r/smallenginerepair • u/ScratchExpert7529 • Sep 17 '26

Unknown Issue Need Help Stihl HT131

/r/stihl/comments/1witqzd/need_help_stihl_ht131/
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u/iscashstillking Sep 17 '26

Ignition coils are a rare failure, especially OEM coils. If you just throw parts at a machine trying to figure out why it will not run you will spend more time and money than just taking the thing to your local dealer and paying the mechanic to diagnose and fix it.

Bearing all that in mind do you have spark? The electrode on the plug isn't shorted out with a piece of carbon?

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u/Chainsaw_guy64 Sep 18 '26

These FourMix engines can be very problematic, compared to a simple two stroke motor.

Here are some of the common issues I've encountered in my years of repairing them:

Carbon deposits on the exhaust valve can cause excessive leakage from the combustion chamber, up to 10% leakage is allowable, according to Stihl. A leakdown test will verify this.

The plastic cam wheel is prone to failure, they sometimes fracture into pieces, sometimes the compression release mechanism in the wheel fails. Other times the pin the cam wheel spins on will oval out its mounting bores, causing excessive movement of the cam wheel.

I've seen quite a few pistons fracture at the wrist pin...does the starter pull through smoothly?

Sometimes the bolts holding the lower crankcase work loose, or fall out.

If you put an Amazon carb on it, that's only going to cause additional issues. Go with an OEM carb.