Hard starting volvo

[.357magnum]

I have a 92 Volvo Penta 2.3L ohc with a 1 bbl solex (with no choke) that is very hard to start in the cold (especially in high altitude where I use it for fishing alot). The boat shop where I bought it said I am not pumping it enough, but I tried pumping it alot, pumping while cranking, cranking after pumping with throttle in various positions, not pumping at all ect. and nothing seems to work. I recently replaced the plugs, points, set timing. I think I am going to try to make a "choke" with a piece of innertube rubber to go over the flame arrestor. Has anyone found a solution to this problem? If so, any advice would be appreciated.