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Hi I have a problem with no 1 cylinder not firing. Had a backfire and
bike has ran on three cylinders since. Have checked for spark and
its ok. However decided to change some things just in case. Changed
the ecu, pickup coil, spark plug and the coil and leads to the dead cylinder
still no joy! Have done a compression check 123all 95psi. Cylinder 4 102.5
engine cold. Decided to do a leakdown test. The gauge I bought registers set
at about 20-30 psi & on this setting all cylinders were in the green OK.However
ehen I increased the pressure through the tester to 50-60 psi cylinder 1 began to
let air escape through a small hole under the exhaust port. It also did this on cylinder 2
but not so bad the other cylinders 3&4 were fine on this pressure. I assume from this
that maybe the head gasket has gone or I have a bent or burnt valve and the head needs
to come off!! Does anyone know what the small holes below the exhaust ports are for, and
if my assumptions as to the problem aee corrrect? Any advise would be appreciated as I
dont want to take the head off unnecessarily!!
Kind regards!!

The holes under the exhausts are to drain oil that accumulates in the spark plug tunnels...nothing to do with the cylinders.

Cylinder pressure should be 99-155 psi. Try a bit of oil in the bore and see if the pressures rise. The last time I did mine they were 120psi give or take 2-5psi.

As for the leakdown test I do mine at 100psi with the crank locked at tdc on the compression stroke for the cylinder been tested, a loss of more than 15% needs investigating and 20% is serious especially if the readings are different across the four cylinders.

Hi Thanks for the reply!! I think ive sorted it now. It was a duff coil. I tried changing the coil with another spare one i had but that must have been faulty too. I only found the problem when I changed both the coils over which were on the bike ie: the good one and the bad one. The problem then
changed to another cylinder!! Ive decided to invest in some dyna coils and taylor leads so hopefully wont have this problem again. Thanks for your advice anyway much appreciated. And especially thanks to cargo he saved me an engine strip by explaining about the holes under the exhaust ports!!