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M running issues and other questions


bitty
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Neighbors one M. 281 engine, 450 style carburetor. Doesn't idle smoothly until he removes the air jet completely (if I am calling it the correct name, red arrow in my last picture) . Does that same thing with his second carburetor. Runs normal with a M carburetor I have. Is this a replacement block as it appears to have a fuel pump location on it and I didn't notice a serial number. IMG_20210829_195054159.thumb.jpg.c8292a5686656c281826e37f41666660.jpg

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Yes replacement block and probably doesn't have a serial number stamped on block. Not a 450 gas carburetor in picture. May be a SM. Screwing out screw leans the mixture on that type carburetor. So it's to rich at idle and the metering is to far out for normal adjustment. Maybe a needle and seat or float problem or someone got happy with a drill bit. Also could have the idle jet missing. Better sealing between throttle body and bowl could help.

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12 minutes ago, TP from Central PA said:

Quite a few of those blocks around if you look................Dumb question.........is it my eyes or is RTV sealing that bowl?  

Bowl had a gasket. We had just dropped the bowl down to replace the needle and seat from the other carburetor. 

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Probably a gob of RTV plugging something.

How does one gently say that you have 100 times more RTV than is needed.

Clean the carburetor and put a quality kit in it.

That will be a start.

And no RTV on the new gaskets.

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49 minutes ago, Diesel Doctor said:

Probably a gob of RTV plugging something.

How does one gently say that you have 100 times more RTV than is needed.

Clean the carburetor and put a quality kit in it.

That will be a start.

And no RTV on the new gaskets.

Pretty sure it's a home made gasket that sticks out the side and is mangled on the outside edge. I know the other carb was a gasket. No rtv was used  I will recheck it tonight when I stop by

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  • 4 weeks later...

Those will idle very well when the carb is clean.

There are small orifices that need to be cleaned out with a drill, etc., then use spray carb cleaner and compressed air.

I picked up a very small set of metric drill bits at Menard's that are cheap and small enough to clean the passages.

Just make sure the passages are clean and never drill out anything bigger.

Make sure the float has no fuel in it. If it does, replace it.

A new needle and seat and gaskets make a big difference.

If needed, watch any of the YouTube videos about cleaning these carburetors.

Both the idle jet you are having issues with and the main jet at the lower front of the carb should be open 1 1/2 turns from totally closed, as a start.

It takes little to plug a small passage in these carburetors. 

Also make sure the sediment bowl is clean and the brass filter is in good shape that is screwed into the carburetor, on the end of the fuel line.

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On 8/29/2021 at 9:25 PM, superih said:

That is a Super M carb.  I bet the idle jet is missing, the small brass stem that screws into the fuel bowl half of the carb

Idle jet was broken off on that carburetor AND his spare carburetor . That's why it ran fine with my used M carburetor but not with either of his 

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