High speed stumble (complicated)

Well, another problem. This 99 of mine has been running nicely, starts first time nearly every time. But, today I had my muffler altered a bit to cut the noise down. When I started it up to give it a listen, I really opened up the throttle once (no load). When I did that, I noticed that it would not "scream" like it should... it went up to something like 3/4 revs and stumbles. I have not lately had room to open it up under load here in town, so honestly I dont know if it's the fault of the muffler mods or not, could very easily have been that way before. So, I guess my question is what else could be causing a high speed falter like that? I have no idea what my air screw (the bottom one) should be, can someone tell me the base carb setting? Does this sound like a vacum leak? Does it sound like a problem that would be caused by increased exhaust resistance? Any suggestions would be very welcome.


Mark Sheldon

Exactly what did you do to quieten it? It does sound restricted. Try a smaller main jet and/or maybe lower the needle (1 thing at a time). Can you try someone elses stock muffler? The fuel screw(not an air screw on a 4 stroke) at the bottom of the float is generally set at 1 1/2 turns out as a baseline. I would swap a muffler with a friend first, or just remove the restrictor and see if there is a difference.

Well, I'm embarrased to say what I let them do to it. I'm in south america and the cops were warning me about the noise so I needed to do something and had very limited options. I let a motorcycle mechanic put a collar in there, and a fin- it can all be taken out. I guess the first thing to do is to remove it all again and start from square one. I will buy a proper muffler insert asap, the ones from pro billet in Idaho look good and they are so cheap thier is no excuse not to use one.



Well at least if you remove it momentarily you can rule out 1 less thing to look for. AND it was the last thing you changed. Test it both ways. If it turns out to be the restrictor, lower your jet size and it will come back around. It will take some fiddling.

I brought the muffler back to original (empty) and it's still doing it. I'm comming back to the states here in a week or two. I'll start with a jetting kit and a good book on the subject, a billet insert, gold valves, new sprockets and chain... whoopie! I heard that yz125 springs soften things up for lighter riders?

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