I would like some help diagnosing the probably cause of this intake valve burn as seen in the picture.

head with burnt intake valve

167fmm engine. 229cc OHV push rod type.

Now here is the rub. this is engine originally 9:1 CR, I have decked the head a total of 35 mill to increase the compression to a calculated 10:1 CR.

Of course i am aware of the increased compression brings higher heat, but running 93 octane, and never experiencing a pre ignition engine knock, i did not/do not anticipate a condition that caused a burn while the intake valve was extended. And rather would expect a burn to occur more on the exhaust valve if due to the higher CR.

But note it was a slow failure as i noticed the valve reducing in diameter as it rotated and burnt away during operation, receding deeper and deeper into the valve seat causing the first manifestation adjustment the valve lash screw higher and higher until there was no more thread for adjustment.

i measured flatness of the head and the rocker assembly, they were fine so i knew it was not a warp or valve train problem. The final failure of course total loss of compression as the large pit obviously didn't allow seal.

My research indicates a burnt intake valve generally due to valve leak, where the hot exhaust gases might slip through and pit the valve.

So Id like any info that might be available.

I should note just prior to disassembly i noticed the valve stem seal had slipped up and off the stem, this perhaps merely indication of excessive pressure due to higher heat.

  • Can you verify the original condition of the valves (did you do the valve job yourself)? Was this to only cylinder with this damage? Was there any other damage? It looks like something may have also struck the valve.
    – CharlieRB
    Commented Apr 10, 2017 at 18:34
  • yes job done myself, with new parts. this is a single cylinder motorcycle engine, no debree found otherwise, it's a rear facing intake but doesn't mean something couldn't find its way in... I've been know to go for a ride without the filter on occasion.
    – axa
    Commented Apr 10, 2017 at 18:41
  • I'm pretty certain that the smaller valve that has burnt is the exhaust valve, not the larger intake valve. A few questions: did you alter ignition timing after increasing CR? And am I right in thinking that the camshaft hasn't been modified since?
    – Zaid
    Commented Apr 10, 2017 at 21:30
  • @Zaid dang you're entirely correct. and I'm not entire sure how I could have been thinking this.. don't do a load of head work but...
    – axa
    Commented Apr 10, 2017 at 21:36

2 Answers 2


As @Zaid points out the smaller valve is the exhaust, thus i was confused to the point of more confusion...

Truthfully i never even thought much about port size variance..

Im thankful for everyone's attention


In my opinion, the CR is higher than 10:1 and the valve burn is due to the higher compression ratio.

You have shaved too much from the cylinder head.

  • Note that the smaller valve is not related to intake .
    – user62737
    Commented Mar 10, 2021 at 14:38

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