Backfiring and Sputtering when hot

[tjokl]

My '74 911 Targa has begun having trouble when the engine is fully heated. It is backfiring and sputtering. Particularly at 3,000 RPM range. The problem gets progressively worse until it stalls out.

Will restart after it cools for an hour or so and can get it home. Let it sit for a day or so and it seems to work ok until it gets hot.

I thought oil level may be too high so I pumped out some oil, but problem persists. Thoughts?

I've heard there are vents for gas to exhaust from heated oil and these may be clogged. Has anyone heard of this? If so, where would the vents for this be?

Other ideas? Thanks

[martidj]

How well does your Targa run at idle? Cold or hot. Does the idle speed change when you remove the oil fill cap? Is your backfire coming from your intake or exhaust? Vacuum leaks are the first thing to look for.

[Targalid]

This sounds like the engine has a correct mixture on start up which does not lean out as it warms up thus giving an overly rich mixture when fully warm. If you have CIS, I would use a fuel pressure tester, observing the pressure profile as the engine warms. A low control pressure that does not change will indicate that the warm up regulator (WUR) is defective. A quick check with a multi-meter can reveal a defective WUR heater. If you have carbs, the same logic applies. You are running too rich.

[tjokl]

I'll do some testing.