Atc90 flat spot

[vroomy]

Hi guys, finally got my atc90 going and its got a real bad flat spot at about 1/4 to half throttle, if you feather it on will rev out fine. Its got a stroker crank and 54mm piston so prob around 130cc now and i have a 22mm mikuni carb. We have gone up and down on main jets, up and down on the needle and played with mixture. All made zero difference

[shortline10]

Short or long manifold ?
I’ve had a couple were no matter what I did I couldn’t tune the short manifold, as soon as I switched to a long style bapp manifold the problem was solved .

[vroomy]

Yeah its a short sudco

[vroomy]

Why would the length create a flat spot? What manifold would i need? It would need to be low profile as there is not alot of room

[shortline10]

POSA , BAPP and Rocky all made a long manifold and in my opinion their much easier to tune . The flat spot is a very common issue .
What mikuni carburetor do you have ? VM22 ?

[vroomy]

Yes correct

[vroomy]

What size main jet would you say it needed roughly?

[DAM shop]

What size main jet would you say it needed roughly?
I agree with shortline on the long manifold. We are running the same set up in several trikes and are running a 100 main with either a 30 or 32 pilot. I am also running a 24mm on a bored out 110 with a ported and matched head with a long manifold. Runs flawlessly.

[vroomy]

Any of you guys have manifold you would be interested in selling?

[Joseph Farrow]

POSA is probably the easiest and best to tune a carb on.

[vroomy]

Whats POSA?

[Jwheeler]

POSA is another manufacture of aftermarket parts from back in the day. They made a long aluminum intake with the mounting studs already installed on one side of the intake.

[HarrisoN*]

Are you certain you have a genuine mikuni carb. There's a lot out there that are fake. I had a similar issue in the last week with a vm26 on a 110. I was way too small on the pilot. Pilot controls idle circuit to about 1/4 throttle and needle controls 1/4 thru to around 3/4. Main is WOT top end. If youre experiencing issues below 3/4 main won't change a thing. Air screw is also only for idle and 1/4. Pilot jet and needle are your issues. If you have a genuine carb.

[ps2fixer]

Here's an image that illustrates what effects where in the carb tuning process.

http://www.jba.bc.ca/JBCycle/Resources/jet-chart.jpg



Here's another site that illustrates it a little differently. Most carbs work under the same basic concept/idea.

http://kawtriple.com/mraxl/carb/mikunicircuit.html

[vroomy]

Thanks for the info. A paid alot for carb and came in mikuni box etc so i hope it is genuine. Its got a 30 pilot, starts easy and idles great. Just seems any adjustments i make just goes bad to worse. Never had such a hard time tuning a bike

[ps2fixer]

Sounds like you're too lean. Assuming the engine is stock, it should have around a #35 pilot and #65 main, atleast that's based on the part numbers for ATC90 K0 (70-71 US90) and the 78 ATC90. The pilot/slow jet is adjustable, how many turns out is it currently from lightly seated all the way in? If it's more than 2 full turns I'd defo say you should go to a larger pilot jet. It should be around 1 1/2 turns out.

Also note the idle rpm should be pretty low based on the service manual I'm looking at, 1200 rpm.

https://www.partzilla.com/catalog/honda/atv/models-with-no-year/atc90-a/carburetor-inlet-pipe

https://www.partzilla.com/catalog/honda/atv/1978/atc90-a/carburetor-inlet-pipe

https://i.gyazo.com/525e102c194dfd463b0eeb04fc5b77b1.png


Based on the image diagram I posted and your first post 1/4 to 1/2 throttle is the point where the carb tapers off from the pilot and starts to really use the main jet. If idle to 1/4 is spot and and perfect, it would suggest the needle position or the main jet needs adjustment/swapped out.

Anyway, I might not be the most qualified person to suggest carb adjustments since I'm just spitting out numbers I can find on the net, but pretty sure you could go to the extreme both directions for each jet/needle setting and find where the limits are and choose the middle of the road jet.

Like take the carb apart, and go up a few sizes of the jet, adjust the pilot screw and record the jet # + screw adjustment and go till it's out of spec (screw adjustment doesn't do anything, or it has to be too far in/out). Once the pilot is picked, start swapping the main jet till you're clearly too rich, and find when it's clearly too lean and pick the middle size. needle groove should give you the fine adjustment to do the final tweaking. It's a lot of work, but it seems to be the way to get the best results. I've seen a guy with a china machine do that and a few other extreme to extreme style adjustments on a moped and he increased mpg, take off speed, and max speed, 100% all win win win for him ignoring the fact he spent a lot of time swapping things around.

Not sure what the OEM size of the carb is for the atc90, but the atc70 is 13mm and the stock size for a 84 TRX200 is 22mm, so maybe the carb is a bit too big making it very temperamental? I suspect it's around 16mm which is the stock size for the TRX90. I couldn't find what a stock size carb is for any of the 110/125cc sized models.

Anyway, good luck with the carb adjustments.

[HarrisoN*]

I'm running the 30 pilot in my vm22 on a 110 and feel I need to go a hair bigger so I wouldn't be surprised if your too small. I tried a 17.5 and 25 because I thought the 30 would be too big out of the box, I was wrong. If you go to the bottom clip of the needle does it get better or worse compared to the top clip.

[vroomy]

It really seems much the same to be honest, i will try a 35 pilot today

[vroomy]

Ok so 35 pilot made it worse it seems, so i went 25 and instantly wanted to rev out with choke on.....too lean then?

[HarrisoN*]

One thing that got me was that choke lever up was choke off, down is choke on. Just to clarify that. You should idle steady or stall out if all you changed was the pilot. Not Rev up. Sounds like a air leak or slide stuck.

[shortline10]

It’s the manifold !
Try making a 1-2” spacer to lengthen your manifold . Can make it out of just about anything just to try it .

[vroomy]

Ok will do