Scary problem today with throttle, any advise? Pic Added

[Private]

First let me start by saying hello to everyone as this is my first post. I am just getting into riding and recently purchased my first bike. It was a beauty Yamaha maxim xs400.
Today I took it out for my first run.. It ran like a charm.
I took it down to a local parking lot to practice some turning and stopping and then took it over to the gas station to fuel it up.

It has recently had an oil change.

When I got home I realized that the odometer was reading the same as when I left and the trip counter was at 0. I read up on this and went to go for another test drive to check it out. Before I got on the bike I realized that I had the Petcock on the Res tank, so I switched it to On this is where the problems started.

When I went to start the bike it was a little sluggish to start but after a couple cranks and some choke I was able to get it idling. I turned the choke down as I was getting ready to take off and then gave it a throttle check to make sure it was still warm from my previous ride . As I let off the throttle the bike started to rev high and kept going till it was really high rpms . I tried to move throttle because I thought it was stuck or something and it just stayed at high revs. I had to use the kill switch to turn it off.

I then put it back on reserve and it seemed to work fine again.

I repeated the above by putting it back to On and had the same idle issue.

Can anyone help me? I put regular fuel in the tank .

 

[HoughMade]

Sounds like a vacuum leak in the petcock causing a lean condition, which will cause the RPM to increase.

 

[Lou Ranger]

2 x post

 

[Lou Ranger]

Today I took it out for my first run...

Hi and welcome to the FORUM.
Sounds like the PO was skillful at keeping the faulty petcock a secret until after you owned it.
If you are in Ont., or one of the other provinces where you need a mechanical fitness certificate to register the bike, you might try taking it back to the certifying mechanic since uncontrolled revving is an unsafe operating condition which he clearly didn't flag properly. Maybe he'll give you a deal to make up for his error.
If not, then at least it is simple repair or something you can live with on RES.

 

[Private]

Thank you for all the replys. I started playing with the bike today, to try and pinpoint the problem.. And I think I may have it, although not sure why it's happeneing.

I noticed that when it was skyrocketing in RPM;s the trottle at the end of the cable was sticking. I noticed there was a little bit of space between the throttle and the idle screw. i was able to just push it back and the idling went down. Now here is my new problem...

So i adjusted the screw so that the throttle had to reset further back, and obviously now the bike will not idle. I lubed up the cable, and all the other moving parts, adjusted the idle till it was around 1000 RPM, and seemed to work fine. But as soon as I put it in gear, it started to rev high again. I took a look at the throttle cable and again there was a small space between the screw and the throttle. I figured I must have gave it a little gas as I was holding onto the handle bars.

After i turn the bike off and rotate the throttle it returns to the screw with no problems.

I have noticed that there is a spring that holds this ( throttle piece and unit) [Sorry do not know the name] together, and it seems to be dirty. I want to undue the screw and clean this all apart... Is this a big job... I am a novice and do not want to mess something up by taking it apart. Any ideas here guys.

 

[Lou Ranger]

... I want to undue the screw and clean this all apart... Is this a big job... I am a novice and do not want to mess something up by taking it apart. Any ideas here guys.

I have an idea, but I am not sure it is a good one. I too am a novice and have a healthy fear of taking things apart. Sometimes they don't go back together. Or a bolt gets over-tightened and a part that was fine gets cracked.

If I thought cleaning might solve a problem but wasn't sure, I'd try doing a partial cleaning without taking anything apart. A toothbrush and some solvent (or gas in this case?) and then maybe flush it out with blasts of brake cleaning fluid. If cleaning it that much makes any measurable difference, then you know you are on to something and can then risk taking it apart. If you get lucky and this cleans it enough and everything works fine again, then you can simply re-lube the parts you cleaned and carry on.

As I said - don't know if it's a good idea, but it's what I would likely do first before taking anything tricky apart. Maybe I would loosen things a bit to make more room for the solvents. If you do, don't forget to note how tight they were and tighten them again when you're done.

 

[Private]

Report -- I took your suggestion, cleaned it all up with a toothbrush. Still had a slight jump in RPM's that wouldn't return to idle. I took the cable out of the spot where it locks in, and ran the bike around the block to see if it would spike in RPM at all... NO spike. put the cable back in, and the bike ran fine. it was almost as if the cable was getting just a tad tighter when it was in gear rather then idle. ( this is without my hand on the gas). I wonder if there is a way to adjust the cable, so that i can make it just a little longer. Thanks for all the help thou...

I am going to start another thread to address my Odometer not working.. hahaha.. I love working on this thing.