Yamaha Champ weak running

This is the second time starting and running. It runs okay when on the centerstand. I tried to ride it and it would only go 5 MPH with me on it. It seems to not have the power that it should.

Compression before starting was 105 psi, and the carb was about as dirty as you would expect. I didn't take apart the auto choke assembly, but did disassemble everything else for a good cleaning. I am using a test tank currently as the petcock isn't working correctly.

Moved from my previous post.

Re: Yamaha Champ weak running

Jack Rutherford /

Still doesn’t sound that good. Probably lack of fuel or perhaps poor spark.

Re: Yamaha Champ weak running

> Jack Rutherford Wrote:

> -------------------------------------------------------

> Still doesn’t sound that good. Probably lack of fuel or perhaps poor

> spark.

Interesting you bring this up. Could this be from a bad battery?

Re: Yamaha Champ weak running

Jack Rutherford /

I don’t believe battery is required for spark on these. Usually it’s the charge coil gone bad on stator if spark is very weak.

Re: Yamaha Champ weak running

A battery is required because without it there isn't a spark. Could a weak battery make the spark weak?

Should I try messing with the carb? I woud like to determine if everything is working properly on the carb before looking into spark.

Are there tests I can run on the ignition side of things?

Re: Yamaha Champ weak running

Jack Rutherford /

Really? I’ve had every other bike in that family - Qt50, mj50, lb50 and lb80 and none required battery for spark. So Yamaha went rogue on the champ and required battery for spark?

Re: Yamaha Champ weak running

You shouldn't need a battery for the bike to run. The magneto make electricity for spark on its own circuit separate from the battery. The battery is charged by a lighting coil to work the lights only....but that doesn't matter, you have spark so let's work on the problem at hand.

Dude, Its fuel. I'd bet money on it. That "duh. Duh. Duh duh" sound when you try to rev the bike is called four stroking. This only happens for one reason, too much fuel. If it still has its original carb and jets the only culprit could be that the choke is on. You need to check out that auto choke.

Chokes work two ways, they either restrict air, with a valve that covers the mouth of the carb, there by decreasing the amount of air in the mixture. Or an extra jet that adds fuel. Point is that a choke enriches the fuel/air mix for easier starting.

Usually a choke will open back up when you pull the throttle. But yours isn't doing that. It could be based off of temperature. But once again. It obviously isn't working.

You have to pull apart the auto choke system or disable it.

Re: Yamaha Champ weak running

> Aaron Blair Wrote:

> -------------------------------------------------------

> You shouldn't need a battery for the bike to run. The magneto make

> electricity for spark on its own circuit separate from the battery. The

> battery is charged by a lighting coil to work the lights only....but

> that doesn't matter, you have spark so let's work on the problem at

> hand.

>

> Dude, Its fuel. I'd bet money on it. That "duh. Duh. Duh duh" sound when

> you try to rev the bike is called four stroking. This only happens for

> one reason, too much fuel. If it still has its original carb and jets

> the only culprit could be that the choke is on. You need to check out

> that auto choke.

>

> Chokes work two ways, they either restrict air, with a valve that covers

> the mouth of the carb, there by decreasing the amount of air in the

> mixture. Or an extra jet that adds fuel. Point is that a choke enriches

> the fuel/air mix for easier starting.

>

> Usually a choke will open back up when you pull the throttle. But yours

> isn't doing that. It could be based off of temperature. But once again.

> It obviously isn't working.

>

> You have to pull apart the auto choke system or disable it.

Good points all around. I threw the battery back in it and started it up. The bike runs rich as you said because the choke is on. When cold this is fine, when warmed up the bike will idle, but runs lazy.

Today was the first time I have actually gotten to ride it down the street. I unhooked the vacuum line from the check valve that runs to the intake pipe. By doing this the bike runs actually fairly decent. It will at least pull me around at around 10-20 MPH.

Watch this video. I apply vacuum to a warmed up engine and it slowly bleeds off. I think the check valve or the BiMetal valve in the head is busted.

Re: Yamaha Champ weak running

Your taking a pretty big risk of seizing doing that. An air leak is an air leak. Just fix or bypass the auto choke.

Re: Yamaha Champ weak running

Steven Bolduc /

No battery needed.

Re: Yamaha Champ weak running

I bought a carb off Amazon for a PW50, it fits but the choke cable is too short. I assume I need to swap over all the jets too.

I have a vacuum leak if you watch my videos. Could be the BMV or something with the check valve. Any ideas?

Re: Yamaha Champ weak running

You created the air leak, you said so in the post before last(removing the line from the check valve....)

That's not the way to fix this. Yes you added more air. But not in a safe way....I'm not sure it even has a full check valve, just enough to restrict vacuum, not block it.

The carb you have is fine! Just fix the choke circuit! Lol

Also, I don't think the jets between those are interchangable are they?

Of you are really dead set on switching carbs get a dellorto Sha 15/15 and intake. It's been done alot before and works great. If you search you'll find threads with the ball park jetting as well.

But seriously, you are missing something simple in the carb. If you aren't going for performance stock is always best.

Re: Yamaha Champ weak running

> Aaron Blair Wrote:

> -------------------------------------------------------

> You created the air leak, you said so in the post before last(removing

> the line from the check valve....)

>

> That's not the way to fix this. Yes you added more air. But not in a

> safe way....I'm not sure it even has a full check valve, just enough to

> restrict vacuum, not block it.

>

> The carb you have is fine! Just fix the choke circuit! Lol

>

> Also, I don't think the jets between those are interchangable are they?

>

> Of you are really dead set on switching carbs get a dellorto Sha 15/15

> and intake. It's been done alot before and works great. If you search

> you'll find threads with the ball park jetting as well.

>

> But seriously, you are missing something simple in the carb. If you

> aren't going for performance stock is always best.

I understand the choke circuit is my issue with the stock carb. The choke is always on. I can put my mity vac on the nipple for the choke diaphragm and it will shut it off. The issue I have is that it doesn't ever route the vacuum properly. When I apply vaccum manually to the bottom side of the check valve, it doesn't hold the vacuum. Check my video above. It bleeds off. It should pull vacuum then block it when warmed up to turn off the choke circuit. That isn't happening I assume because the BMV in the head is busted or the check valve is bad. Both of those parts are NLA.

I bought a chinese PW50 knock off carb. I am going to take the jets, and needle from my carb and put is in the PW50 carb. The bike runs and drives pretty good with the jets that came with the carb. It went about 25 MPH pulling me around. I will keep this thread up to date so others know there is another option that fits the stock intake and airbox.

Re: Yamaha Champ weak running

punkrock randy /

Aww shit, you in KC currently?! I’ll hafta bring out my Champ once yours will rip. I’m in Columbia, hit me up if there’s any organized rides 🤙🏼

Re: Yamaha Champ weak running

> punkrock randy Wrote:

> -------------------------------------------------------

> Aww shit, you in KC currently?! I’ll hafta bring out my Champ once yours

> will rip. I’m in Columbia, hit me up if there’s any organized rides 🤙🏼

Do you have a champ?

Re: Yamaha Champ weak running

Jack Rutherford /

He does

Re: Yamaha Champ weak running

punkrock randy /

I do :-)

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Re: Yamaha Champ weak running

> punkrock randy Wrote:

> -------------------------------------------------------

> I do :-)

Nice whip. What carb are you using?

Re: Yamaha Champ weak running

punkrock randy /

Still stock, maybe one day tho. Goddamn intake shape sux.

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