Aloha, I have a MHR Replica, 21mm carb, Yasuni R on my Minarelli engine and just cleaned up the ports. For those who have ported and know this cylinder, can you advise me?
Here is what I did and the results:
1: I matched the exhaust outlet to the Yas pipe
2: The case outlet openings were 2.5mm wider than the inlets to the cylinder, so I matched them as well as the base gasket.
3: All the ports (exhaust and transfer) had a 1mm "lip" (on the top or head side) as they went into the combustion chamber. (something you can catch with your fingernail). I removed 1.5 mm from the exhaust and smoothed out the whole port, while not changing the shape. I did flaten out the port a bit on top and made it about 1mm wider on each side. Since I flatened the exhaust port, I also put a very slight chamfer so I would not snag a ring.
4: I also removed the lip from the transfers, thereby raising them by 1mm. And I checked their heigth to be sure they opened at the same time, so the right ones I think I raised 1.5 mm and the left only 1mm. The rear I left alone.
I did not change the direction of the front transfers that cause the gases to hook to the back of the cylinder instead of toward the exhaust.
5: I cleaned up the transfer ports by the base by smoothing and knife edging the divider in the port.
The results?
Before: Cylinder head temp (yes it is water cooled engine) was running 325 F at 1/2 to 3/4 throttle cruising speed. 90kph
Before: bottom end and midrange seemed more snappy
After: Cylinder head temp now runs at 215 F. WHY? Maybe since I removed the lip on the tranfers, the direction of gases now seem a little more up toward the head-spark plug and less toward the piston, thus more cool head. ?????
After: Feels like it is running lumpy and a bit rich at low speed and mid. EGT needle barely moves.
After: Top end now has no quams hitting-overrevving down a slight incline at 120kph and 12500 rpm. Runs 9500 and 85kph easily 3/4 throttle up the hill.
My conclusions: Drop about 1 gram off 3 weights, lean out the pilot a bit. maybe drop a point or two off the main jet, OR drop the needle one clip.
What do you all think. I would especially like to hear from those of you that have ported the MHR replica.
thanks Frank
Here is what I did and the results:
1: I matched the exhaust outlet to the Yas pipe
2: The case outlet openings were 2.5mm wider than the inlets to the cylinder, so I matched them as well as the base gasket.
3: All the ports (exhaust and transfer) had a 1mm "lip" (on the top or head side) as they went into the combustion chamber. (something you can catch with your fingernail). I removed 1.5 mm from the exhaust and smoothed out the whole port, while not changing the shape. I did flaten out the port a bit on top and made it about 1mm wider on each side. Since I flatened the exhaust port, I also put a very slight chamfer so I would not snag a ring.
4: I also removed the lip from the transfers, thereby raising them by 1mm. And I checked their heigth to be sure they opened at the same time, so the right ones I think I raised 1.5 mm and the left only 1mm. The rear I left alone.
I did not change the direction of the front transfers that cause the gases to hook to the back of the cylinder instead of toward the exhaust.
5: I cleaned up the transfer ports by the base by smoothing and knife edging the divider in the port.
The results?
Before: Cylinder head temp (yes it is water cooled engine) was running 325 F at 1/2 to 3/4 throttle cruising speed. 90kph
Before: bottom end and midrange seemed more snappy
After: Cylinder head temp now runs at 215 F. WHY? Maybe since I removed the lip on the tranfers, the direction of gases now seem a little more up toward the head-spark plug and less toward the piston, thus more cool head. ?????
After: Feels like it is running lumpy and a bit rich at low speed and mid. EGT needle barely moves.
After: Top end now has no quams hitting-overrevving down a slight incline at 120kph and 12500 rpm. Runs 9500 and 85kph easily 3/4 throttle up the hill.
My conclusions: Drop about 1 gram off 3 weights, lean out the pilot a bit. maybe drop a point or two off the main jet, OR drop the needle one clip.
What do you all think. I would especially like to hear from those of you that have ported the MHR replica.
thanks Frank
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