running A2

1959 - 1978, M151, M151A1, M151A2, Technical questions and discussions, regarding anything related to the M151.
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Hanselandgretel
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Lean running A2

Post by Hanselandgretel » Sun Oct 02, 2016 6:26 pm

My 151A2 needs about 3/8 inch choke at all times to run smoothly at cruise. Accelerating seems OK, acts like it is running too lean. Are there several main jet sizes and where can I find that info? Carb was overhauled, valves were ground, all gaskets and lines are new and tight I think. Ideas? Do I need to find a richer jet? Or drill out the one I have? Thanks Al Hansel


Rickf
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Re: Lean running A2

Post by Rickf » Mon Oct 03, 2016 6:13 am

These carburetors were designed to run lean due to emissions regulations. There were only two jet sizes, 28 and 30. I never found any real running difference. What I have found to be the biggest difference is to reset the idle mixture just a hair to the rich side instead of centering it at best idle. Also all of the other engine parameters have to be right on, valves, timing. These engines are famous for a vacuum leak around the intake manifold so I usually bring the engine speed up to a fairly high idle with the hand throttle and then spray carb cleaner around the intake ports and listen for any change in engine sounds. Be sure to spray under the manifold as this is where the leak usually is.

The idle mixture tubes also tend to get dirt in them and clog very easily and this will cause the lean running issue and usually a bog at midrange. This entails removing the carb and cleaning the entire idle circuit from the carb throat all the back to the bowl. Do NOT lose the tiny gasket on the tube in the bowl!
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

Hanselandgretel
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Re: Lean running A2

Post by Hanselandgretel » Sun Oct 09, 2016 6:23 pm

Thanks Rick. Carb was just cleaned and new gaskets put in, plus all the rest of the manifold gaskets etc. I will try the cleaner trick in case I have a leak. Thanks again. Al

Rickf
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Re: Lean running A2

Post by Rickf » Mon Oct 10, 2016 5:13 am

If exhaust gaskets were put in and manifold torque was not exactly followed then there is probably an intake manifold leak. The clamping design on that intake/exhaust manifold setup is very poor and if the torque is not done just right they will leak. And the A1 and A2 use different torque settings since the A1 did not use exhaust manifold gaskets.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

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W. Winget
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Re: Lean running A2

Post by W. Winget » Sun Oct 16, 2016 9:54 pm

At the risk of being ragged on by some, every M151 I've ever driven (lots) restored (a few) has run best with the choke about 1/4" off the dash. I normally try to adjust the choke cable so the choke knob sits flush after pulling it 1/4" out so it doesn't annoy the driver....(after tuning the engine properly)
You can dremel tool polish the throat of the carb to smooth the airflow up to the venturi, you can add packing (mouse pad cut up into rings) under the throttle linkage to reduce air intrusion through the shaft, you can shave the throttle and choke vane leading and trail edge to give it a more aerodynamic flow. All of these little things will help smooth that carb out.
Worst thing you can do is park it for more than about three weeks without starting and the Ethanol will start blocking things up in the carb. STARTRON (the blue stuff at Walmart and others) seems to help best on the MUTT for fuel staleness, but starting every three weeks for even a few minutes keeps the carb flowing.
V/R W. Winget

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Re: Lean running A2

Post by Grasshopper » Fri May 26, 2017 4:44 am

W. Winget wrote:At the risk of being ragged on by some, every M151 I've ever driven (lots) restored (a few) has run best with the choke about 1/4" off the dash. I normally try to adjust the choke cable so the choke knob sits flush after pulling it 1/4" out so it doesn't annoy the driver....(after tuning the engine properly)
You can dremel tool polish the throat of the carb to smooth the airflow up to the venturi, you can add packing (mouse pad cut up into rings) under the throttle linkage to reduce air intrusion through the shaft, you can shave the throttle and choke vane leading and trail edge to give it a more aerodynamic flow. All of these little things will help smooth that carb out.
Worst thing you can do is park it for more than about three weeks without starting and the Ethanol will start blocking things up in the carb. STARTRON (the blue stuff at Walmart and others) seems to help best on the MUTT for fuel staleness, but starting every three weeks for even a few minutes keeps the carb flowing.
V/R W. Winget

Sage advise. I'm quickly coming to the same conclusions after much tinkering. Mine always seemed to run best with the choke out a 1/4 inch, so I think I'll just live with that and reset the cable to compensate.

Rickf
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Re: Lean running A2

Post by Rickf » Fri May 26, 2017 7:29 am

All of the carbs I have set up have not had to run with the choke out. Once the rest of the engine is set up correctly you can set the idle mixture just a hair to the rich side and it will run just fine with no choke at all.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone


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