Fuel in cylinder -HELP

[dazwatson]

Hi folks, running a 2ltr air cooled CU. I have fitted a set of twin 40 Dellorto's which have been running well. I have been running a too big main jet simply because that's all I had at the time of tuning, I have now bought what I believe to be the correct size.

A couple of weeks ago on my return journey from Bugjam on the motorway the van started missing and gradually got worse and nearing home was struggling that I was nearly at a stand still. I have checked all the plugs and No.4 seems to be oiling up, I've checked the spark & also changed the lead and cleaned the plug, as soon as I pull off its ok for a small time but soon starts missing again, as soon as I remove the plug it's oiled up again.

Yesterday I did a compression test and thankfully all cylinders are running similar psi's of around 100 -115. What I did notice while turning the engine over was the No.4 that seems to be the issue, when I looked over the open plug hole was neat petrol spray and fumes being pushed out.

Any ideas of what to look at next??

Thanks

Daz

[dazwatson]

Bump

[bigherb]

What plugs are you running? If they are NGK ditch them anyway. If they are Bosch W7CC change them to W8CC they burn off the oil deposits on older engines better. If it is petrol in the cylinders could be a leaky carb float valve or to higher fuel pressure or oil could be a worn/cracked inlet valve guide.