Rainy Day Auto said

If you decide to rebuild the slant, it might be a good idea to diagnose it properly.

If it runs good then dies out after 35 minutes, then a rebuild may or may not solve your problem. It sounds like another factor is the problem. If the engine needed a rebuild, it more likely would never run decent at all.

Does it smoke, have you done a leak down test??

Can you set the timing accurately??

A rebuild or engine replacement sounds like it might be a shot gun diagnosis

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