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Discussion Starter #1
Hello All. Working through some tuning options on a 2002 XR70R project, and I could use some fueling input.

Stock carb was a 12mm (!) with the stock aribox. It held a nice, steady, low idle, but the bike felt absolutely strangled when I tried to spin it up/rev it out.
I picked up a PZ22 with pod filter from eBay, and installed it with no issue. Bike revs out now, and seems stronger from 1/4 throttle on up.

...but there is a problem...
I can't get it to idle cleanly at revs below clutch-engagement-speed. No matter how I tweak the fuel mixture screw, it just won't stay running at a low idle like it did with the stock carb.
I've seen similar on 125 two-strokes that were over-carbed for high-end performance, and I'm familiar with sizing down on woods bikes for better low-to-mid. The stock 12 was/is just ridiculous.

Is a PZ22 too much carb for a stock-bore 70 to allow a stable low idle? I don't wish to go back to the stock 12, as I would like it to cleanly rev out. What do the keepers of all things Honda-Horizontal say is the best option/sizing for the XR70R, given a completely fresh, but stock size & C/R, top end? If the PZ22 is indeed the recommended size for this bike, what should I do to baseline jetting to get a clean, below-clutch, idle (Central Texas, massive temp swings, 1000 feet above sea level)?

Thanks for any guidance anyone can provide.
 

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Discussion Starter #4
Sorry that it took a while to get the picture up...Idle adjust (slide) on left-side. Air screw on Right/exhaust-side, and it is definitely an air screw based on the taper.
271552
 

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Going by your picture, is the bike used on the street or a kart track? Only asking due to the screen door air filter on it, which may be your problem - too much air getting in overwhelming the mixture to the point of stalling it out. If you have the air screw all the way IN to richen, and it still stalls, you might need to go up a size (or 2) on your pilot jet.
Off the top of your head, did you happen to pull the jets out and get a look at them?
A guy over on ChinaRiders has a PZ22 issue with his, he has a hard time keeping it going, along with a sputter in the 3/4 to wide open range. He sent me a pic of his pilot jet, shown below, with his dimensions for it. It is a #35, the carb is Chinese, an XLJOY brand, from Amazon. The closest style jet I could "match up" was a Keihin N424-74C going by the dimensions. He wanted a size or two smaller than the #35, but the Keihin jet doesn't go any smaller than a #35.

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Discussion Starter #6
I haven't pulled the jets yet. Only worked the fuel/air screw and the idle adjust.

The K&N-style pod that came with the carb is temporary, and I'm waiting on a dual-layer UNI foam pod. With the angled intake, I can't run a standard airbox, and the stock airbox was as restrictive as the stock carburetor itself, which would defeat the purpose of its use. I'm sure a dual-layer UNI foam with light oil will flow just about as much as the treated-fabric-style (cloth under the mesh).

The carb will allow the bike to pull smoothly from a "high idle" all the way up to what I am comfortable revving to with a fresh top end (not broken in yet). It just that the idle is right where the auto clutch starts dragging just a bit. Below that, the bike stumbles and dies...period. Never happened with the smaller carb.
 

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Try using the stock air filter. my atc70 hates all aftermarket flame arrestors..
 

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Discussion Starter #8
Try using the stock air filter. my atc70 hates all aftermarket flame arrestors..
I can't use the stock airbox. Larger carb neans backbone clearance issues, hence the angled intake tube. Stock airbox is intended to ride inside the frame-mount plates, straight back.

Also, the stock airbox won't fit the bell-mouth of the 22mm carb.

Besides, the stock airbox and carb are what kept it from revving out in the first place.

Looking for the right carb sizing/jetting for a pod-only carb to get a reasonable stable/low idle while still revving out like this one does now. Someone must have gotten this right.
 

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Discussion Starter #10
This bike has this carb kit on it, with a Kitaco 75cc big bore kit and an Ultima Jr. pipe. I took it out of the box, set the air screw at 1.5 turns out, and put it on the bike with the stock jets. Starts and idles fine, and moves the bike right along.
Thank you. researching now...I'm thinking the step down from 22 to a 19 may be a big deal if TBOLT themselves considers this the "big carb" for under-100cc variants of the motor.
 
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