Another point to consider is that even though the mixture adjustment screw is towards the engine/intake side, it may actually be an air screw, and turning it OUT may lean the mixture, and, as such, will make the engine run hotter than normal. The engine will run strong...but not for long.

Update # 2
 

I pulled the plug today, and sure enough, the electrode was showing a very lean mixture as it was whitish.

1. I lowered the needle clip to the bottom setting per ya’ll’s advice. There is only three settings and it was already in the middle setting. It ran like a champ.

2. I put the K&N style filter back on, and it ran like dirt. Sputtering and spitting in 1/2 to 3/4 throttle range. Pulled the plug and it was sooty and black. Too rich.

3. I decreased the main jet from a 95 to an 85 and it ran similarly, but was lean again per the plug.

4. I put a 90 main jet in, and it ran a little better, but still bogging at 1/2 to 3/4 throttle.

Yes, I adjusted the mixture screw each time to get it “right”. It seems to like 1.25 turns out.

After that, it was time to quit.

I am at a loss as to what to do at this point.

A couple of questions, if you don't mind...


1. Why did you switch to a K&N filter if the bike ran good after the needle clip adjustment?

2. Why do you make such big swings in jetting changes? Going from a 95 to an 85 is five size differences, why not go to a 92 and then a 90 and then an 87? You would then see if those slight changes are for the better or worse.

To answer your questions:

1. Because the K&N filter set up is the stock set up on the bike.

2. Because I don’t know what I’m doing.

Thanks for your input. It is much appreciated.

1. If you're riding off-road, you should be using a two-stage foam pod air filter that is properly oiled for better engine protection.


2. Since the bike seemed to run so good with the 85, if you think there is room for improvement, try the 88 and reset the needle clip back to the center notch, and adjust from there

Did you get the valves done yet? They usually come pretty tight from the factory. Getting valves set correctly should be your starting point. Not saying it's your issue but it's one less thing to worry about when getting things set up.

I am waiting for my feeler gauges to come in from eBay. I bought the flexible ones recommended earlier that allow you to access the recessed tappets that come with these bikes.

I gotta tell ya, this is a wonderful learning experience. It can be frustrating, but I *do* enjoy it as I like a challenge and am mechanically inclined.

If your carb is a china PZ22 your needle should have 5-notches for the c-clip. Wonder why your's only has three???

Not sure. My carb is a "Molkt" PZ22 clone (Keihin clone).

Adjusted valves...no change.
 

(1) I adjusted the valve lashes to proper specs: .003” intake/.004” exhaust. I couldn’t get the gauges in at all upon initial inspection. They indeed do come from the factory too tight. Now they lightly sound like a shutter on a sewing machine.

I ran the machine and it still bogged at 3/4 throttle. I pulled the plug and the top was still slightly white. This was with a 95 main jet.


(2) I then moved the needle clip to the middle position and while it would idle just fine, it wouldn’t even run when in gear but *completely* bogged down and cut off. I pulled the plug and it was still a little white but frankly I don’t think it ran long enough to get a good read.

(3) I changed the jet to a 98, and now, it really won’t run without starter fluid and is very hard to kickstart.

I had already worked on it for about 3 hours, so it’s time to quit today.

Making some progress...
 

1. I rechecked the valves and while the exhaust was in spec, the intake was still too tight. I must have accidentally set it too tight by turning the tappet while tightening the valve lock nut. There is just a little play in the rocker arm when I move it with my hand when in TDC.

2. I couldn’t get the bike to idle longer than a few seconds, so I checked the carburetor bowl drain screw to make sure I was getting fuel. We are getting gas. The idle screw was properly set too.

3. I checked the pilot jet to see if it may be clogged since it controls idle to 1/4 throttle (or a little more). Sure enough, the pilot jet was clogged. I blew it out with my air compressor, reinstalled it in the bike, and viola! She idles great now.

She runs great from idle to 3/4 throttle. She loses power and bogs down at 3/4 to WOT. That is with a 98 main jet, middle position on needle (only 3 positions) and mixture screw @ 1.5 turns out.

I’m going to stop tinkering with the carb at this point as I’m going to order a 2 stage pod air filter.

My suggestion to you after you've oiled and installed the pod filter would be to leave the settings as they are now until you see how it reacts with the pod filter in place.

If your below 800 ft sea level, you will probably need to be around 110 -112.5 main jet.

Thanks Douglass. I'm definitely keeping the current "tuning" the same until I get the pod filter installed. I should be able to get it installed in the next few days as it is coming via Amazon Prime. I am currently at 938 ft above sea level according to Google.

I appreciate your input. All of this is very helpful! Thank you.

Pod filter installed, won't start
 

(1) I got the pod filter installed, but now it won't start.

(2) I left all of the carb settings the same as before the install (95 main, needle clip in middle setting (only 3 notches on the needle vs. 5).

Interestingly, the bowl had a bunk of junk floating in the gas in the bottom of it and the in-line filter is getting the same junk in it.

Thus, it's time to pull the tank for a thorough washing out with mineral spirits, replace the fuel lines, and get a new in-line filter. I may even purchase a genuine Mikuni carburetor in a few weeks as I'm not sure of the quality of this Molkt carb (Keihin knock-off).

QUESTION: Does anyone here know off the top of their heads what the typical size of 125cc pit bike fuel lines is?