Viper 150 carb advice

[Britt]

I've got a new Viper/Hawk 150, and I'm having trouble getting it to idle reliably. (Though it seems to run OK when not being forced to just idle.) Sometimes it will idle fine, albeit a little fast, and other times it will just die out, either immediately or after an undetermined amount of time. Yes, it pops a bit on deceleration when I just rev it. Granted, I've only got a couple hours on the bike thus far, if that matters.

The carb is a PZ27. I have access to the Idle Speed Screw in the middle on the side, but the Idle Mixture Screw is not exposed, and the hole it should extend from (on the underneath of the carb on the engine side) seems to have some type of black resin/glue/epoxy inside it.
If the carb is not easily tunable (the seemingly plugged Idle Mixture Screw, and it's my understanding it isn't easy to find jets for the PZ27), can anyone recommend a decent tunable carb and/or advice on how to make tunable the one I have and where to find appropriate jets? Or would I be best off by just buying another cheap PZ27 that actually has an adjustable Idle Mixture Screw?

I don't have any real carb experience, just a PZ Tuning Manual I found from Walbro that covers the PZ26--yes, the PZ26 rather than the PZ27, but they look identical.
Thanks in advance for any advice!

[Thumper]

SHORT ANSWER... Shimming the jet needle and adjusting the airmix screw might be all you need to do to solve the problem(s).

Longer answer...
I deleted the advice on the pilot jet/"airmix" screw, because a knowledgeable member on this site tells me the pilot jet is a fuel jet, not an airjet. These are different depending on the carb. Opening my "airjet" screw seems to help the idle and made it easier to start. For now, I will leave it at that.

For top end, a larger main jet would probably help. The PZ27 is OK. It is a Keihin clone and it's new. Either muck around with the jets, or buy a new carb and muck around with the jets. Your choice.....
I would just adjust the PX27 first, but it's your new bike.

Making the adjustments isn't difficult.
The PZ27 and PZ30 carbs are built the same way, with same parts in the same design. There is a nice description of which circuit work 0-1/4 throttle, mid-range, and more than 3/4 throttle in the video linked in the following post
http://www.chinariders.net/showpost....5&postcount=39

The popping on deceleration usually means lean in the midrange. The main jet may need to be bigger (for top rpm mixture), but try shimming the needle up FIRST to enrich midrange gas/air mix with a washer or two to fix that. It is actually easy, just kind of a mystery at first. Here is a link to the jets (JET KIT Fits STOCK Carb):
https://sites.google.com/site/motocheez/brozz-250
Yes, Brozz 250 uses the same type of carb.

You can figure this out...To remove your carb, unclamp the intake side (airbox side) and remove the two 10mm nuts that hold the carb on the hard rubber flange that is connected to the head. You will have to pull the gasoline supply line off and then work the carb out of there. Then unscrew the cap with the throttle cable going into it, and just gently slide it out. You will see the slide/needle in this assembly. You will need to shim that (later).

The three screws on the bottom of the carb need to be removed to get to the the bowl off for access to the pilot/primary and main jets. But they are break-off screws and what's left behind after they build the carb is a rounded surface. I used a dremel to cut the flathead screwdriver slot that everyone cuts to open it. No biggy. If you can gently pinch the carb in a vice with the bowl up (protect the ends with cardboard and do NOT overtighten the vice!), the screws are right there. You can replace those screws with M4x16, .7 pitch (.75?). Just take a screw to verify the pitch.

Once open, you should be able to read the jet size on the main stamped on the end on the top surface. The Brozz 250 and my Bashan Storm both had a 105. If you get the jet kit, choose a bigger jet, and swap it out. Go up one or two sizes. But main jet really only restricts gas flow at the 3/4 to wide open range. The needle height can make the midrange perfect even with a main jet that is too small for top rpm performance.

For the needle shim, the throttle cable needs to be removed from the slide assembly (see this in the video above). Compress the spring and it can be pulled down and the nub on the end goes outside the sleeve and through the larger hole at the top end of the slot. The spring will fall off now, so carefully set it in a clean place. The needle is held down with a spring clamp, and the large spring holds that clamp down when reassembled. Remove the spring clamp (tweezer, hemostat, needle nose, etc), and the needle will fall right out of the slide now. Place maybe 1, 2, or 3 mm (thickness) of tiny washer(s) to shim the needle up that distance, drop it back into the slide, and put the spring clamp back. 1 or 2 mm is usually enough. If it is the same as the PZ30 in the Brozz and Storm, the larger spring holds that down in place so the needle is pinned when reassembled.
****As I mentioned above, the needle shim will help midrange even if you do not replace the main jet. The main jet limits the high end performance (more than 3/4 open throttle).

Put it back together and you are done. The bike should be otherwise running right for this adjustment to be done (valves, good battery/spark, etc) and I hear that different grades of gasoline can also affect carb tuning.

[Thumper]

FYI- I think you will find that your pilot/primary circuit jet is a two piece jet. Easy to find out... just remove it. If the second half is still down in the carb body, it's is the two piece jet. Here is a Japanese made PZ27 with the two piece pilot jet:
https://www.chinariders.net/showpost...16&postcount=1

And here is the same jet in my PZ30 (and the Brozz 250):
My PZ30 is branded "YouALL"
https://www.chinariders.net/showpost...7&postcount=40

I am going to get the chinese jets. For now, I opened the airmix screw. I am going to check my valve adjustment before I finalize the airmix screw and carb jetting.

[Britt]

Thanks immensely for all that info. I think I saw another thread from a while ago in which someone with one of these Vipers just put on another PZ27 that actually had the Idle Mixture Screw, and they were happy with it. But if I get a good recommendation for another carb that would suit better than the PZ, I'll likely grab it. Either way, I'm going to end up tuning something, and your info will help tremendously.

[Ruben353]

I got the 2020 viper 150cc model this year the carb leaks gas when petcock is turned. I took it apart and found out they broke off the screws to take carb apart. You may be better off buying a new one. I've seen them on Amazon for $30