I would think that the WT-433 should have worked out well. If it ran for 20 minutes good and then went all flaky. man that sure sounds like bad fuel, crud in the fuel or something like that. What fuel and oil are you using? I am running the high end synthetic oil from Amsoil, their Saber Pro oil. If you used a low grade mineral oil it could foul the plug out on you. Bad gasoline could do it too. Some of the Chinese cheapie spark plugs do not seem to last very long at all too.
Another thought is the CDI module is overheating. it works until it heats up. Also all of my CDI units so far are rated for 6.0v maximum voltage. A 5 cell Nicad or NMH pack exceeds that and can cause the ignition unit to fail.
I almost forgot that sometimes the little hall effect sensor can go bad to.it may be heat sensitive. I think they have some little test gizmos for it as well as a tester for the CDI too.
A SV 17cc engine I got used had a WT-694 carb on it. That has a 12.7mm venturi in it. When I ran the engine I could not lean it out at all. It stayed extra rich the entire time I ran the engine with it. But I had thought that the 17cc engine needed something around 9mm for a venturi size to work right. So I rummaged through my little collection of carbs and found a WT-520 carb (no choke, but it uses a external primer bulb). So I tried out the carb and the engine ran great with it. So since the engine ran so well then, I put it on a Escapade .60 plane and I have been flying it a lot like that now.
Now my RCV 26cc engine looks quite a lot like a SV 26cc engine too. It has a Chinese clone HiLo carb on it. Hobby king does sell the carbs too.
But you can do like I do and research Walbro carbs with this link here and find a carb with a venturi size of interest and options you want too.
http://wem.walbro.com/walbro/group2.asp?FamilyName=WT
You can see here where the 17cc engine uses a external tube to route the crankcase pulses to the carb's pump cover.
Here is how I took care of the external primer bulb for the engine too.
My RCG 26cc engine looks like a SV 26cc engine. But it uses a Chinese Hilo carb clone of a Walbro.
You can just see how they routed the external pump tube from the engine to the carb's pump cover in this pic here.
They crimped off the external primer bulb line on this carb.
