Elapsed ticks is the worst way to do that. Furnace now can smelt items.

patches/minecraft/net/minecraft/tileentity/TileEntityFurnace.java.patch

@@ -88,7 +88,7 @@
          return 64;
      }
 
-@@ -166,12 +199,31 @@
+@@ -166,9 +199,12 @@
 
      public void func_73660_a()
      {
@@ -97,32 +97,12 @@
          boolean flag1 = false;
 
 +        // CraftBukkit start - Use wall time instead of ticks for cooking
-+        int elapsedTicks = MinecraftServer.currentTick - this.lastTick;
 +        this.lastTick = MinecraftServer.currentTick;
-+
-+        // CraftBukkit - moved from below - edited for wall time
-+        if (this.func_145950_i() && this.func_145948_k()) {
-+            this.field_174906_k += elapsedTicks;
-+            if (this.field_174906_k >= this.field_174905_l) {
-+                this.field_174906_k -= this.field_174905_l; // Paper
-+                this.field_174905_l = this.func_174904_a((ItemStack) this.field_145957_n.get(0));
-+                this.func_145949_j();
-+                flag1 = true;
-+            }
-+        } else {
-+            this.field_174906_k = 0;
-+        }
-+        // CraftBukkit end
 +
          if (this.func_145950_i())
          {
--            --this.field_145956_a;
-+            // --this.furnaceBurnTime;
-+            this.field_145956_a -= elapsedTicks; // CraftBukkit - use elapsedTicks in place of constant
-         }
-
-         if (!this.field_145850_b.field_72995_K)
-@@ -180,13 +232,21 @@
+             --this.field_145956_a;
+@@ -180,13 +216,21 @@
 
              if (this.func_145950_i() || !itemstack.func_190926_b() && !((ItemStack)this.field_145957_n.get(0)).func_190926_b())
              {
@@ -150,56 +130,32 @@
                          flag1 = true;
 
                          if (!itemstack.func_190926_b())
-@@ -196,29 +256,31 @@
-
-                             if (itemstack.func_190926_b())
-                             {
--                                Item item1 = item.func_77668_q();
--                                this.field_145957_n.set(1, item1 == null ? ItemStack.field_190927_a : new ItemStack(item1));
-+                                ItemStack item1 = item.getContainerItem(itemstack);
-+                                this.field_145957_n.set(1, item1);
-                             }
+@@ -202,7 +246,7 @@
                          }
                      }
                  }
-
--                if (this.func_145950_i() && this.func_145948_k())
-+                /* CraftBukkit start - Moved up
-+                if (this.isBurning() && this.canSmelt())
+-
++                
+                 if (this.func_145950_i() && this.func_145948_k())
                  {
--                    ++this.field_174906_k;
-+                    ++this.cookTime;
-
--                    if (this.field_174906_k == this.field_174905_l)
-+                    if (this.cookTime == this.totalCookTime)
-                     {
--                        this.field_174906_k = 0;
--                        this.field_174905_l = this.func_174904_a(this.field_145957_n.get(0));
--                        this.func_145949_j();
-+                        this.cookTime = 0;
-+                        this.totalCookTime = this.getCookTime(this.furnaceItemStacks.get(0));
-+                        this.smeltItem();
-                         flag1 = true;
-                     }
-                 }
-                 else
+                     ++this.field_174906_k;
+@@ -219,6 +263,7 @@
                  {
--                    this.field_174906_k = 0;
-+                    this.cookTime = 0;
+                     this.field_174906_k = 0;
                  }
-+                */
++                
              }
              else if (!this.func_145950_i() && this.field_174906_k > 0)
              {
-@@ -229,6 +291,7 @@
+@@ -229,6 +274,7 @@
              {
                  flag1 = true;
                  BlockFurnace.func_176446_a(this.func_145950_i(), this.field_145850_b, this.field_174879_c);
 +                this.func_145836_u(); // CraftBukkit - Invalidate tile entity's cached block type
              }
          }
 
-@@ -269,13 +332,13 @@
+@@ -269,13 +315,13 @@
                  {
                      return false;
                  }
@@ -215,7 +171,7 @@
                  }
              }
          }
-@@ -289,14 +352,39 @@
+@@ -289,14 +335,39 @@
              ItemStack itemstack1 = FurnaceRecipes.func_77602_a().func_151395_a(itemstack);
              ItemStack itemstack2 = this.field_145957_n.get(2);
 
@@ -259,7 +215,7 @@
 
              if (itemstack.func_77973_b() == Item.func_150898_a(Blocks.field_150360_v) && itemstack.func_77960_j() == 1 && !((ItemStack)this.field_145957_n.get(1)).func_190926_b() && ((ItemStack)this.field_145957_n.get(1)).func_77973_b() == Items.field_151133_ar)
              {
-@@ -315,6 +403,8 @@
+@@ -315,6 +386,8 @@
          }
          else
          {
@@ -268,7 +224,7 @@
              Item item = p_145952_0_.func_77973_b();
 
              if (item == Item.func_150898_a(Blocks.field_150376_bx))
-@@ -530,4 +620,23 @@
+@@ -530,4 +603,23 @@
      {
          this.field_145957_n.clear();
      }