[Ls1cameric]

I have an ATC70 with the TB 88 big bore kit and it never ran very well with the Chinese carb it came with so I put a mikuni VM 20 on it after some recommendations.
I've got the idle and pilot jet nailed down but it was still really boggy mid and upper throttle.
I moved the needle down 1 notch from the middle and that improved quite a bit but it still bogs at WOT. I bought 2 different jets kits and it seems to run best with the 185 main but still bogs. I've gone up to a 230 and didn't go further because it ran terrible and the same when I drop the main jet below 170..

Im beyond frustrated with this thing and hoping someone can offer some advice..

 

[loopiemclooperson]

I am thinking you are a bit rich. Most guys are closer to 20/140 or 22.5/150 depending on conditions.

 

[kirrbby]

X2

You have to be rich.
I did the same thing before with a VM 20 on a 117cc. Kept going bigger...turned out I had stepped past the right sized jet thinking I was still lean. I think I ended up between 140-160.

 

[Ls1cameric]

Ok, I'll drop the main jet and see what results I get. I've got all of them so I'll keep going lower.
I was worried that I may run it yoo lean.

How can I tell if it's too lean vs too rich?

 

[Ls1cameric]

I remember why I quite going lower on the main jet.
It was because when I lower the main jet, the engine started revving really high upon start up and will then slow down a bit but I was thinking it's because it's too lean

Do I need to possibly adjust the pilot jet as a result to this? I believe I have a 25 pilot now and seems to run good low end..

 

[Ls1cameric]

Update:
I don't think I was running the engine long enough to see the changes previously. I lowered the main jet to a 150 and see a noticeable improvement and will continue to play with them.
I think I may be too rich on the pilot and needle now because even though it runs pretty good low end and mid range, there's a noticeable power gain about mid throttle even with my 200 lbs riding it.
I'll continue to tinker with it but the thank you all for the advice to steer me in the right direction 👍

 

[SS LS1]

Update:
I don't think I was running the engine long enough to see the changes previously. I lowered the main jet to a 150 and see a noticeable improvement and will continue to play with them.
I think I may be too rich on the pilot and needle now because even though it runs pretty good low end and mid range, there's a noticeable power gain about mid throttle even with my 200 lbs riding it.
I'll continue to tinker with it but the thank you all for the advice to steer me in the right direction 👍

Did you get your carb sorted out then?

Reading your last jet settings I think you were still way too rich based on my recent experience with a larger engine.

I just freshened up my TB 114cc big bore kit with a highly ported race head, matched intake porting, large duration Z40 camshaft, high flow exhaust, UNI filter and ported inlet Mikuni VM20 carb. So it's definitely moving more air than an 88cc engine and my final jetting is much leaner.

In cold 34° F weather my VM20 ran best at a 20 pilot, 120 main jet, needle clip 2nd from bottom (one step richer than center clip position) producing a clean carmel colored plug.This was the best throttle response jetting running crisp throughput the rev range with no bog or sputtering on WOT.

Tryed a 125 main and it was a little blubbery (rich) under WOT in this cold weather and would only get worse in the summer heat.

The 114cc engine started, idled and ran very good with a 17.5 pilot but the idle air screw was only 1/4 turn out so a 20 pilot was the better choice with zero stumbles just off idle. Same with the needle jet in the center position, it ran good with an occasional low RPM transition stumble, but ran perfectly with the clip down one step richer.

I had originally guessed the 114cc VM20 jetting would be 17.5 pilot and 115 main just by math comparing engine size to my perfectly jetted Piranha 150cc VM26 engine. In truth its not far off from where I achieved optimal 114cc jetting once all said and done. 😉

A VM20 is a pretty big carb for an 88cc, and mathematically your main may end up down around 100 or slightly Larger as the carb signal will not be as strong as my bigger cc engine.

How big is your intake valve diameter and how How black or sooty looking is your spark plug now? 🤔

 

[Fast50s Crew]

I have an ATC70 with the TB 88 big bore kit and it never ran very well with the Chinese carb it came with so I put a mikuni VM 20 on it after some recommendations.
I've got the idle and pilot jet nailed down but it was still really boggy mid and upper throttle.
I moved the needle down 1 notch from the middle and that improved quite a bit but it still bogs at WOT. I bought 2 different jets kits and it seems to run best with the 185 main but still bogs. I've gone up to a 230 and didn't go further because it ran terrible and the same when I drop the main jet below 170..

Im beyond frustrated with this thing and hoping someone can offer some advice..

Vm20 is a better idea than an overseas replica carbs of any brand that I can think of or have had to work on. The reason the Keihin's and Mikuni's are better is they have gotten better and better over the years being the OEM carbs on so many bike brands. The "replicas" are just copies of something good. They look "good" but rarely work good.

The needle isn't gonna help on the wide open as the needle regulates the mid range. The wide open is regulated by the main jet as it appears you know... The 230 sounds far too big in my opinion, I would try to go no larger than 195 or 200 nor lower than a 175 or 180 (Also you said the 170 was no good) You're probably a lot closer to the right jetting than you think... I have at times chased the perfect setting because I could not get the right jet into stock at our shop or out of my pile of jets amassed over many years in film roll canisters. Curious do you have a 175 & 180 or 190 & 195? I always buy more sizes to ensure I can make my bikes run well. I have NOT EVER looked at my probably 100's of dollars of brass and thought "What a waste of money" I love all that brass because it makes my bikes haul ass... LOL

When you're all done and close the air screw should be at about 1.5 turns out so that tells you if your pretty close.
I have very often (I am embarrassed to say) on my own personal bikes got the jetting "close" where the airscrew is at only 1/2 out and that's fine for right then to race THAT day in a hurry but I have really no adjustability to clean up the tuning when somewhere else and it's not quite right which I hate. So in a perfect world if the airscrew is at only a 1/2 out you would change your jetting to make it 1.25-1.75 turns out...

Don't forget the airscrew can be used as a tool to see what direction you need to go with a jet.. You hold the throttle in the "bad" spot and turn the airscrew each way and usually it tells you what way to go with your jets as it will either get worse or clean up with the screw.

And not at all knocking the one poster who said a 20mm is "pretty big" for the 88cc but we use 17mm carbs on STOCK 50's and they absolutely rip. Back in the day (BITD) when one company was first importing the "S" Stage low compression 88cc kits from Takegawa they had the PC18 carbs in their kits. When we started importing them, we put the PC20 (Off the XR/CRF80 which externally is the SAME carb but with 2mm more inside) and we had a RIPPING 88cc that was better on top end. Then we put a high compression piston in that same kit that we made and that was even waay better. Now another brand sells that EXACT piston design in there stock head 88cc kit and it's also ripper only they have a replica carb in there kit...

Basically with all that being said, I am saying a 20mm carb with an 88c could be on the "small" side...(Side note: We had an 88cc SuperHead build with some compression and a nice hand cut cam that ran a VM26 and it absolutely SCREAMED) The bottom end power was "ok" but where it was ok you never rode it there EVER, so the mid and top end power it had more than made up for that...
Another thing, we have seen bikes that are not quite perfect (let's say "static" on the stand) on the bottom end or if you rap the throttle super fast but when you ride the bike (in "action" no sitting) they run perfect because you are rolling the throttle on pretty much no matter how aggressive you are as a rider.
Remember these small carbs we mostly use are NOT pumper carbs.

Ok, that was 72 cents worth of into now I think!!

-Fast50s

 

[SS LS1]

No arguments with the post above stating a 20mm or 26mm carb can work out well on an 88cc for WOT or mid to top end RPM racing needs. But for the OP they are not running a race pit bike at WOT going high speed with both front and rear suspension.

They are asking about carb tuning on an 88cc engine used in a heavy, rigid suspension ATC 70 with significantly more rolling resistance (likely heavy semi auto crank mounted clutch also) that will need not only to work at WOT "Screaming" RPMs, but also have good throttle response at low end RPM speeds for their specific needs. Thus why I said a 20mm carb is getting on the large size of what they may want to use on an 88cc in an ATC 70.

Totally their choice of course on what size carb to use, but they should be honest with their specific application and not just focus on what screams out on a race track.

A VM20 works remarkably better at all RPMs on my highly modified 114cc ATC 70 than the VM26 Mikuni I tried previously. It is running a 25mm intake valve diameter and a good basic rule of thumb is to use a carb size that is 80% the diameter of the intake valve.

Example:
25mm intake valve x .8 = 20mm Carb