Jerking at full throttle

[Catlish]

So, I've got a Chinese 200cc enduro from a local brand that has a PZ30 carburetor that just behaves odd.

By now, I'm pretty sure it's a jetting problem, definitely not coil or plug.

Basically, when you open the throttle past 75% the bike might suddenly "cut out" and starts to jerk pretty violently until you let go the throttle a little.

It works perfectly fine on low and medium throttle and the spark plug is a nice brown color, so no doubt it's working fine here.

I have no way to know how it's burning at full throttle because I can't drive it at full throttle for basically any period of time.

So, any suggestions? Could it be too lean or too rich?

I'm planning on getting some carb rebuild kits to mess around a little with the jetting.

[Megadan]

past 75% throttle is entirely main jet. You need to determine if you are rich or lean and adjust accordingly. Generally the symptom of cutting out and getting all herky-jerky is a rich issue but it is best to verify if possible.

[Thumper]

They must have rejet it. Yup. What he said. Get a jet kit and drop by one size, see what happens.

Have you cut slots on the (probably) breakaway tiny bolt heads to remove the float bowl yet?

Once you get them off, replace with Allen bolts or Philips if you can't find Allen bolts.

I will bet there is a 125 in there, or even higher. If so, drop to 115. Unless it has aftermarket exhaust and/or airbox removed/pod filter, it shouldn't need higher. It probably came with 105 or 110.

[Mudflap]

I've seen similar symptoms when the main jet came loose and fell out into the float bowl.

[Catlish]

Thanks for your suggestions.
So, today I went ahead and got a pz27 and a pz30 rebuild kit. The were cheaper than I thought and since I could afford both, all that was left was to experiment with the jets.

Pulled the carb, it says Keihin? I guess, as the logo has a star separating the word. I think these are good? How do I know if it's an original?

The carb disassembled pretty easy. It's all Phillips screws, idle needle had no cover.
Gave it a good clean and as far as I'm aware, the first one in 10-ish years.

So, I went and used the PZ30 rebuild kit. Replaced both jets. (the new ones have no numbering). Visually, the pilot jet is the same as the old one (old one says 40). And the new main jet (old one is hard to read but I think it says 110) is slightly bigger, so, richer mixture.

And after a good cleaning, I put everything back and fire it up.
No more jerking on high load WOT. It no longer cuts out, but I don't feel like it accelerates there. So I'm guessing it might still be a little lean. Waddya think?


So, idle is now messed up. It idles really rough, and when it warms up, rpm's go waaay up. Also, when coasting down a hill, I hear popping on the exhaust.
This is my main problem now as the a/f needle is the same as it was when I cleaned it (1 and 1/2 turns out)
I reckon the idle circuit had a LOT of gunk and now is clean.
However, it's impossible to move this needle with the carburetor on the engine. There is just no way my hand fits in there.

I really don't fancy taking the carburetor out to make tiny adjustments and try for myself while burning my hand on the exhaust.
What do you guys think. Is this a lean or a rich problem?

[Thumper]

Gee, we both (me and Dan) thought it might be too rich on the high end, but you think slightly larger main helped!

I don't think cleaning it would make it leaner at high end so puzzled now.

On the idle, maybe adjust the screw. I believe this is indeed a replacement carb already. So you may need a set of jets to get this jet. Try going to one turn on the idle mix screw, then after a test ride to assess the affect, go the 2 and a half. If nothing else, it may help you decide if the pilot jet is too big for this engine in current airbox/exhaust condition.

[zero_dgz]

High/climbing idle and popping while coasting or on deceleration are classic symptoms of the idle mixture being too lean. Your main jet may now be correct(ish) but you also have the pilot jet to contend with which controls fuel mix at idle. Another readily identifiable symptom will be that at idle your exhaust header will get unusually hot while the engine is idling -- sometimes even hot enough to visibly glow in dim light! This is obviously bad.



The rough breakdown of what controls the mixture when on these types of carbs is:


0% throttle/Idle: Size of pilot jet *and* position of idle mixture screw (not the idle speed screw, which is different). If you run out of adjustment on the mixture screw without any improvement, swap out to one size bigger pilot jet and try again.

up to ~25% throttle: Pretty much entirely the pilot jet.
25% ~ 40%: height and taper angle of needle, size of main jet.
40% ~ 75%: size of main jet, taper of needle. (reverse of the above, really)

75% ~ Wide Open: Size of main jet exclusively.


We can quibble over which range of "percents" each jet controls for a particular carb but at the end of the day, the pilot jet and mixture screw control idle and low throttle, needle and main jet mid throttle, and main jet alone wide open throttle.


Remember that when coasting with the throttle closed your fuel/air mix is the same from the carb's standpoint as it would be at idle -- just with the added difference that the piston is also being cranked along by the wheel rolling while you're in gear.


I'm sure Megadan will come by with a ton more info than what I've outlined here.


Not all brands/manufacturers of Chinese jets are actually the same bore size for the same number stamped on them. Also, Chinese clone jet sizing does not correlate directly to the same sizes as genuine Mikuni or Keihin jets. If you want to not drive yourself nuts with tuning, do not mix brands/manufacturers of jets while you're messing around with it because if you do you'll basically be throwing random numbers at the sizing unless you know for sure (e.g. via measuring) which "size" is bigger than which between which brands/manufacturer.

[Megadan]

Originally Posted by zero_dgz

High/climbing idle and popping while coasting or on deceleration are classic symptoms of the idle mixture being too lean. Your main jet may now be correct(ish) but you also have the pilot jet to contend with which controls fuel mix at idle. Another readily identifiable symptom will be that at idle your exhaust header will get unusually hot while the engine is idling -- sometimes even hot enough to visibly glow in dim light! This is obviously bad.



The rough breakdown of what controls the mixture when on these types of carbs is:


0% throttle/Idle: Size of pilot jet *and* position of idle mixture screw (not the idle speed screw, which is different). If you run out of adjustment on the mixture screw without any improvement, swap out to one size bigger pilot jet and try again.

up to ~25% throttle: Pretty much entirely the pilot jet.
25% ~ 40%: height and taper angle of needle, size of main jet.
40% ~ 75%: size of main jet, taper of needle. (reverse of the above, really)

75% ~ Wide Open: Size of main jet exclusively.


We can quibble over which range of "percents" each jet controls for a particular carb but at the end of the day, the pilot jet and mixture screw control idle and low throttle, needle and main jet mid throttle, and main jet alone wide open throttle.


Remember that when coasting with the throttle closed your fuel/air mix is the same from the carb's standpoint as it would be at idle -- just with the added difference that the piston is also being cranked along by the wheel rolling while you're in gear.


I'm sure Megadan will come by with a ton more info than what I've outlined here.


Not all brands/manufacturers of Chinese jets are actually the same bore size for the same number stamped on them. Also, Chinese clone jet sizing does not correlate directly to the same sizes as genuine Mikuni or Keihin jets. If you want to not drive yourself nuts with tuning, do not mix brands/manufacturers of jets while you're messing around with it because if you do you'll basically be throwing random numbers at the sizing unless you know for sure (e.g. via measuring) which "size" is bigger than which between which brands/manufacturer.

Nope, I have nothing to add.to that. Pretty much spot.on.

My only addition would be.
Coasting along with the throttle just slightly open (around 1/8th or less is entirely pilot jet. Pure decel with the throttle closed is mostly idle mixture though the high vacuum will sometimes draw a little fuel up past the needle

[Catlish]

Once again, thank you all for your help.

I took down the carburetor and adjusted again the idle mixture screw. This time, I took it 1/2 turn out, so it now sits at 2 turns.

The idle got way better. And it doesn't idle as rough now. I do get an ocasional misfire, so I'm guessing it might be still a little lean. But that's something I'll have to correct another time.

As for the main jet. I do think this carburetor is an original Keihin (like 90% sure of it) as per the last owners words: it's the original carb, no one ever touched it.
Also, the jets have the Keihin logo engraved. I checked them again and the original main jet is a 105, not a 110.

I still think it lacks some power on full throttle. So maybe another day I'll start experimenting a little with the main jet. Probably I'll have to use a drill or something as this new jet just has no numbering.

I was also confused about the fuel mixture being too rich or lean because I thought it was a little rich and was unable to burn fuel. But at the same time it started to ping a little when hot. (<- I didn't mention this before because fuel in south America sometimes does this just because).
In the end the pinging also stopped. So not the fuel, obviously.

So, TL;DR: right now the bike is running 'ok' and definitely better than before. When I have some free time, I'll mess up a little with the carb to see if it improves.

Thanks for the help.

[TominMO]

Copied and pasted post #7 to a document file just to have it handy. Great job there zero_dgz