Sunset, the most common and readily available true yellow flowering clematis belong to the tangutica group and they all have small flowers as you have undoubtably already discovered. Right off the top of my head, I can't think of any true yellow large flowering hybrids. There is one called Lemon Chiffon but it is more of a creamy white than yellow.

Your best bet to finding one would be to search on the COTW search engine linked below and put in yellow for the tepal color. You might find something with true yellow tepals listed there (the names with + by them have pictures) but my guess is if you do is that it is not something that is going to be readily available.

Good luck!

Here is a link that might be useful: COTW search

True yellow flowering clematis tend to be species clems and so offer rather small flowers - tangutica, orientalis, cirrhosa, thibetianus, ladakhiana and serratifolia all offer yellowish flowers, often in profusion, but small. The LFH's that are reputed to be "yellow" - 'Lemon Chiffon', 'Guernsey Cream', etc. - are really much more of a creamy white and not a true yellow.

While it can be a bit of a thug, Clematis tangutica offers a very long bloom season for me, producing flowers from June through September. While there's always some color showing on this vine during the summer, it's not a big show. What is most noticeable are the seed heads which are far more showy (IMO) than the flowers and really give you an indication of how heavily this vine blooms over a course of several months.

Didn't mean to step on toes, Miguel! We were apparently writing at virtually the same time.

No problem gardengal. You can never tell when someone else is posting at the same time! LOL