I realize everyone has an opinion on how to go about this, but what to do is NOT the question! You need some background info before I ask though.....
Two or three years ago, I read a post on this forum from a guy who did port work for Boyesen (can't remember his name). He stated that the easiest way to de-tune one of these mini's was to put a washer between the cylinder and exhaust pipe. The amount of de-tuning would be determined by the size of the hole in the washer. At the time I thought this was way too easy. My goal then was to convert my son's CR80 into something his younger (middle) sister could ride. We ending up selling the bike and buying an XR100, so no more problem. Just last year she decided to try the racing thing and we went back to the 2 stroke. She had no fear and it was never an issue. She stretches the throttle cable...
Now to the present. That sister (middle daughter) has moved up to a big wheel and My youngest daughter is now on that hand me down YZ80 (It already had a 2mm overbore when we bought it, so it's 89cc). She cannot, nor does she have the desire to ride it at the level it's capable of. She shifts way before the power comes on because she can't deal with the "hit". I thought to myself, here we go again. Problem was, she didn't want an XR-TTR, she wanted the "real" thing. With Eric not doing much these past few months, I was in a dilemma about what to do. Then I remembered the post about the washer and thought - "It can't hurt to try".
I found a washer very close to the correct diameter and ground it down until it would slide easily into the exhaust flange. It had about a 3/8" hole in the middle which I opened up to 1/2". I also drilled four 1/4" holes around the outside of the center hole. I put the pipe back on figuring there was no way the thing would run. I was wrong. It cranked first kick. And it was very quiet. I rode it down a dirt road and from 0 to 3/8 throttle, it felt just as before - maybe slightly more responsive. Above 3/8 it was very rich. I dropped the needle to the last position and that cleared up a lot. I've bought a leaner main (270 -- stock is 300), but have not installed it yet. The transformation was pretty amazing. When you twist the throttle and the build of power is very linear. There is NO hit whatsoever. Yes, top end power is way down - probably 30%. It'll still outrun any of the 125cc 4 strokes, but not with the authority it used to have. Believe it or not it feels like the old Enduro bikes.
Now to my question.....reliability. Having decreased the size of the exhaust opening so much, what detrimental effects could result? Will I get reed flutter because there is more air in the cylinder that the exhaust opening can get out? Will it lessen the amount of lubrication to the crank since it's not pulling as much fuel mixture through? Will it run hot? What am I overlooking? She's ridden it for about 2 hours with no problems, but I'd really like to know what the experts think before I turn her loose. I'd hate for a shade tree modification that I did cause her to get hurt from a component failure.
I am NOT recommending this mod in this thread. This is an experiment that I've tried, that up to this point seems to be working. Now I want to know what the guys who know what they are doing think. Rich -- Eric??
Two or three years ago, I read a post on this forum from a guy who did port work for Boyesen (can't remember his name). He stated that the easiest way to de-tune one of these mini's was to put a washer between the cylinder and exhaust pipe. The amount of de-tuning would be determined by the size of the hole in the washer. At the time I thought this was way too easy. My goal then was to convert my son's CR80 into something his younger (middle) sister could ride. We ending up selling the bike and buying an XR100, so no more problem. Just last year she decided to try the racing thing and we went back to the 2 stroke. She had no fear and it was never an issue. She stretches the throttle cable...
Now to the present. That sister (middle daughter) has moved up to a big wheel and My youngest daughter is now on that hand me down YZ80 (It already had a 2mm overbore when we bought it, so it's 89cc). She cannot, nor does she have the desire to ride it at the level it's capable of. She shifts way before the power comes on because she can't deal with the "hit". I thought to myself, here we go again. Problem was, she didn't want an XR-TTR, she wanted the "real" thing. With Eric not doing much these past few months, I was in a dilemma about what to do. Then I remembered the post about the washer and thought - "It can't hurt to try".
I found a washer very close to the correct diameter and ground it down until it would slide easily into the exhaust flange. It had about a 3/8" hole in the middle which I opened up to 1/2". I also drilled four 1/4" holes around the outside of the center hole. I put the pipe back on figuring there was no way the thing would run. I was wrong. It cranked first kick. And it was very quiet. I rode it down a dirt road and from 0 to 3/8 throttle, it felt just as before - maybe slightly more responsive. Above 3/8 it was very rich. I dropped the needle to the last position and that cleared up a lot. I've bought a leaner main (270 -- stock is 300), but have not installed it yet. The transformation was pretty amazing. When you twist the throttle and the build of power is very linear. There is NO hit whatsoever. Yes, top end power is way down - probably 30%. It'll still outrun any of the 125cc 4 strokes, but not with the authority it used to have. Believe it or not it feels like the old Enduro bikes.
Now to my question.....reliability. Having decreased the size of the exhaust opening so much, what detrimental effects could result? Will I get reed flutter because there is more air in the cylinder that the exhaust opening can get out? Will it lessen the amount of lubrication to the crank since it's not pulling as much fuel mixture through? Will it run hot? What am I overlooking? She's ridden it for about 2 hours with no problems, but I'd really like to know what the experts think before I turn her loose. I'd hate for a shade tree modification that I did cause her to get hurt from a component failure.
I am NOT recommending this mod in this thread. This is an experiment that I've tried, that up to this point seems to be working. Now I want to know what the guys who know what they are doing think. Rich -- Eric??
