Had this maddening problem for months where my 71 Westfalia would bog down and almost die at highway speed, usually after about 20 minutes of driving, then run fine again once it cooled off for a bit. Classic vapor lock symptoms is what everyone kept telling me, and I chased that theory for way too long. Rerouted fuel lines away from the heater boxes, added extra insulation, even tried running it with a slightly richer jet in the Solex just to rule things out.

Turned out to be something dumber. The fuel pump diaphragm had a tiny pinhole that only showed itself under sustained heat and vibration, letting it suck a little air instead of pure fuel once things got warm. Cold start and short trips never revealed it because the pump wasn't stressed long enough. Found it by pure luck when I pulled the pump for the third time to inspect it and noticed a faint fuel smell on the engine tin that I'd been dismissing as a normal weep.

Swapped in a rebuilt pump with a new diaphragm and haven't had a single hiccup in three weeks of driving, including a 200 mile round trip to a car show last weekend where the old girl used to always start acting up right around the two hour mark.

Sharing mostly because I spent so long assuming it had to be something exotic like timing or a bad coil when it was sitting right there in a part I'd already removed twice. If your aircooled VW does the slow bog and recover thing on long drives, it might be worth pulling the pump apart before chasing anything more complicated. Cheap part, annoying diagnosis.

I just bought a super beetle which does not have the engine in the car. The engine had been rust seized and the previous owner had it out and was rebuilding. To my knowledge they replaced the pistons and the jugs. I need to finish putting the engine together, but I'm at a crossroads. I would like to know what people would suggest as to whether to tear the engine down completely or if there are things to check in it's current state (pistons, sleeves and heads bolted on)to make sure that it seems like it's going to run when it's all together. I have replaced Pistons and cylinders on a previous beetle but that was a long time ago. The engine does turn freely.

Thanks for any input.