thanks guys,
ive re looked at the back plate and found that two of the three tapped casing holes that the back plate is screwed into are stripped and one of the back plate mountings is cracked and is a bit shaky when the engine is running, this has now been repaired and seems to have solved the issue.... fingers crossed .....
with regards the unleaded fuel, i know you can buy lead balls which can be added to unleaded fuel,( drop them into your fuel tank) to allow you to run old cars and bikes with out a head conversion. 
JD
Good sleuthing!
The lead issue: these heads do not rotate the valves like pushrod engines or DOHC engines usually do. Because of that, we don't get valve recession into the seats with unleaded fuels. One of the accidental advantages of the SOHC4 design... But, the guides on the post-1975 bikes can wear quickly with unleaded fuels, because they were switched (for the most part) to cast iron in 1975 so save a few $ at Honda.
Today, we can handily solve this problem several ways:
1. Add a bit of top oil (like Marvel Mystery Oil or Lucas Top Oil) when fueling up. I've done this since unleaded took over, and at last check (126,000 miles) my 750 still had less than .0020" average valve clearance (mine has stellite guides, though, from 1972). They are original valves and guides, never been turned.
2. If you do a valve guide replacement to ensure REAL long life, the 550 has just one option that I know of: you can install bronze sleeves in the guides, like on old Fords and Chevies, beginning just recently. AFAIK, there are no bronze guides available for the 550, but someone could prove me out-of-touch in this regard? The bronze sleeves will deliver less friction, self-lubrication in the absence of lead, and will hold their tolerance much longer than cast iron guides.
I have just started offering this option to 750 heads that come this way: the machine shop thinks we can also do it with 550 heads. I hope to find out for sure, soon. It's quite a bit le$$ expense than replacing all the guides.
