#4 sparkplug fouled--valve seal prob?

[Taiju Kobayashi <kobao@xxxxxxxxxxxxxx>]

Hi Digesti,
I just wasted my precious track time & $$$(or \\\)  due to tech problem with
my racing GTV.
I have already gone through basic inspection shown below, but none of them
worked; now I suspect it must be failure in valve seats/guides.
Before removing the head off (the last thing I want to do), could any of you
advise me the solution, tips, or just insights to shoot what the trouble is?

<the trouble>
Very hesitated acceleration above 3000rpm
#4cyl sparkplug get fouled, while others are fair

<basic inspection I've done>
Idles fine, and rev up well while unloaded (standing still)
All the sparkplugs get equally fouled a bit if idling continued for a while
Carburettors are equally balanced when measured.
No spark leak in #4 wire
No spark plug failure
New distributor cap
Throttle plates are equally opened
No air leak in inlet manifolds
Carburettor venturis/chokes are securely fastened
Main jets/air correctors have been tried in some different combinations

<major engine modification>
Engine is recently overhauled with new pistons & head gasket
12:1 Shankle pistons/13mm Shankle cams/ RML ignition with optical triggered
distribution/ported head etc.

Thanks in advance,

Taiju Kobayashi
Tokyo, Japan
'66 GTV "Corsa"
'91 75 TS

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