OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions on Other Engines Suzuki 2 Stroke Surging

I have looked at the other post regarding this engine and I have a similar (but different) issue with this engine. It's a Rover fitted with the Suzuki 2 stroke motor. It starts first time every time but surges at low revs but running fast seems ok. I have stripped and blown out carby with air hose, fuel mix screw (undamaged) has been set to one a half turns out, made all new carby/manifold gaskets from automotive gasket paper (oil and fuel proof) and sprayed with the solvent based spray before assembling again. Adjusted the governor as per a post I saw on this forum, checked engine crankcase seals (all ok), new spark plug, no bent linkages or springs and no links are binding or rubbing on anything to restrict movement, all governor springs are in ok condition and fitted properly as per the photos of same setup I've seen on one of your forums. The only other thing is that I've used a piece of foam from Clark Rubber as a filter element as the mower was not fitted with one originally, but have soaked in 2 stroke oil and then squeezed out excess as one would need to do. As I said starts and runs on fast really well but surges when down to idle, I have also tried as an alternative solution making the gaskets from the bottom muffler gasket material from A Victa 18 muffler system as I thought a bit more body would give a bit more seal when tightened, so I have tried 2 different options for the gaskets. The only thing I can say is when it is running on fast when you put it on choke slightly it starts to die (as one would expect). I'm running out of ideas so I need new light shed on this. Any suggestions sincerely appreciated. Cheers all and thanks. Mike

Hi Mike, welcome to Outdoorking.

From your description, you have lean mixture at idle only: mixture is OK at higher speed, and the choke works. To verify this, I suggest you try warming up the engine then adjusting the choke at idle. If the problem is indeed lean mixture, you should be able to find a choke setting that gives you a perfect idle, then the idle should deteriorate with either less or more choke than that. Please report back when you have tried this.

There should be no problem using a pre-filter made from urethane foam from Clark Rubber. At worst, it would be restrictive and would make the mixture rich. Since your symptoms suggest lean mixture, the pre-filter cannot be blamed. Incidentally, your wording suggests that this was the only air filter, rather than a pre-filter. The service manual specifies a urethane pre-filter in front of a paper air filter. Please confirm that you have the correct paper filter in place.

Hi Grumpy that is excellent advice, see it pays to ask an expert. I understand your advice re the choke settings, if I move the choke to get a perfect idle, one would assume that it is running too lean, the air filter box only has the foam fitted, nothing else, when I got the mower it had no filter material at all. If your advice on testing when warmed up and closing the choke slightly to get correct idle, what do you think would be the solution to this issue when the throttle control is off choke and back to the normal idle position? Cheers!

Mike, if it runs properly at all speeds and loads except idle, and the problem at idle can be overcome by applying some choke, the engine has lean mixture at idle only. That means either it has an intake air leak at idle only, or the carburetor's idle system is not passing enough fuel. Because carburetor engines have high intake vacuum at idle but much less vacuum at higher speeds, an air leak is a possibility, but you have worked on the intake system to eliminate leaks. So, the best bet is the carburetor's idle system. The Suzuki M120X uses a Mikuni BV18-15 carburetor, which is fairly complicated for a mower engine. As you know, it has an idle mixture screw, and you should try adjusting it to see if you can improve the idle. Do not take the "initial setting" too seriously in this situation. If you find the mixture screw can make it worse but not better, there is a strong possibility that you have not got all the crud out of the complicated idle system. I suggest you read this thread:
https://www.outdoorking-forum.com.au/forum/u...mp;Words=M120X&Search=true#Post50386

Note that Toro recommends that carburetor maintenance for that engine consists mainly of soaking the carburetor (with plastic parts removed) in carb cleaner - you won't get all the crud out just by squirting the cleaner here and there, let alone just by blowing air through the passages. (Carb cleaner is powerful stuff by the way: if you haven't used it, you'll be surprised at its effectiveness.) The internet is full of reports of problems with that carburetor, and most of them seem to be consistent with them still having dirt in them, after conventional cleaning.

You need to put a proper paper air cleaner on that engine before you make any use of it. It is a good enough engine to deserve a reasonable chance of survival, which requires an air cleaner that works the way it should (including foam pre-filter, and paper filter).

Thanks Grumpy, very useful information which I will put into practice. Thanks again!

I pulled the carby down on this again and stripped it and found there is a small jet that runs vertical next to the mixture screw. I removed this and there are 2 holes in the sides slightly offset and 1 very small hole straight down the centre of the jet that is only small enough for a piece of wire from a wire brush, I cleaned this out and used carb cleaner on the rest of it and used the compressor air gun to clean up before reassembling, using gasket solvent on the gaskets then I reassembled. I must be on the right track because it runs heaps better, throttle more responsive but still "hunts" slightly when throttled down towards idle but nowhere near as excessive. Maybe I should soak the carby for a day or so. It does idle down almost ok when slight choke is applied although does smoke a bit more due to choke being on slightly. Still in progress, very frustrating!!!! Cheers all!

I suggest that you should soak it in carb cleaner for a few hours, then blow it out, just to be sure you have removed all the gum. From your report, it sounds as if fouling in the idle system has been the problem. Since you have only half-done what you are supposed to do, and that has only half-fixed it, doing exactly what is in the manual seems to be the next step.

I have had a carburetor that I sprayed carb cleaner through a couple of times, then found that the main jet and emulsifier were still completely blocked. Hard gum is really difficult to remove. Soaking should fix it. My way up to now has been to use the correct diameter jet drill bits for each hole, and re-create all of the missing holes that way, but generally I think soaking is gentler and more practical.

Thanks Grumpy, will do just that! Thanks again!

Hi all, got this issue sorted with the Suzuki, took all advice, problem fixed, had to clean out main jet with very small drill bit and it's right as rain, thanks to all for steering me in the right direction. Cheers!

Thanks for reporting back on that Micky. It validates what the workshop manual says, and agrees with a good proportion of the various internet reports too: it is a good, smooth running engine that has the misfortune to have a carburetor that is difficult to clean properly. However, I don't recall personally hearing of a "problem engine" Suzuki M120 that was not due to a simple carburetor fouling issue, or at least might have been (in many cases the owner never did really clean the carburetor the right way, so we don't know if it would have fixed it).

You've given us a clearly documented case now on Outdoorking. I think we had one before that was carburetor, and one where the member put the job aside when simple maintenance didn't fix it. I don't recall any more than those two cases. So, yours is a really useful addition to our archives. And congratulations on sticking with it until you found the answer.