MadMat

Motor is a 330hp carbed engine. Thunderbolt IV ignition.
Compressions all read in the 140+ range. No.1 cylinder won't fire. Have changed plugs, ignition module, distributor, leads & carb. No.1 still won't fire. Checked lead order (it's only no.1 missing). Cam lobes also look good.
Have pulled inlet manifold off in case I left a rag in - nothing.
rocker covers off, valves appear to go up and down as expected.
Anyone have any clues where else too look - the fact that it has good compression, and I can even see the spark firing inside the cylinder using a "colortune", but it's just not igniting the mixture - I'm very confused.

stageharbor

I just had the same problem on a customers boat. It ended up being a defective sensor wheel (rotor) it was brand new from mercury. I changed it out and problem went away. I still cant explain why. It looked fine. good luck

MadMat

Thanks - thought about that , so swapped distributors, no change.

GOODT

verify spark using a spark tester or similar method, then try and use a timing light on # 1 to verify spark timing

I dont know what color tune is, I m guessing an oscilloscope. but go to basics

if you have air, fuel, spark, and compression and all is in time it pretty much has to fire

MadMat

Colortune is a glass spark plug - so I can see the spark inside the cylinder, and also see the flame colour when it burns.
.
So I know the cylinder has a spark in the combustion chamber, and have only just checked the timing

GOODT

ok spark tester


was timing 8 deg

MadMat

Yeh, on the nose.

GOODT

has 140 psi compression, how about a leak down test.......

MadMat

Slight problem - I don't have a leakdown tester - yet.

I think there is some evidence pointing to an air leak on the intake gasket to no.1, so I'm gonna get a straight edge on it and see how it looks.

GOODT

while running the mtr spray carb cleaner where you think the intake is leaking and listen for a change