I have a push-behind trimmer with a standard 4-cylinder engine. It cranks easily, then its RPM oscillates very predictably between standard operating RPM and almost dying. Why is this happening?
I’m not actually asking what is broken. What I want to know is what’s actually happening in the carburetor to make the RPM drift up and down like it does.
I’ve done a lot of research and I am now somewhat educated about carbs. Ignorantly, I left it fueled over the winter with ethanol-spiked gas. I have now learned this is a bad thing to do in general, and this model is especially sensitive to it (Cub Cadet ST100). I guessed that the engine is probably fuel-starved, so there’s probably something upstream fouled. I guessed it was something in the carburetor.
Now I have refueled with ethanol-free gas, removed the carb, cleaned it by soaking it in good gas, and I made sure the pilot jet isn’t clogged. It’s still oscillating. My next guess is the fuel filter is fouled.
But my overall question is, what’s actually oscillating inside the carb? I really want to visualize what’s happening to cause that up-and-down drift so that it will help me think through solutions.
But my neighbor does. He works on these things for a living and noticed my mower was doing the same thing when I was out mowing.
He instructed me to get the appropriate carburetor rebuild kit (just a few gaskets I think) and he rebuilt/installed it for me in less than two hours.
I think something was sticking or leaking where the float wouldn’t smoothly open the fuel valve, but it would pop open, flood the bowl, close off, then nearly drain the bowl before it would ‘pop’ open again.
My small engine guy says this is the usual cause. He thinks the ‘stickiness’ of the mechanism is just poor design, it should be able to settle into a steady fuel flow even if it’s a little slow in filling and draining the bowl. Some of the ones on riding mowers have gotten ridiculously complicated and still have the same problem.
Not my area of expertise, but my understanding of it is that there can be quite a few different causes of it.
What I believe happens is that any kind of fuel starvation causes the carb to be unable to keep the bowl filled with gas. The bowl empties, the engine gets starved for gas, the RPMs drop, now the carb can get enough fuel in to keep up with the load (since the RPMs are so much lower) and the bowl fills back up, the engine now has more gas available so it’s RPMs go back up, and then the engine drains the bowl again, and the cycle keeps repeating.
Alternately, as was already mentioned, sticky floats can cause the same thing as the floats allow the bowl to drain too far before opening the fuel valve, starving the engine and causing a similar surge cycle.
I think you are going down the right path. Carb cleaner will do a better job of getting the gunk out than fresh gas. Plus carb cleaner comes in a spray can with a nozzle you can really spray it into all of the little holes and places where gunk can hide and force out the gunk.
My 2c on this. Gas with ethanol is the culprit. I switched all my small engines on ethanol free gas and they have never had this problem again.
Check if there is any gas station in your area selling ethanol free gas. It may cost 50c more per gallon, but you will save a lot on carb cleaners and engine life.
I used to have a problem with my motorcycle not starting in the spring (back when I had a motorcycle…) Apparently if I didn’t drain the carb in the fall, the gas in the carb bowl would get sticky (and in the spring, it smelled like turpentine). After a while the engine finally started, the gas would eventually wash away the sticky for a while.
Another culprit may be that you left the carb full of gas over the winter. Options may be fuel stabilizer in the fall, or some form of carb cleaner in the gas now, if old gas is causing the carb float to stick. Float sticks in the UP position, closing off the fuel flow, until the carb sucks so much fuel out that the float weight and vibration causes it to un-stick and fall down and lets the bowl fill. Full bown lifts the float up. Rinse and repeat. Goal is to get rid of the sticky. Question is - why sticky?
Solved! Pulled the fuel filter, soaked it in starter fluid (ether), and it works like a champ.
Likely cause, overwinter storage with a full tank of 10% ethanol gas. I’ve never had this happen to a mower before, and I’ve read other posts suggesting this model is more picky than most mowers.
But I’ve definitely learned something today, thanks all! Nothing but 100% premium dead dinosaurs for this baby from now on.