In TCM engines, the cause of the hot start problem is the heat-soaked engine driven fuel pump.  Get it cooled off (by running the boost pump in ICO for at least 60 seconds) and you no longer have a hot start.  It's a normal cold start.  In Lycomings this does not work.  You're on your own!

Yes, there is a fundamental difference in the two fuel systems and any "hot start" discussion has to be specific to the type of system.  However, there is a common thread - the fuel between the fuel control and the injectors remains in the system after the engine is shut off and when hot that fuel can vaporize, discharging fuel into the engine and causing the flooded condition during the restart.  So there is too much fuel in the engine and not enough fuel in the distributor portion.  The distributor in both systems is supposed to shut off cleanly and be able to hold against a little back pressure (on the Lycoming system it is about 2 psi).  If the line between the fuel control and distributor is well insulated it might not develop enough pressure during the cool-down to vaporize.  I found on most Lycoming systems it is very important to insulate that line and for that reason I use the ugly. loose-fitting fire sleeve instead of the nice-looking integral sleeve in this location.  This line is very short in the IO-550, so maybe insulation doesn't matter.

It was also posted by someone that their fuel control doesn't completely shut off the fuel when the mixture is pulled.  All the certified systems (Bendix/Precision Airmotive) have a mixture control that shuts off the fuel, but it is not unheard of that this valve leaks, causing an uncertain shutdown.  I suggest you check the fuel control - I can't imagine someone would build a system that is incapable of shutting the fuel off.  And it is certainly true that shutting the selector off will result in a difficult restart, since the fuel pump will pull a vacuum, resulting in a lot of vapor being created that has to be purged through the system.  It is certainly true that adding the aftermarket purge valve at the distributor will shut the engine off, but that is one reason I didn't add it to mine - I don't need one more way for the engine to stop running - I've got enough already.

Gary Casey