XIO-320 rpm surge

Before anyone blasts me I searched forum for info on this and didn't find anything. I have RV-4 with the XIO-320 and I occasionally experience rpm surges after stArt up during taxi and only at lower rpm. Never happens during flight. My first experiences were after a hot start. But it has happened on occasion after a flight during taxi back to hangar. I have had it happen done a normal run up and mag check then flown with no issue what so ever. Any advice.? The first time it happened we did the bottle test on the injectors all had simular fuel flow. Love to hear Any and all ideas. Thank you in advance.

One possibility is vapor lock. Take a look at the routing of the fuel lines FWF. A great deal of heat up there with little airflow in the situations you mention. Are the lines wrapped in firesleeve (insulator) and routed to avoid high heat sources (exhaust)?

How many hours on the engine?

PS - I searched and found nothing as well.

If by surge you mean that the engine slows a bit then speeds back up I think this is similar to something I experience in the past.

I experienced this same problem with a 320 that I put the Airflow Performance injection system on. After a lot of troubleshooting discovered that in the low RPM conditions, the fuel line from the servo to the flow divider would develop vapor bubbles that would collect in the line until the bubble got big enough that the flow moved it. That is when the surge or stumble would occur.

What was happening is that with a low power setting, the pressure drop across the metering of the servo reduced the pressure and the fuel was warm enough to vaporize slightly.

Set about insulating all lines firewall forward and using forced cooling on the fuel pump and so on which never fully solved the problem.

I determined that the flow divider was only holding about 1PSI in the inlet line. What worked best was having the flow divider pressure set up to 3PSI (I think). That kept enough pressure on the fuel that it didn't vaporize in the line. Problem solved.
Don at Airflow Performance can help you with this even if you have a different brand system.

Stuart, on a hot start I will turn the boost pump on after startup and leave it running. This will keep the fuel under pressure and prevent formation of bubbles. We have extremely hot temperatures in Redding, Ca. Vapor lock is a fact of life here. Learning to prevent it from causing trouble is key.