I referred you problem to one of our tech reps, here is what he said:

I don't have a lot of experience with piston engines but it sounds entirely like  a fuel delivery problem. At this point I think I would be having the carberator gone through.

Best of luck

Thought of this right after I selected the "post" key, Have you checked the wiring and ignition switch for grounding. Try disconnecting the "P" leads, for Slick mags, or insert test pigtail leads Bendix, run the engine and see if you can duplicate the problem, as you probably know with the "P" leads disconnected the mags are "HOT" and the prop will bite if not given respect.

Hi Mike;

Just a thought. Because it seems to only happen after the engine has run for a while and gets hot, have you checked the intake manifold for security?  If you are getting any warping of the intake tubes or manifold, it could be leaking air and you would not get enough fuel into the cylinders even with adequate fuel flow to the carburetor.

Another thought is with the valves, which can be a problem on the O-300 engine (mostly due to the higher lead content of 100LL).  If you have enough lead buildup on the valve stems, they may be paritally seizing when the engine gets hot. When the engine cools, they are free to move and the engine will start and run normally again. The valve lifters could also be sticking in their bores, and causing the valves to not close completely when the engine gets hot.

Some people have had success using Marvel Mystery Oil in the fuel to free up sticking valves. Otherwise I would think you would need to pull the cylinders and remove the valves and lifters to check the valve stems, guides, and bores for lead buildup and clearance.

If you have fuel/air mixture, a timed spark, and compression, you should get combustion. A leaking manifold would affect the first, and the valve problem the last. The magneto checks above should cover the timed spark. Hope this helps.

Jon

I had a very similar experience with my 172H.  The engine would run fine in flight, and then occassionally would just stop during the landing roll and couldn't be restarted. 

I am on a farm strip and would have to tow the aircraft back to the hangar. I did all the tests you did and got all the same results. Good fuel pressure, spark, timing correct etc. 

Like you, I also wondered about heat soak in the main fuel hose. 

I the end I put on a new Slick ignition kit from Aircraft Spruce and have not had a problem since.  I can only conclude that the coils in the original Slick 664 mags, which had been on the aircraft since 1972 were breaking down when hot and at idle revs.

The new mags have made a world of difference to how easily she starts too.

Well, gang, here is an update. Since the last power-out landing we:

1. Replaced the carb with a factory-rebuilt unit.

2. Found and fixed loose valve guides in two cylinders.

3. Installed a new gascolator (Steve's Aircraft).

4. Replaced the flex hose from the gascolator to the carb.

5. Installed a carb air temp gauge.

6. Repaired carb air box.

7. Mags tested by IA and new points installed.

I'm going from memory, but I think that's it.

We taxi tested on Monday for about an hour and ran it up several times.  We also did a few high-speed taxi runs. No problems.

Today we did some more run-ups and taxi tests, no problem.  So, we took off and climbed to about 3300 feet MSL and stayed over the airport.  The engine ran fine--for about half an hour, then a few occasional misses (nothing violent), followed by a rapid loss of power. Carb air temp was well above the yellow arc.  Landed uneventfully and rolled to the taxiway, where the engine died and would not restart.

We checked the new gascolator, plenty of fuel at its drain.  Turning over the prop by hand felt OK, suggesting normal compressions.  Tried primer and cranked-it showed no sign of starting.

The only change this time is that it would not restart after sitting for about half an hour, unlike the previous episodes. Hoping whatever is acting up has failed altogether, so we can find the problem!

We need to revisit you folks' previous suggestions in this thread--just wanted to provide an update.  We're going to uncowl the plane tomorrow evening and investigate.  Stay tuned, I'll keep y'all posted! And if anyone has any additional ideas, please chime in!

First, my apologies for taking so long to give you folks an update.  The problem turned out to be...you guessed it--the mags, as suggested by many. I was under the impression that the mags had been bench-tested while hot, but that turned out not to be the case.  When they were tested hot, each of the coils failed within about 10 degrees of each other.  We installed a new Slick system and that took care of the problem.  So, while it is *unlikely* that both mags can fail in the same way at the same time, it is not *impossible*  !!  Thanks to all for the advice.