I heard a similar thing from “normally reliable sources” too. As they describe it, there’s a valve in the fuel line that opens on priming. The pump delkivers initially a sligthly higher pressure to open the valve and then falls back to “normal” prime pressure. This voltage drop is handled by a switch. The initial spike in fuelflow as seen on the gauge could be explained by this. The suggestion to improve cold start fuel delivery (though improve suggest that there is something to start with… excuse me the pun) was/is to ‘bypass’ the switch and let the primepump deliver the higher pressure for fuelfumes to reach the cylinders.
Bypassing (or disabling) the switch obviously isn’t in the officially recommended cures and most likely would void the warranty on the engine and perhaps everything else on the plane…). BUT, if there is some consistency in what we and our maintenance folks see, there may be a published cure in a not too distant future…
FWIW, it’s warming up in Europe…
Learned something this morning when I brought my bird up to East Coast Aviation for maintainence. It seems most Cont. IO-360’s have a fuel diverter valve (before the spider) that aids in starting. If I understand it correctly, during priming this valve diverts some of the fuel to lines plumbed directly to the intake pipes which pours fuel directly down into the cylinders. When not priming the fuel simply passes through the diverter on its way to the spider assembly for normal distribution to the cylinders.
It seems the 360-ES we fly doesn’t have such a valve (according to East Coast) which requires a seemingly ludicrous amount of priming for cold starts.
I wonder if the IO-550-N has such a valve; I thought I read somewhere that Paul Traina’s POH calls for 60+ seconds of prime for cold starts. (I suppose I should’ve verified with CD first before posting.)