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Sans1 07-15-2010 10:41 PM

ATV Only Idles - What happened?
Specs 1999 Yamaha Scrambler 200.

Symptoms – Idles, but dies if you crack the throttle. Rapidly opening and closing the throttle, RPMs come up very slightly, but cannot sustain.

What happened: I was following a new rider. She had some trouble knowing when to shift, but seemed to be doing ok at a steady speed. I was following on dirt bike, Quad smelled and sounded fine.

Manually mixing 32:1
Rode about 12 miles
Climbing very slight hill (like 3% grade max) rider began slowing, saw her downshifting, but quad stalled. Just assumed she was in 3rd or 4th going too slow. When she couldn’t restart, I tried. Would only start with no throttle, and died instantly if gave any throttle.

Swapped plugs with same temp, no change.
Checked air intake (even removed filter temporarily) no change

Removed and cleaned carb (Mikuni) – no clogs but I did notice the slide was black like you might expect to see inside the cylinder, this was the first time I had this carb apart (only had quad one season – been running fine – most recent ride was two weeks before). Reason I noticed the blackening was because my DT 175 dirt bike also has a Mikuni carb which has nice and bright shiny brass slide).

Checked the reeds, no apparent damage (they were clean and seemed to completely close at rest)

Emptied fuel tank and refilled with fresh gas. Although fuel in it was only 2 weeks old.

Examined piston as best I could on the inlet port while the carb and reeds were removed and slowly rotating the kicker. What little bit I could see did not appear to have any damage.

Reassembled no change in symptoms. With it idling I sprayed carb cleaner all around the engine looking for air leaks – none found (no change in RPMs).

Just to verify, I sprayed carb cleaner into the air intake and engine stalled (what I expected)

Plugged exhaust with hand – seems like plenty of pressure, and will stall if I completely plug it.
Not sure what to do next. Ideas? Thanks in advance for any input.

Sans1 08-12-2010 09:29 PM

Can't believe no replies to my post, been out of town for work, but finally had a chance to give it one more try tonight.

Solution: There was a broken wire up near the ignition switch. Why it mattered? The carb has an electrical connection on top enclosed in a housing that activates the throttle slide. As long as the throttle is not pressed, this connection reads a short (0 ohms). Pressing the throttle changed this to an open (i.e. no connection). In effect it was just like activating the kill switch anytime I depressed the throttle. To verify this was the cause, I placed a jumper across the terminals, started the engine then depressed the throttle, engine revved (behaved like it was supposed to).

From there it was just a matter of locating the broken wire. Once I found it, I reterminated it and we are back on the road!

Probably would have been an easy fix if I had a Chilton's or Clymer manual just haven't ordered one for this toy yet.

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