Yamaha Champ/QT50 Update3

Its me once again. I have been unable to continue my frustrating attempt at getting my Yamaha Champ (very similar to QT50) since I have been waiting for a head (thats "A" not "some"!), and one has been found so I am ready to get back at it. I stripped the threads popping plugs in and out over and over. But there has been a new discovery adding to some more head scratching and thinking on my part. I spoke to the previous owner of the moped (there was only 1 ever) last week, and towards the end of his ownership described having the same problems that I was having originally which is starting, idling, and running fine for a few hours or miles whichever the case, and then coughing out and not restarting and finding a fouled plug. And judging by the blackness discovered with the original plug that I removed, as well as in cylinder and around bowl of head where combustion occurs, I take that it was indeed running too rich which is also where I am deriving this hypothesis. He also tells me that he invested approx $400 into it at a local Yamaha shop to get it into its running condition which included among a few other things not related, a ring job, new carb and cables, which would explain the extreme cleanliness of the carb upon disassembly.

As I noted before this "bike" is very similar to the QT50, but its main difference with exception to the tire size, is that my Champ (thats its name a.k.a LC50G) has an "auto" choke on it which operates off of vacuum and a device called a BVS valve which opens and closes based on the temperature which it gets from sitting on top of the head which is then sent to a check valve. Is it possible that any of these devices along the "auto" choke (dumb ass thing) are faulty? And if so how can I tell and/or how can I bypass it or rig up a manual one of my own? The QT50 has a manual thumb control, but that I asume would involve a carb from a QT50 as well. I have replaced all hoses involved with this operation so that factor has been eliminated. Anyone?

Re: Yamaha Champ/QT50 Update3

I don't know exactly how that auto-choke works... but I would not trust it while it is acting like you describe.

Try to disable it.

Yamaha 2 strokes use Mikuni carbs almost exclusively... Mikuni carbs usually have a choke that is a plunger which goes up and down...

when the plunger is down > no choke

when the plunger is up > choke is on

disconnect the linkage or hose(s) running to the plunger and see to it that the plunger stays down (wire or tape?)... the choke will be off and now you should run it to test it.

this is mostly guesswork on my part... hope it works

Re: Yamaha Champ/QT50 Update3

Reeperette /

Dude, I think Fred's nailed it solid.

That sounds exactly like a stuck auto-choke.

Figure out how it works and look for weakened or worn parts, and follow Freds advice here.


Re: Yamaha Champ/QT50 Update3

Well the head is on and .....NOTHING. Afew sputters, thats it, oh and one sore ass leg! HELP! Yes it is a Mikuni carb (brand new at that), can you please further describe the plunger I am looking for. I will sound silly since I do not know all of the terminology but here goes; there are 2 hoses off of the (stupid) auto choke that go to the carb, one near the top there it is a triangle shaped head and I can see the edge of what looks like a black rubber diaphram, and the other goes below it near the middle of the carb, both of these enter on the right hand side of carb when sitting on bike. The manual says to troubleshoot disconnect vacuum hose from intake (the vacuum that starts it all) and if it starts its either the BVS valve (controls opening and closing of choke mech) or the fuel petcock. But could you please help me and tell me what to tape down to attempt to dissable the choke. Will this just make it harder to start but run ok once running if it is problem? According to diagram in book regarding to the triangle shape I described on top of carb with diaphram, there is a diaphram indeed, a spring that rests on a piece of metal then another diphram. Any suggestions are welcome. I'll kick my ass off if I have to just as long as it runs. Compression is awesome now, definately getting gas (very wet plug and cylinder) exhaust is clear, and spark is staying strong.

Re: Yamaha Champ/QT50 Update3

Well the strange thing is that even with a sticking choke... if everything else is alright... it will still start... but will foul a plug within 5 minurtes of running.

On a typical mikuni.. the plunger goes in from the top.. so try to pull off the 'triangle shaped' thing and see what is under it... if I am guessing correctly... there will be a litle brass shaft with a barrel on the end down inside the carb... and it gets pulled up and down by the diaphram.. if you get to it.. take the whole shaft/barrel piece out and make sure it goes all the way to the bottom of its travel.. then wire it so it can't come up.

OK... more thoughts... some of what is happening here is just not right... maybe your flywheel has sheared its key and the spark is arriving at the totally wrong time.

You will prob have to remove the flywheel to find out.

(I say this because if this carb is new then the choke shouldn't be the prob)

You aren't running diesel fuel innit are you ? :)

Re: Yamaha Champ/QT50 Update3

Nope, no diesel fuel in 'er. But I strapped on its new head (because I keep stripping threads popping plugs in and out since I foul so often) and after a few kicks she sprang back to life and idled and reved fine for approx 3-5 minutes then slowly died down to nothing. After trying again for a while I removed the plug and it was extremely dark/wet. So black that it almost looked like motor oil! And this was a brand new NGK plug prior to this. So I ran to get 3 more plugs (like I say I go through them) and I put one in and immediately it was fouled and so I put in the next and again it ran for awhile then died again! I have the mixture screw to its leanest, no air filter, and yet it still runs rich(my assumption due to color) ! Like I said before the carb is brand new. Could it be one of the auto choke parts or a jet that is too big? I tried removing different hoses as it was idling and it began to sputer and die as I did so. There was a good vaccum from all. As I stated before there are 2 vaccum hoses going to the carb, one on the front side to top diaphram and another just below it to the middle of carb, appears to be in mixture chamber or just above. So does this help? I'm about to give up, moped/scootering is a BLAST but is it suposed to be this hard!?


Re: Yamaha Champ/QT50 Update3

Extremely old thread!

Well, I am in exactly the same position. A champ with brand new Mikuni VM18 that wont start. Plug fouling.

Any input from anyone would be apprecited.

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