I too own a 1950 Buick Super Riviera 2 dr hardtop. It has a 263 engine in it and I also have a spare 263 enigine and transmission. I have considered having the spare rebuilt but hae not done so as of yet. One thing you need to do is a compression check on all cylinders. When I first bought my 50 Super, it had sat outside under a carport for 14 years and the engine was stuck. I poured Marvel Mystery Oil in all the cylinders and the engine freed up with a little rotating back and forth with a breaker bar on the crankshaft end bolt. Once free, the engine started and ran fairly smooth until the smoke of the MM oil was cleared out. I drove the car and after awhile I notice a loss of power. I did a compression check on all cylinders and found one to be out of range on the low side and all the rest were within and close to each normal. I then took a small box wrench and loosened the nut on the threaded end of the valve adjustment on the rocker arm for that cylinder ( determining first if it was an exhaust or intake valve leak and then using a flat tipped screw driver I turned the rocker adjustment stem until I heard the engine smooth out. I then retightend the nut and did another compression check and BINGO! the cylinder came back in range and the engine ran fine with the necessary power. Remember though this is a car that is equiped with a DynaFlow or I like to sometimes call it a Dynaslush transmission, it is not quick off the line like the other cars of 1950 such as Chevys, Merc's and Fords. It was not designed to be quick off the start but rather to be a smooth shockless transmission that you don't sense the shifting like other cars. Once the car gets moving though the Dynaflow is a powerful and smooth operator!!. Check for engine compression loss and besure to determine if it is the intake or exhaust valve so you can adjust the correct one to bring compression back. if not sure adjust each one as the engine is running at idle and besure to count the turns and direction of turn incase you need to put it back where it was. If it does not return to proper compression after adjustments of the valve pressure then the rings may be stuck or worn, but the engine would most likely smoke or burn oil..so check the compression first before deciding to rebuild what may be an engine with lots of life left in it. I made those adjustments right after I bought the car in 1983 and it still starts and runs like a top ever since then. In fact once my engine is started and it starts everytime with a few cranks, I can then set a glass of water on the valve cover and it will not shake the glass. The Buick Straight 8 is a geat engine, the dynaflow makes it slow and shockless at first. If unsure have the car taken to a mechanic and ask him to do the compression test and adjustments. It is really simple to do. It may end up saving you a lot of money in the end.
I hope this was of some help.
Best Regards
Jack
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