ATC90 (132cc) help - idles but can't take throttle (FIXED!) now need seat advice

[pismorat]

So I've been working on a '78 ATC90 for some time now and trying to get it running so its usable. It has a Bandito frame, 132cc kit, large Mikuni carb, Powroll intake, and a few other period-correct trick parts. I have put new points, condenser, and coil in it. The valves are adjusted and the timing is spot on. It starts with the choke on and the idle fluctuates a bit till its warm, then with the choke off it idles perfectly and will all day long. Any time I try to crack the throttle though, it starts sputtering and wants to die. I let off, and it idles again, so that tells me the idle circuit is fine, but something it wrong with the main? With the air filter off, gas almost comes spraying backwards out the inlet as I try to increase the throttle. I have taken the carburetor apart and cleaned it more times than I can count. I have played with the needle in all positions, messed with the air screw (idle circuit), and cannot seem to make it any better. My father-in-law ran it as is 25 years ago, so I am thinking the jetting should be close enough. Does anyone have an idea what to go after next?

[shortline10]

Sounds like jetting to me , to big possibly . What size main jet ? It should be somewhere around a 100-130 ?

[pismorat]

The jet is in the end of the bowl drain screw. Its a 90.

[shortline10]

Yes the main jet is in the bowl screw . Must be a vm22 or 24mm mikuni .
They can be a pain in the butt to get right .
I’ve had several were the shorty manifolds would not tune no matter what I did to the carburetor .
The longer aftermarket manifolds are much easier to set up .
The bog off idle is a very common problem with the 90/110 modded motors .
Lots of threads on this site about the fixes .

[pismorat]

Dang, I've searched a ton (I thought) and not found much. It is a vm22. I wonder if the o-ring that seals to the bowl, even though I replaced it, could be leaking fuel by? The bog I'm experiencing never goes away, and the engine will just die if I hold it wide.

[shortline10]

That O-ring definitely has to be perfectly sealed or the jet does nothing .

[DAM shop]

I agree with shortline on this one has to be the jet. I can give you a few settings on some of ours.
Bored and stroked 90 vm 22 long manifold 100 main.
Big bore only 90 vm 22 100 main, long manifold. Both of these not over 125cc
Bored and stroked 110 vm 24 100 or 105 main, about 140 or so cc runs perfect.
I would as shortline said look at the main jet size. On that cc I would start with a 130 or so main and adjust from there.
I also wonder if the bowl is filling enough with fuel?

How about a pic of this beast?:

[ATC200X4716]

are you sure the exhaust pipe and muffler is not clogged?

[pismorat]

I've checked the pipe, good thought there! It has a Supertrapp on it, and it breathes fine. The bowl is filling with enough fuel. I have the floats set right and every time I pull the bowl screw after running it out of gas, more still comes out. I have ordered a new o-ring as the other one looked soft and may have been rolling up? Then I'll start opening up the jets if it needs more. Thanks for all the help thus far!! I'll get a picture posted tomorrow.

[pismorat]

Its got a new front tire and wheel setup since here. A few things left, but if I can figure out the carb, I'm ready to flat track!

[Jwheeler]

Its got a new front tire and wheel setup since here. A few things left, but if I can figure out the carb, I'm ready to flat track!

Good evening.Lets see more pics of the frame ?

[pismorat]

Still waiting on the o-ring to arrive. Until then, here are a couple more pictures.

[Gumbo]

That’s got some great old school parts on it


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[MrConcdid]

Very cool machine, its like a time capsule.

[pismorat]

Thanks all! So I got the new o-ring...it's no better. So now I have larger jets on order that will hopefully help me troubleshoot more of get it running. It's amazing how well it idles, and then starts stumbling with a sporadic rev, without revving up, as you crack the throttle. At full throttle, it just dies. I've never had this kind of issue with a main circuit before, it's always the idle circuit that kicks my butt!!