Greetings Surferwood,
I recently had a similar problem with my 1999 Generac Q-55G generator and it turned out to be a fuel delivery problem (I think). The previous owner had replaced a section of fuel line and did a pour job of splicing dissimilar sized rubber line. I think it might have been sucking air. While I had the fuel line apart I blew into it to make sure there was no obstruction in the line. I could have cleared a partial obstruction in the line also, but I'm not sure. Anyway it has run great since then.
While trouble shooting my generator I got pretty familiar with my unit and the Owner's Manual. My manual lists these five items in the troubleshooting section for the problem "Engine starts, then shuts down"¯.
1. Engine oil level low
2. Bad control relay (CR1)
3. Bad low oil pressure switch
4. Bad control relay (CR2)
5. Water in fuel
My generator has two switches designed to protect the unit. One is a Low Oil Pressure Switch and the other is a High Temperature Switch. You might want to verify that these are not causing your unit to shut down.
With regards to relays CR1 & CR2, I'm not sure where they're located. They are not shown on the electrical drawing in the Owner's Manual. Fortunately it appears not to have been my problem.
Here is a website you might find useful:
http://www.guardiangenerators.com/Pr...al.aspx?src=qp
They have a list of generator manuals which include some of the older models.
Good Luck! I hope this is helpful.
Warren