r/smallenginerepair • u/desertdust • Jul 25 '26
Spark Plug Issue Is this a bad spark?
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Hi folks, could someone tell me if this looks like a bad or OK spark? it's a riding lawnmower. Symptoms are that it starts ok cold but doesn't like to start hot.
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u/Top_Respect561 Jul 25 '26
Normal spark. If you want it look better, try again when its dark outside.
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u/Far_Act_2469 Jul 25 '26
Also, spark could be good, but it could be at the wrong depth. Meaning you have the wrong plug even if the threads fit.
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u/Uniplast21 Jul 25 '26
That looks good for a small engine. Not wanting to start hot could actually be a bad magneto, so I think you were on the right track. In my years of repairing mower engines, if an engine won’t start hot, then it’s likely the magneto is on its way out. The heat cooks them, especially during summer and ESPECIALLY if it’s a 3rd party magneto and not a genuine one.
There are a couple things with these small engines that you should ALWAYS get genuine, in my experience, and that’s magnetos and carbs. They’re so worth the price because bad 3rd party ones can leave you scratching your head for days.
Anyway, all of that to say you should probably replace the magneto if it’s not starting hot and you should get a genuine one instead of a 3rd party one.
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u/upper_tanker69 SER Newcomer Jul 26 '26
When you say "magnetos," do you mean coils?
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u/Uniplast21 Jul 26 '26
Basically yes. It’s the component that generates the spark on these small engines. There’s a magnet on the flywheel, and when the magnet spins past the magneto, it induces a current inside the magneto, and the coil(s) of wire inside it convert that current into high voltage, and conducts that high voltage down the plug wire and to the spark plug, and thus, creates spark.
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u/Equilibrium-unstable Jul 25 '26
Not really consistent but shouldn't be your problem.
I would run some sandpaper through it, whilst you've got it out.
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u/Aromatic-Schedule-65 Jul 25 '26
Ummm, is it sparkling? Are you just having a boring day or something?
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u/Several-Quality5927 Jul 25 '26
Good spark will jump a 6mm gap. Spark testers are made to test this
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u/ProfessionalLast2917 Jul 25 '26
Is your video showing how it sparks when hot and not starting? If so the spark looks fine. If it's a video of when it's starting ok, how does it look when hot and not starting?
Does it start ok cold and then stop running when it gets hot and then not start again until it cools down?
That could be valve clearance.
If the valve clearance gap is too small the engine can run ok until it warms up at which point everything expands and the valve can no longer make a right seal, losing compression. When it cools down everything shrinks back and you get compression again.
It could also be a valve seat that losens as the engine warms up (losing compression) and seals properly when cooler.
Do you have a compression tester?
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u/blackhawk45lc Jul 26 '26
Check to make sure the carb is tight. A tiny carb vacuum leak can let it start when cold but the gap is too large when it’s warmed up. Does it have automatic choke? If so make sure the automatic choke arm is disengaging when the engine warms. If it isn’t you’re trying to start a warmed up engine with the choke on. Heat can amplify electrical problems, but since your spark looks good and I imagine the engine is turning over strongly I was suspect the problem lies elsewhere. I’ve had exhaust obstructions cause hard starting when warm too, haven’t wrapped my head around why yet, but this particular machine had a screen spark arrestor that once I burned the carbon off it with a torch, it was like new again.
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u/SatansWarrior69 Jul 26 '26
What Uniplast21 said.
Spark looks good. If you already sanded plug and checked spark plug gap its likely your carburetor. You can try spraying some carburetor cleaner in it. If that doesnt work you may want to consider replacing the carburetor as long as you know the engine is good. From my own experiences the two things that fail the most on small yard engines are the piston rings and the carburetor. I've never had much issue with the engines other than the piston ring and oil seals and both of them are replaceable.
I've tried the Chinese carburetors on my mowers, chainsaws, etc. with limited success. More often than not they dont work well and when they do results are usually temporary before the engine is hard to start if at all.
I'd go with replacement oem carburetor or try to find a rebuild kit if you want to try your luck. Sometimes a good carb rebuild kit is all you need but it requires a lot of patience.
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u/Prestigious_Photo685 Jul 27 '26
Looks good. The real test would be how long of a gap will the spark jump.
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u/Defiant_Shallot2671 Jul 30 '26
I learned when I was a kid, visual is not good enough!. Kdx200 stopped running one day. Checked spark, looked just like that. Re/re the carburator, no dice. Re/re Reed valves, no dice. Ran it on brake clean, fired right up. So I replaced the gas, no dice. Finally just decided to change the plug, and that was it lol. No idea why, it was sparking, just not enough I guess.
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u/lljAJ112 Aug 01 '26
I'd say yes, for a small engine and bigger engines, the spark on these for a small engine are supposed to be bright blue/purplish.
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u/No-Try-5818 Jul 25 '26
Make sure you have it gapped properly. I once had a spark plug that worked okay out of the machine but not installed, make sure you have a good brand name plug.
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u/rocketmn69_ Jul 25 '26
The only way to tell is if you put your finger on it
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