diff --git a/src/devices/machine/6522via.cpp b/src/devices/machine/6522via.cpp
index 240de545092..ddbd4ecf965 100644
--- a/src/devices/machine/6522via.cpp
+++ b/src/devices/machine/6522via.cpp
@@ -776,13 +776,19 @@ u8 via6522_device::read(offs_t offset)
 			LOGSHIFT(" - ACR: %02x ", m_acr);
 			if (SI_O2_CONTROL(m_acr) || SO_O2_CONTROL(m_acr))
 			{
-				m_shift_timer->adjust(clocks_to_attotime(8) / 2); // 8 edges to start shifter from a read
-				LOGSHIFT(" - read SR starts O2 timer ");
+				if (m_shift_timer->expire().is_never())
+				{
+					m_shift_timer->adjust(clocks_to_attotime(7) / 2); // 8 edges to start shifter from a read -- use 7 for a mac128 issue to be fixed later
+					LOGSHIFT(" - read SR starts O2 timer ");
+				}
 			}
 			else if (SI_T2_CONTROL(m_acr) || SO_T2_CONTROL(m_acr))
 			{
-				m_shift_timer->adjust(clocks_to_attotime(m_t2ll + 2) / 2);
-				LOGSHIFT(" - read SR starts T2 timer ");
+				if (m_shift_timer->expire().is_never())
+				{
+					m_shift_timer->adjust(clocks_to_attotime(m_t2ll + 2) / 2);
+					LOGSHIFT(" - read SR starts T2 timer ");
+				}
 			}
 			else if (!SO_T2_RATE(m_acr))
 			{
@@ -972,13 +978,19 @@ void via6522_device::write(offs_t offset, u8 data)
 		LOGSHIFT(" - ACR is: %02x ", m_acr);
 		if (SO_O2_CONTROL(m_acr) || SI_O2_CONTROL(m_acr))
 		{
-			m_shift_timer->adjust(clocks_to_attotime(8) / 2); // 8 edges to start shifter from a write
-			LOGSHIFT(" - write SR starts O2 timer");
+			if (m_shift_timer->expire().is_never())
+			{
+				m_shift_timer->adjust(clocks_to_attotime(8) / 2); // 8 edges to start shifter from a write
+				LOGSHIFT(" - write SR starts O2 timer");
+			}
 		}
 		else if (SO_T2_RATE(m_acr) || SO_T2_CONTROL(m_acr) || SI_T2_CONTROL(m_acr))
 		{
-			m_shift_timer->adjust(clocks_to_attotime(m_t2ll + 2) / 2);
-			LOGSHIFT(" - write starts T2 timer");
+			if (m_shift_timer->expire().is_never())
+			{
+				m_shift_timer->adjust(clocks_to_attotime(m_t2ll + 2) / 2);
+				LOGSHIFT(" - write starts T2 timer");
+			}
 		}
 		else
 		{
diff --git a/src/mame/drivers/mac128.cpp b/src/mame/drivers/mac128.cpp
index 884f29500bb..43b184c2c81 100644
--- a/src/mame/drivers/mac128.cpp
+++ b/src/mame/drivers/mac128.cpp
@@ -698,7 +698,7 @@ void mac128_state::via_sync()
 	uint64_t vpa_cycle = cur_cycle+2;
 	uint64_t via_start_cycle = (vpa_cycle + 9) / 10;
 	uint64_t end_cycle = via_start_cycle * 10 + 4;
-	m_maincpu->adjust_icount(cur_cycle - end_cycle);
+	m_maincpu->adjust_icount(cur_cycle - end_cycle - 4); // 4 cycles already counted by the core
 }
 
 uint16_t mac128_state::mac_via_r(offs_t offset)