20hp Intek V dying when hot

Elkins45

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I bought a used mower primarily for the purpose of scavenging the transaxle and deck. The seller told me the engine had been rebuilt after throwing a rod. It starts and runs fine but after 20-25 minutes it started to miss more and more and eventually died. After sitting for 10 minutes or so it starts up and runs fine for another 10-15 minutes before doing the same thing. The starter turns freely so it’s not locking up. Removing the gas cap doesn’t fix it so it isn’t a venting issue. It seems completely reversible once the engine cools down.

My initial thought is that its an electrical issue, like maybe the coils are getting hot. The only thing that perplexes me is that it would be both coils at the same time. Could it be the stator? That doesn’t seem likely to me because I thought the stator is just there to charge the battery and not to provide spark.

Thoughts? The model number is 407777.
 

Elkins45

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probably not the stator, Might be a valve lash issue.
i would buy two spark testers with the bulb in them (https://www.lislecorp.com/specialty-tools/inline-spark-tester) cheaper an scAmazon.
and fit them both inline and run it, when it starts to die, see if it's sparking or not.
Do you know of a convenient source for valve clearance specs? That seems like a very easy thing to check, and I would need to buy any new gizmos to do it.

Your suggestion about testing for spark is a good one.
 

Elkins45

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Valve lash makes sense because the guy told me it had new heads. I sort of assumed he meant head gaskets because throwing a rod wouldn’t cause you to need new heads, but if the repair guy just bolted on a couple of heads from a scrapped engine it’s quite possible they are far out of spec.
 
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