Bought a new huasheng carb to replace defective one. Now it produces even more wicked backfire. How do i "Adjust the jet"?

Deliveryfoot

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https://www.motorizedbicycle.ca/catalog/product/view/id/179/s/carburetor-assembly/category/17/ Here's the one i bought.

It ran great the first run. Except the throttle would stop responding when nearly wide open.

Now even though nothing except a new day has changed and it's totally refusing to behave properly on full open choke and will actually die when i try to open the thottle. And at various levels of choke adjustment, it seems to want to die and has serious response issues. I'm back to only being able to run the engine on closed choke. . .

It'll idle fine at open choke, though. Till i try to touch the throttle.

I hear lots of people say "adjust the plastic idle screw" when all that seems to do is just push the throttle to be more open. Not impact the jet.

Like here: https://motorbicycling.com/threads/adjusting-the-huasheng-142f-49cc-carburetor.41561/ wtf, there's only 2 screws. One to empty the bowl. The other to adjust idle speed???? Is it not like 2 strokes where you pull out a needle and readjust the clip?
 
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darwin

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There's a brass plug to the lower left of the face of the idle adjustment stop screw, pull that plug out gently, screw is in back of that. If that doesn't work clean out the black plastic idle jet under the idle stop screw. Use a fine wire from an electrical cord or wire brush. Remove it by gently twisting motion and pulling upwards at the same time. On adjusting the jet I always start by screwing it all the way in gently then backing it out 1-1/2 turns for a start. Usually within a half turn of that you'll find the sweet spot when warmed up. Revs all the way warmed while riding under load no hesitations.
 

CrazyDan

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Use a small drill bit to carefully drill a hole in the plug, then run a screw (I used a wood screw) in the hole till it has a good bite and use pliers to yank it out.
Normally when you get a backfire it's because your air/fuel mix ignites before the intake valve is fully closed. Check valve lash.
 

Deliveryfoot

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Use a small drill bit to carefully drill a hole in the plug, then run a screw (I used a wood screw) in the hole till it has a good bite and use pliers to yank it out.
Wow, okay. I think i understand now. The brass plate covers a screw up used to adjust the mix. That's really strange.
 

CrazyDan

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Wow, okay. I think i understand now. The brass plate covers a screw up used to adjust the mix. That's really strange.
I believe they set the engines lean from the factory and cover the adjustment screw to prevent you adjusting it yourself for EPA restrictions.
The correct one is the one on the left side closest to the engine block, not the one behind it closer to the black plastic idle screw.
You might have a partially clogged main jet. It sounds like you have a fuel issue at WOT, so that's something wrong with the main jet. It's either not the right size or clogged. To get at that take the bowl off and unscrew it from the center. Use the same cleaning process @darwin mentioned.
 
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CrazyDan

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I thought I mentioned that the intake valve was open when ignition occurred.
Normally when you get a backfire it's because your air/fuel mix ignites before the intake valve is fully closed. Check valve lash.
Maybe his flywheel key sheared too, and timing went off.
 

Deliveryfoot

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Just an update. I installed the new carb and ran into a lot of issues. Ran like s**t and i came here. Learned about the carb hidden tuning screw and still had issues.

So i took the carb apart, needled the jets with a wire and used gumout carb cleaner and put it back together. Runs flawlessly (Why do i have to clean a brand new carb??). Though im having trouble knowing where exactly i should ideally tune the carb screw, because it seems to run perfectly at various turns. Should i be turning it clockwise or counter for fuel efficiency? It's rich when its being turned clockwise, right?

Also, thanks so very much to those who helped answer my questions here. It's always so very wonderful to run into a group of bros who are able to lend a few words of wisdom to someone so clueless as me. Just never been so happy to finally have my engine working like it's supposed to now.
 
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Deliveryfoot

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it clockwis

I thought I mentioned that the intake valve was open when ignition occurred.

Maybe his flywheel key sheared too, and timing went off.
(i might not know what im talking about here) Gasbike.net sent me a keyway that wouldnt fit the shaft. Contacted them about a proper fitting replacement and they ignored me. So i sanded it down best i could to make it fit and shoved it in..with a hammer... Didn't know it being lop sided could result in backfire.
 
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