I have been very lucky!

[Jono]

Following on from this thread ...

http://www.rtoc.org/boards/showthread.php?t=6078

it developed a miss fireon the second to last run at 3 sisters sprint event on Sunday. One compression test later and number four has about 10psi.....doh .

A few brews later and the head is on the floor and im looking at an exhaust valve with a hole right through the middle. The seat was perfectly intact , though with stress cracks running through the valve head. I then managed to then drop the removed valve and a big piece broke off! I guess it wouldnt have lasted too much longer in the head and I was lucky it didnt break up on the way home and do untold damage. Does anyone want to tell me how I managed to burn a hole through the centre of the valve?

[dave r5]

at a guess, very hot egt's

do u have an egt guage
if so wat was it reading?

[Jono]

I have only been running 15 psi. I have an AFR and it has been running rather rich.

Im just suprised it has blown/burnt a hole through the valve rather than eat away at the valve seat.

[dave r5]

could it be running that rich that its still burning on the exhaust stroke, thus having an oxcysetalin affect to the valve

[paul b]

lucky boy indeed......:agree:

[Andrew Cooke]

has the head been modified?

[Jono]

Yeah a little .

ports cleaned up and matched to the manifolds.

[Andrew Cooke]

Yeah a little .

ports cleaned up and matched to the manifolds.

what about the exhaust guides and seats?

for whatever reason it looks like you've got that valve too hot.

[Jono]

no they have not been touched.

Im wondering if I had the clearences a bit tight. I reset them in a hurry the night before the event as they had got a bit noisy.

[Andrew Cooke]

possibly. You might want to replace all the exhaust valves just in case. Check that your guides aren't worn.

I did read that you're more likely to burn valves if you're using too much oil. No idea how true that is, but might be worth keeping in the back of your mind.