Weishaupt Boiler very smokey?

My Weishaupt powered Ideal Standard boiler (WL15A burner type) was running very rich one night. I noticed the brown smoke against the night sky and investigated. I don't use this boiler much at all, having plumbed my central heating system into my wood stove. So the Ideal Standard is used just to get the rads warm and nice and then the fire takes over. Anyway, I swept the chimney and mainly stone dust came down and settled in the steel flue pipe. I cleaned this out and also got my hand in the rear of the flue chamber and noticed it was quite sooty. I would like to clean the burner chamber, but cannot get the WL15A off. There are two long bolts which would appear to hold it on, but it does not want to budge. I feel the main chamber is likely to be sotted up due to a poor flue output. Does anyone have any ideas. The age of the Wl15A appears to be 1994. When it burns smoke comes form the peephole shutter which rattles and lets the smoke out, also there are smoke marks around the burner which means it was not breathing properly. All I need to do is get it off and then clean it out, if it still smokes after this I will renew the jet. Many thanks