Main jet on Super M

Hayfarmer · Feb 24, 2017

Can someone give me the size of the main jet on a super M? I replaced the one on my SMTA but I think it is for an M instead of super. I think I need to open it up a little
thanks for any help

charlieu · Feb 25, 2017

(quoted from post at 04:14:15 02/25/17) Can someone give me the size of the main jet on a super M? I replaced the one on my SMTA but I think it is for an M instead of super. I think I need to open it up a little
thanks for any help

I did some looking for you in my Carburetor spec book GSS-1309 and this is what I found for size of Main Metering Jet, sizes are listed in thousands.

M gas carb #58553DB 0.058
M Distillate/Kerosene carb #47387DC 0.059

SM gas carb #356948R92 0.058
SM Did/Kero carb #357231 0.059

What part number did you get for the main jet that you replaced?

PretendFarmer · Feb 25, 2017

Strange how its the same size yet a different number. Putting an M jet in there shouldn't matter.
If the tractor has an overbore at 4 1/8", you could actually drill out that main jet to .072-.082" as well as the metering jet and replace it with a 29/32" venturi.
Reason being is if you are adjusting the carb and you crank out that adjustment screw all the way and don't get black smoke, the carb isn't supplying the maximum amount of fuel.
You should be able to have someone hold the throttle wide open, then adjust the screw out till it smokes, then turn it back in till it stops smoking for maximum power.

pete 23 · Feb 25, 2017

The reason for different part number but same size is the taper of adjusting screw. Jet tapered to match. Fire crater packages came with the different tapered screw and jet also.

Hayfarmer · Feb 25, 2017

A couple years ago I was using my SMTA on a swather and was having trouble, seemed like it got "tired" as the day went on. Someone here suggested drilling out the main jet and metering jet a bunch. It never ran out of power but was overfueling. I changed out the metering jet but left the main jet. I recently replaced the main jet but can't get it to run smooth like it did before. I have checked fuel to carb and it seems ok. Main jet was NLA from IH, finally found one at McDonald carb but think it was for an M. Anyway probably main jet size isn't my problem so will hae to look elsewhere. thanks for the help on this

D Slater · Feb 25, 2017

Jet # 350001R1 was .051 from IH. Think for some reason jet # 56646D was made in size .0545 and .058. When making changes the best thing to get is a small numbered bit set marked in thousands and a small micrometer. See what goes or doesn't go in passages. Some passages will be between sizes. Something as simple as a close hanger can be turned down to get between some sizes if needed. Most can be checked okay with a set or within .002. I check sizes every time a carburetor is apart and still can't tell you for sure what standard was after all the changes made over the years. Find a lot of different sizes. For a SM changed to 281 size working tractor .060 or .061 should be the most you need.

PretendFarmer · Feb 26, 2017

I suggest the best way to see how big the jet is, is to use pin plugs. Most machine shops would have them. With our pin plugs I should be able to get accuracy within .0005" or better.

High Octane · Feb 27, 2017

I remember you doing this back then. Don't over drill it this time.
Drilling the bottom two holes of the super h thru mta style stems only.
I'd look at a .063-.073 bit sizes for main jet bit sizes, if your compression is below 150 psi.
Bottom two holes of the metering stem,
Which these holes made too big, or drilling all of them, can cause over fueling. Too small of venturi n big jets can smoke.
Keep loyal to your favourite gas. Don't use from bunch of different stations.
Make sure that bowl is clean. Could have fine shavings in circuit under the idle stem.
There's passages in the throttle shaft, that makes the change over from idle to main jet circuit.
Wrong shaft position by wrong idle stop setting can mess with your fuel settings.
If you have messed with the governor shaft vertical linkage length, you may have to recheck that adjustment to make sure the gov is letting the carb go into idle.
A 281ci, at 150 psi of compression, running up to ten percent over stock rpm, using a Mta carb, 87 octane gas, for full load operation, main jet start point be. 065, and same for bottom hole of stem.
Yeap, someone...

Main jet on Super M

Hayfarmer

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charlieu

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PretendFarmer

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pete 23

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Hayfarmer

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D Slater

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PretendFarmer

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High Octane

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