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? :wondering

[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?:pics::pics::

[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!
269905
269906

[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!
269905
269906

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.
269916
269917

[Gumbo]

That’s got some great old school parts on it[emoji1303]


Sent from my iPhone using Tapatalk

[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!!

[shortline10]

Look for a long style aftermarket manifold and I bet that fixes your problem .
I have my Doubts that larger jets are the fix but it can’t hurt to try .

[DAM shop]

You never did say what size main you are running

[shortline10]

Said he was using a 90 main .

You never did say what size main you are running

[pismorat]

I pulled the 90 and put in a 120. No change. Then I upped it for fun to a 150, still nothing. Makes me wonder if there is a jammed passage in the carb still or the timing is off, even though its set right. It will idle all day long, just starts missing when the slide goes up and doesn't increase engine speed. I'm going to try a compression test this weekend...not sure what that'll tell me though.

[shortline10]

Long style manifold .
If your 100% sure the electrical is ok then it’s the manifold .


https://www.ebay.com/itm/363918270712?hash=item54bb3820f8:g:aloAAOSw8tBi2x4 J&amdata=enc%3AAQAHAAAAoApiqdoFL7NXh4gKGx7L5sJu8lriS QqBXSH9sIrtOhRAbAR7mz%2FVN1bXanYzNXhn4%2B9blCkfrDl ASgI8s9wu8LDZUVfXHXMTdPFY54WCDbPUaPHhavrrl7bzouz7j kQpm%2B6tNK7Aqqq2ErYFxGWQeCCcv3nFi%2B4ewrsWD68B9JZ fLHtF6ZW%2Fod7VEXerZFwfuvo7%2FJsngxAjDI%2BLgbZ%2B2 a4%3D%7Ctkp%3ABk9SR9Kh9vP-YA

[HairyJR]

I agree with shortline10, long manifold. It's been years since I've jetted 90's but as a stretch remember a friend who had cleaned their carb and lost the needle jet, gave the same symptoms.

"HJ" 269942 :beer

[pismorat]

Hey all. thanks for sticking with me here. I disassembled the carb again over the weekend and boiled it, then blew all the passages out again to try and, for the umpteenth time, verify it is clean. No changes. I did a compression test and its at 175psi. This makes me wonder if I shouldn't have the timing set at "0", or if the advance is working. What's an easy way to test for that...before I go after a long manifold?

[Dave8338]

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? :wondering

Not knowing anything about engines, ignition systems, or carbs, rubber band powered airplanes, model trains, or hydroplanes; I would suggest replacing your condenser and while you’re at it, throw a cheap coil on it too.

If you’re burping up fuel you’re not short on fuel. Just my ignorant.02 for what that’s worth.

[pismorat]

I have put new points, condenser, and ignition on i at the beginning. I am starting to lean towards a timing or advance issue since it was able to rev up a few weeks ago, though not great, better than now. Then I did some things to it, setting the timing being one of them, and now it is far worse. Maybe the thing needs more advance or the mechanical advance isn't working right. That'll be my next area or fous.

[shortline10]

If you don’t want to purchase a long manifold just make a makeshift one to try lol .
Needs about two more inches for the fuel to mix better .
A 2” block of wood and a hole saw and can be made up in a few minutes , this would be just for temporary trial of coarse .

[Joseph Farrow]

Long manifold, Long Manifold, Long manifold...... POSA or Long BAPP work the best. It will make a difference. The short manifolds and a Mikuni are a female dog to get right.

BTW nice oldschool bike. The frame is a Nicholson and it appears to have HD front forks.

[shortline10]

Hey Joseph maybe I should of said it like a broken record to :lol:
But what the hell do I know , I’ve only built a few hundred of these motors :crazy:





Long manifold, Long Manifold, Long manifold...... POSA or Long BAPP work the best. It will make a difference. The short manifolds and a Mikuni are a female dog to get right.

BTW nice oldschool bike. The frame is a Nicholson and it appears to have HD front forks.

[pismorat]

It's fixed!! I ran it this afternoon and its screams (for a non-suspended ATC 90). I am sure it'll run better with a long manifold down the road, but I found the true issue. I dove back into the timing and went all the way to the camshaft. The pin coming out of it was cocked, so as everything from that point out was set from the misplaced pin, the timing was way off. I set the timing by eye (using a mutli-meter was measuring continuity even when separation started) and the thing started and revved no problem. Had a good time blasting it around my in-laws' property today and now will button up a few loose ends so I can take it flat tracking in a couple weeks. More pics to come...

Thanks again for being patient and to all those who helped me track this gremlin down!

[pismorat]

Now I need help with the seat... I have the set of fiberglass fenders and made new seat pan attachment pieces, but I cannot find seat foam for it anywhere. I can get a cover off eBay, but that does me no good without formed foam. My usual upholstery guys don't have the time to work on it before I need it done in 10 days. What have you all used for seat foam, anything you can order that is close that need minimal trimming? I'd hate to take this thing to the flat track having to sit on a pillow, but I will if I have to.