diff --git a/BizHawk.Emulation/Consoles/Nintendo/NES/Boards/SxROM.cs b/BizHawk.Emulation/Consoles/Nintendo/NES/Boards/SxROM.cs
index 56e5cbe160..c4ba2a08aa 100644
--- a/BizHawk.Emulation/Consoles/Nintendo/NES/Boards/SxROM.cs
+++ b/BizHawk.Emulation/Consoles/Nintendo/NES/Boards/SxROM.cs
@@ -207,19 +207,26 @@ namespace BizHawk.Emulation.Consoles.Nintendo
 		//configuration
 		protected int prg_mask, chr_mask;
 		protected int vram_mask;
+		const int pputimeout = 4; // i don't know if this is right, but anything lower will not boot Bill & Ted
 		bool disablemirror = false; // mapper 171: mmc1 without mirroring control
 
 		//state
 		public MMC1 mmc1;
-		/// <summary>cpu cyle when the last MMC1 serial write occured</summary>
-		int lastwritecycle;
+		/// <summary>number of cycles since last WritePRG()</summary>
+		uint ppuclock;
+
+		public override void ClockPPU()
+		{
+			if (ppuclock < pputimeout)
+				ppuclock++;
+		}
 
 		public override void WritePRG(int addr, byte value)
 		{
 			// mmc1 ignores subsequent writes that are very close together
-			if (NES.cpu.TotalExecutedCycles - lastwritecycle != 1)
+			if (ppuclock >= pputimeout)
 			{
-				lastwritecycle = NES.cpu.TotalExecutedCycles;
+				ppuclock = 0;
 				mmc1.Write(addr, value);
 				if (!disablemirror)
 					SetMirrorType(mmc1.mirror); //often redundant, but gets the job done
@@ -265,7 +272,7 @@ namespace BizHawk.Emulation.Consoles.Nintendo
 		{
 			base.SyncState(ser);
 			mmc1.SyncState(ser);
-			ser.Sync("lastwritecycle", ref lastwritecycle);
+			ser.Sync("ppuclock", ref ppuclock);
 		}
 	
 		public override bool Configure(NES.EDetectionOrigin origin)
@@ -359,6 +366,7 @@ namespace BizHawk.Emulation.Consoles.Nintendo
 			chr_mask = (Cart.chr_size / 8 * 2) - 1;
 			if (!disablemirror)
 				SetMirrorType(mmc1.mirror);
+			ppuclock = pputimeout;
 		}
 
 	} //class SxROM