Bigger Carb

I have that old 88' Yamaha Razz, & The 12mm carb is crappy on it, so Ithought I would try the 14mm Yamaha Jog carb I had in my back yard, so I cleaned it all out, put it on, & it worked good.... almost. As soon as I hit about 20-23 it wont go any faster, it doesn't really die down, but it just stays at that speed & sounds shitty. Please help on how to fix this? I

Re: Bigger Carb

Chris Robertson /

Sounds like your fuel/air mixture is too rich --your engine is getting more gas than it can use so it begins to four cycle. This is characterized by the engine's pitch suddenly dropping and you feeling a loss of power. When the moped slows down a bit, you'll get your power back until you hit the "magic" speed again.

A smaller jet might help.


Re: Bigger Carb

So too big of a carb?

Re: Bigger Carb

So, would you recommend getting a smaller carb, or just trying to fix the older 12mm one? I would Like to stay with the 14mm Jog one if I could. Can I tune the fuel richness screw so it will adapt to my moped? I tried to turn that screw the other day, & it just wont turn, I feel I am going to strip it. Any help, is needed, Thanks.


Re: Bigger Carb

Chris Robertson /

The fuel mixture screw only affects your idle. It won't help you on the top end.

Fred knows more about this topic than I do, if he's around, I'm sure he'll help you out.


The problem is.

The problem is... that I don't know enough about a Jog's carb to give a good answer...

Is it a Mikuni?

Is it a slide type?

Does it use the standard (larger style) jets?

If the answer to all those questions is "Yes"... then I could more than likely get it to run good (especially if it had a tuned pipe to go with the bigger carb) ... IF!... I had the ped in my hands.

Trying to describe what to do to someone else is a lot harder.

One thing you need to do Heath... Is to do 2 plug chops... One at its current top speed(23mph?).. and one at say..17mph... then compare them and tell me what they look like.

And are you running an air filter?

What ever you are doing with the air filter... do the opposite.. and describe what happens when you add it or remove it.

One of the things you will discover is that... modifying is work.

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