Code 001136 Exhaust emissions - Adjustment screw fell out. Can I just cap it?

Perhaps it has no adjustment screws - there was a period when exhaust emissions were of a greater concern from small engines, especially 2-stroke engines. During that period some small engine carbs had no facility for adjustment. Other carbs had the adjustment hidden.

I suggest you try cleaning the carb again - completely dismantling and cleaning every air and fuel passage with compressed air.

Possible causes
- Missing or loose fuel cap
- Incorrect fuel filler cap used
- Fuel filler cap remains open or fails to close
- Foreign matter caught in fuel filler cap
- EVAP canister or fuel tank leaks
- EVAP system hose leaking
- Fuel tank leaking
What does this mean? Tech notes
The P0455 it's a very common code, because the code is set if the gas cap is loose or missing. Start by tighten the gas cap and clearing the code. In most cases the code can be clear by disconnecting the car battery for about 30 minutes.
What does this mean? When is the code detected?
Evaporative Emission System (EVAP) control system has a very large leak such as fuel filler cap fell off, EVAP control system does not operate properly.
Possible symptoms
- Engine Light ON (or Service Engine Soon Warning Light)
- Possible a noticeable fuel odor caused by the release of fuel vapors
P0455 Mitsubishi Description
This diagnosis detects leaks in the Evaporative Emission System (EVAP) purge line using engine intake manifold vacuum. If pressure does not increase, the Engine Control Module (ECM) will check for leaks in the line between the fuel tank and EVAP canister purge volume control solenoid valve under the following vacuum test condition.
The vacuum cut valve bypass valve is opened to clear the line between the fuel tank and the EVAP canister purge volume control solenoid valve. The EVAP canister vent control valve will then be closed to shut the EVAP purge line off. The EVAP canister purge volume control solenoid valve is opened to depressurize the EVAP purge line using intake manifold vacuum. After this depressurization is implemented, the EVAP canister purge volume control solenoid valve will be closed.

Read more: http://engine-codes.com/p0455_mitsubishi.html#ixzz3NlVyARyX

I'm sure you already tried adjusting the fuel/air mixture screw or screws. Look at the adjustment screws again, is there a plastic cap on the screw? The cap will have a slot for a screw driver. Look at the cap closely is there a tab that limits the adjustment of the screw? If there's a tab limiting how far you can turn the adjustment carefully pry or pull the plastic cap off. Be careful you don't want to bend the adjustment screw. Under the cap you should find a slot in the actual head of the adjustment screw. Turn the screw about a 1/2 turn counter clockwise then start the saw and let it warm up. Once the saw is warmed to running temp adjust the screws slowly a little at a time until your saw runs at normal idle speed by itself. I was amazed after I did this with my saw. Once my saw is warmed up I can set it down and it will idle until it runs out of gas. You may have to play with the adjustments to get it where you like. Basically the plastic caps on the air/fuel mixture screws are an EPA thing to reduce emissions, in other words keep the engine running as lean as possible. Running to lean causes stalling and since 2 cycle engines are lubricated by the oil in the gas to lean can wear the engine.

p0456 was likely the gas cap, evaporative emission alert.

p1273
Either the exhaust is leaking or management needs reset.

'Clear the Self Learning Data' procedure. Anytime fuel component is replace, this procedure needs to be done. (exhaust emissions as well)

1. Unplug MAF sensor
2. Start car for about 3 seconds. Turn off car. confirm P0102 code
3. re-connect MAF sensor. Clear code. START car.
4. Run car for 10 mins.

there are plastic caps on the adjustment screws they are there so you can't adjust them but if the fell off you can

That screw is probably the pressure regulator adjustment screw, this screw is to adjust fuel pressure. Regulating fuel pressure is very important in newer engines. Suggest going to shop good with fuel pressure problems. Not easy fix for a DIY.

Small emission leak would indicate to me that the exhaust is leaking, maybe, throwing off the O2 sensor.

If it's an evaporative leak, then I would make sure the gas cap is on tight, and that the filler tube is connected with a vapor tight connection to the tank, and that the tank has no leaks in it.

What was the code, exactly, if I might ask?

The code PO420 you have is Catalyst System efficiency is below threshold bank one. Usually replacing the O2 sensor that's the front most in the exhaust system will cure your problem, Your Check Engine light is on and you find a P0420 code for a catalytic converter fault. Does that really mean your converter has reached the end of the road and needs to be replaced? A new converter can cost $600 to $1000 or more if a new car dealer or repair shop does the job, or maybe half that amount if you do it yourself. It's an expensive fix that may or may not be necessary. The problem, in most cases, is an emissions issue, not a performance issue that affects the way the engine runs.
A P0420 diagnostic trouble code is a "generic" fault code that is set when the Onboard Diagnostic II (OBD II) system sees a drop in converter efficiency. The OBD II system monitors catalyst efficiency by comparing the switching activity of the upstream and downstream oxygen sensors in the exhaust. The upstream O2 sensor in theexhaust manifold reflects the condition of the exhaust gases as they exit the engine. The downstream O2 sensor in or behind the catalytic converter reflects the condition of the exhaust after it passes through he converter.
The catalytic converter is like an after-burner. It oxidizes (burns) any residual fuel vapors (unburned hydrocarbons or HC) in the exhaust. It also burns any carbon monoxide (CO) in the exhaust. The exhaust must meet federal emission standards, and if a problem exists that causes emissions to exceed the federal limits by 150%, the OBD II system is supposed to catch the fault, set a code and turn on the Check Engine light.
Thanks.