OutdoorKing Repair Forum Forums Restoring, Customising and Modifying Tech Talk Rover Suzuki 2 Stroke

This is a picture from the Walbro WA Technical Manual. You can see that at idle, it emits fuel from a single idle port near the throttle butterfly. As you open the throttle, first two more idle ports chime in progressively, then the main jet joins in, and eventually the idle ports stop emitting. This multi-port system is used to get a smooth transition from idle, using just the first idle port, to fairly substantial throttle openings, when the main jet is in full song.

Not all carburetors have an idle progression system like that. Also, even if they do have that kind of system, there can still be lean spots in the progression if air is sucked in through the main discharge nozzle before petrol begins to flow out of it. Some of the Walbro carburetors have a one-way valve under the main nozzle to prevent this.

The carburetor is not the only possible cause of a mid-range flat spot like yours. It can be due to a small air leak, or retarded ignition timing, or leaky ports in a 2 stroke. However knowing that you have been having mixture problems, I'd start off with that as the most likely cause. It is interesting to note that we have had another thread about an M120X with lean mixture problems, but I don't think that one was ever solved.

It may be that your engine is particularly sensitive, or something may have been missed. I'm pretty confident that Suzuki designed an engine that was capable of running properly. I'd be continuing to focus on the idle system for the present. Are you sure that all of the jets are not only clean, but are still the sizes shown in this page from the workshop manual?



Edit: You might try reading this very long report on that Suzuki engine in a Toro mower:
http://forums2.gardenweb.com/forums/load/lmower/msg0423075515023.html
As you can see, the consensus is the same as I've said previously: aside from a problem with the fuel tank cap, the only problem causing surging in that engine is gunky jets, fuel passages, and air passages in the carburetor, presumably due to old oil going hard and staying put. I see one guy says you have to "boil it in carb cleaner". Not sure how you do that safely, but lots of people soak carburetors in carb cleaner, with good results. We are all headed in the same direction.

Here is an extract from the Toro workshop manual for the Suzuki M120X:


I suggest you just do exactly what Toro recommends, including remembering not to expose the plastic parts to carb cleaner.