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ATC with TB-88cc WOT bogging

864 Views 8 Replies 5 Participants Last post by  SS LS1
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..
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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 馃憤
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掳 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. 馃槈

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? 馃
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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

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