Idle screw setting and bogging issue

Idle screw setting and WOT bogging issue

I have a china "Raw 80cc" engine that I need to have the idle screw pretty much screwed in all the way to have the engine idle without dying out. I find this abnormal because my other "raw 80cc" engine idles fine with the screw roughly half way in or so.

I am not sure what exactly is the problem, but I am going to throw out some ideas I have in mind and hopefully someone can help me:

I been having issues where the engine bogs (hesitates) slightly at WOT (~30 mph), but continues to pull and goes even faster if I back the throttle off slightly (up to ~34 mph). Because of this, I know I am not maxing my CDI or engine. Just to clarify, in my case, the bogging is more of a hesitation than full on loss of power.

1) Fuel frothing: Rejected this idea, because if it was indeed fuel frothing in the carb bowl, I shouldn't be able to go even faster (more vibration) when I back the throttle off slightly. All mounting bolts are tightened well.

2) Fuel starvation: I read in another post that this is indicative of fuel starvation. I pulled my plug a few times and noticed it's a nice golden brown, so the fuel/air ratio should be correct. I even bought a new carb and tried it out, but still have the slight bogging at WOT. But I have yet to try another jet. This is where I am unsure do I need to go with a larger jet (0.72) or smaller (0.68). I believe the stock NT carb jet is 0.70. So far, this idea seems to make the most sense in my case. Will a new jet mess with my air/fuel raito, requiring me to tinker with the c-clip?

3) Possible air leak: I tried spraying carb cleaner around gaskets of intake, carb, etc during idle to see if the idle changes. The engine didn't change in idle when I sprayed carb cleaner, meaning there are no air leaks.

4) Exhaust flow restriction: rejected this idea because I can go faster when I back the throttle off slightly.

So these are my hypotheses and my reasoning behind each. Perhaps, the fact that I have to screw in my idle adjuster pretty much all the way in is somehow related to the problem (not necessarily the cause) I am having with the bogging at WOT.

GearNut said:

If you back off on the throttle from WOT and the engine performs better that is a sure sign that the fuel jet is too small. It cannot supply enough fuel to the volume of incoming air that occurs at WOT.

Is the idle screw long enough to actually raise or lower the slide position as you adjust it? Have you watched this happen?
If so, the idling issue could very easily be the angle of cut on the bottom of the slide. It is too acute, or not steep enough. Some folks have re-angled the bottom of the slide with sand paper and flat glass while others have simply cut a notch on the engine side of the bottom of the slide, starting at about 1/16" and slowly file it wider a teeny bit at a time until the darned thing works correctly. Do NOT notch the air cleaner side as that is not where the problem is.

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Awesome, thanks for the reply. Well, with the new carb, it has the same issue: have to screw in the idle screw almost all the way in. It does raise the slide as intended, because as I adjust it, it does change in idle, it just the position needed to keep the engine idling it pretty much all the way in. With a new jet, do I have to change the c clip position or can I leave it alone?

I have no problems from idle to 3/4. Awesome, hopefully the sick bike parts dellorto .72 jet fits my NT carb.

rohmell said:

Simply switch carbs between the two engines See if the problem follows the carb or stays with the engine.

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I have a spare carb that I bought and tried, which produced the same problem.

I checked for airleaks and didn't find any. I decided to try a new air filter that I got from sick bike parts:

Sick Bike Parts

Now, it seems my engine tops out at 28mph. It doesn't bog or anything but seems to pull comfortably at 28 mph and doesn't want to go faster. If I am going down the slightest decline, then it will pull faster up to the 34mph I can normally get before. I don't know what is wrong now.