M idles on main jet

Dalrymple

Member
I bought a SMTA with a reportedly fresh motor and a carb problem. I went through the carb and I'm confident I found the problem, some kind of sticky thick gunk running down out of the manifold had the throttle plate practically stuck, plus there was some bug carcasses in passages here and there.

I soaked it and cleaned out all passages, etc, and put in new gaskets but I have to run the main jet adjustment in to about 1 turn off the seat to get the tractor to idle properly. Out any further and it's rich, and gets richer the more you back it out. Two turns out will kill the engine.

When the main jet is run in to idle good there is some activity in the idle mixture circuit, but I can't kill the engine with it, just make it rough.

I haven't measured the idle RPM's but it's nice and slow with the idle speed screw barely moving the throttle plate. I've adjusted the float level to about 1 7/16" with no effect.

Obviously the tractor won't work at high idle with the main jet screw in that far, and getting off the tractor to adjust the carb when changing throttle settings is too much exercise for me!

I should probably mention that the guy I bought the tractor from also has some M's built for pulling, and I wonder if this carb is modified for that? He had this engine overhauled 5 years ago, and the t/a, and had the sheet metal painted, but stopped on the project and never even started the engine until I went to look at the tractor a few weeks ago. I'm pretty sure that they were not particular about making sure this carb came off this tractor back when they disassembled the engine for overhaul.

Any ideas?
 
Have you tried it at WOT? The main jet may be drilled out but if you can adjust it to where it runs at WOT it should be ok. Set the idel screw after wards.
 
Clean out the idle circuit! Remove the idle air jet and run a fine wire (soft, such as copper from a piece of taillight conductor)through the passage AND jet. Use brake cleaner (evaporates completely) to spray through each passage, blow dry with compressed air, and re-assemble.
Set the idle air adjusting screw at 1 1/2 turns out and main jet screw at 2-2 1/2 turns out for starting point.
 
The main jet is internal, and not adjustable. The power adjustment supplements the control of the main jet by adding more or less fuel to the main in a range of lean to rich.
It should never have an affect on idle richness.
The internal idle passages/ports and adjustment screw is used for that. If they make little difference, and the float level is correct (and not sinking) the problem is in that idle circuit. (IIRC) the idle richness screw is rich in lean out. Jim
 
...The power adjustment supplements the control of the main jet by adding more or less fuel to the main in a range of lean to rich.
[b:c63ac14a94]It should never have an affect on idle richness[/b:c63ac14a94]....

Exactly, but it does. That's the problem I'm trying to solve, why it's pulling fuel through the main jet when the engine is at idle.
 
If the PO modfified it for pulling, hard telling what you have. Take the carb off, disassemble, soak it overnight in real carb cleaner, the type that will peel your skin off, not parts cleaner. Blow it out with compressed air. Every inlet to the air must come out somewhere. You may have to clean the passages with wire as a last resort, I don't like to manually clean jets, etc, but you may have to. If the choke control is on the inside of the carb it is a SM-up carb. If it is on the outside it is an M carb. The SM carb is not adjusted like the M carb (main jet screw in until rough, out until smooth). Instead max power is with the screw 5 turns out, in a turn or so with light loads. I doubt if it will pull anything if 3 turns out. The main jet is supposed to have nothing to do with idle. Your idle system is full of crud. If you cannot get it clean, you need another carb. or at least put the right jets in it, provided you can get the old ones out.
 
It's clean. I'm a veteran at carbs, just not a veteran at tractor carbs. I now know I have the wrong carb for this tractor though, if the choke control belongs on the inside. This one's on the outside.

I want the tractor to finally be correct, so I'm now in the market for a SM carb rebuild-able core if anybody has one. If I get that and still have a problem, I'll go from there.
 
You need to measure the idle jet size. Idle jet
should be a #72 which is a darn small hole. Main air
bleed will also affect it. Hard telling how many
ports have been drilled out if used as a puller.
 
Dalrymple send me a e-mail with your Phone # and I will call you. Think I have a SM carb sitting somewhere in my farmall parts.
 
In classic look to right in the posts. Will have send email or no email. Dave has the send email checked, just click on that to send.
 
(quoted from post at 07:49:26 05/22/12) In classic look to right in the posts. Will have send email or no email. Dave has the send email checked, just click on that to send.

In classic view I see [No Email] in Dave's post, and in yours.
 
Thats strange I see send email in both. Checked modern and see email for the last post made by you and none for Dave or I. Good luck.
 

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