Surging Engine, despite usual remedies

[fultonwa]

Hi, I have a customer’s Mountfield mower ST120 in with a STIGA engine. It had trouble starting and running so I initially gave the carb a deep clean and emptied/cleaned out the tank. It still surged so having looked for any other issues such as leaking gaskets, cracked inlet manifold, air filter etc. I put on a new carb. Same problem, surging and the governor arm moving in and out every second or two until it dies. Anyone any ideas please? Thanks in advance.

 

[EFSM]

Does the surging go away with additional fuel?

 

[fultonwa]

No, the surging continues whether the tank is half empty or full. I’ve also run with the fuel cap loose in case it’s not venting properly but that doesn’t reduce or eliminate the surging.

 

[EFSM]

No, I mean if you put the choke on partway (if it has one) or slowly push the primer bulb.

 

[fultonwa]

No, I mean if you put the choke on partway (if it has one) or slowly push the primer bulb.

Ah, sorry, I misunderstood. It has an automatic choke and no primer. It looks to be a Honda clone carburettor.

 

[EFSM]

I would be highly suspicious of the plastic jet in the top of the carburetor, although you've probably checked that on the old and maybe the new carburetor. That's the smallest orifice in the carburetor and the most likely to plug.

 

[Sloughfoot]

Ah, sorry, I misunderstood. It has an automatic choke and no primer. It looks to be a Honda clone carburettor.

I feel your pain. Those Honda type carbs for mowers have a lot going on to sort through. You tried fiddling with the bend on the linkage? I bent my throttle linkage to get a little higher rpm and it threw the choke off. Used to run well summer and winter. Now it surges till it warms up in the winter and runs well summer only.

 

[fossil]

If the carb has a low side mix needle, open it up.

 

[fultonwa]

If the carb has a low side mix needle, open it up.

Thanks, will do, I’ve got some micro drills and will try tomorrow and let you know how I get on

 

[Motorka]

Had one engine that would cut keys on flywheel look at this.

 

[Brad Button]

Is the new carb an OEM or aftermarket? Watch this video, it may help.

 

[fultonwa]

Is the new carb an OEM or aftermarket? Watch this video, it may help.

Thanks, yes I watch Steve’s YouTube videos routinely. He’s one of my top go to people. The carb is aftermarket though the engine itself is a Chinese/Honda (CHonda). I’m beginning to think it’s the ignition coil as I’ve looked at every fuel/air issue. It most definitely gets worse after a few minutes of running so I plan to heat up the coil in an oven then check readings with an ohm meter. Watch this space .

 

[GMB74]

I think you were on the right track to begin with. The engine is running lean.
Have you modified the air filter in any way? Too little restriction will not pull enough fuel through the jets.
Is there an air leak in the intake system? An air leak will make it run lean for sure.
I don't know about the Chinese made Honda copy engines sold in Europe, but the ones sold here in the US are leaned right out to meet EPA emissions.
The quality of the replacement aftermarket carbs is erratic, often they are jetted too lean and will surge.
Did you try opening up the pilot jet the way Steve described in the video?
Sometimes you have to open up the main jet with a slightly larger micro drill bit.

 

[fultonwa]

I think you were on the right track to begin with. The engine is running lean.
Have you modified the air filter in any way? Too little restriction will not pull enough fuel through the jets.
Is there an air leak in the intake system? An air leak will make it run lean for sure.
I don't know about the Chinese made Honda copy engines sold in Europe, but the ones sold here in the US are leaned right out to meet EPA emissions.
The quality of the replacement aftermarket carbs is erratic, often they are jetted too lean and will surge.
Did you try opening up the pilot jet the way Steve described in the video?
Sometimes you have to open up the main jet with a slightly larger micro drill bit.

Thanks everyone for your thoughts and suggestions. I’ll increase the idle jet size as has been suggested and run some ohm meter tests in the coil after heating it up in our oven (while my good lady isn’t around ). I’ll report back soon. Thanks again.

 

[Sloughfoot]

Thanks, will do, I’ve got some micro drills and will try tomorrow and let you know how I get on

Picked up one of these at the local welding supply house for such things. It's gotten a carburetor or 3 working for me.

Uniweld TCM Tip Cleaner Set Tip savers Master Sizes 75-30 26064 21 Cleaner Wire | eBay

Model : TCM.

www.ebay.com

 

[el33t]

Before you start reaming the jets, I would advise you to replace the spark plug first, if you haven't already done so. You didn't mention anything about the ignition system before, and you've replaced the carburetor, so of course, in addition to that, inspecting the high-voltage wire and checking the resistance of the coil windings are pretty obvious steps. Although the most sure thing would be to swap the coil to check.
I would also still check the ignition switch.

 

[RCBS]

I've taken to ordering older main jets for my Honda carbs. Seem to run more smoothly. Main jet dimensions have not changed for decades, only the size of the holes. Yes I have used a tip cleaner to slightly enlarge not the main jet, but the orifices on the body of the main jet.