Nibbi pwk 28 carb tuning/minibike

[MadMaxCart12]

Hello Everyone looking for jetting/tuning advice on a Nibbi pwk 28 used on a Stage 4 hemi 212cc 1 Cylinder engine, head milled .050", aggressive port and polish from AGK in Santa Barbara, Ca. So far I'm having to use a 140/38 jet setup with needle on the bottom, 5th position, and it's still slightly lean. It idles and accelerates well, returns to smooth idle, does'nt stumble or stutter in 1/2- WOT , but still slightly lean no matter which spark plug I use. The round slide 24mm carbs worked well but the difference in size and throttle response vs. the pwk is night and day. Keep in mind this head has 27/25mm valves and I've used their round slides, not this size of pwk's before. Even them I had to keep choke open 1/2 turn not to run too rich under a slightly faster idle, roughly 150-200+rpm vs. normal idle, the ported head /with carb demands a little more fuel to idle properly. Is there anything else I can do to get a slightly better plug condition with this carburetor?

 

[panchothedog]

You say that it is running lean no matter which spark plug you use. Do you have any other jets for the carburetor, or a source of where to get them? If you're sure it's running lean ( reading the plug ) then try to richer up the mixture. Put the next size up of available jets and see how it performs. I would think that would change things more than a different heat range of sparkplug.

 

[Rat]

It's lean because 28mm is too big, the reason the 24mm works so well is because it's cmf is spot on for the engine displacement and intake tract.

A PWK 26 is the biggest you can use and still tune fairly easily and not have it be fussy with drastic ambient/altitude swings... 24mm is just the perfect size especially if you delete the powerjet assembly fed off the bowl (it's a 2t thing that had no benefit on a 4t).

The a carb is too big for an engine it goes lean because the negative intake pressure struggles to pull fuel up the jets. Excessively rich jet sizes can correct but it is fickle tuning best avoided with the correct part.

LIKEWISE
When a carb is too small for an engine it goes rich because the negative intake pressure is too strong thus pulling too much fuel up the jets too easily. Excessively lean jet sizes can correct but it is fickle tuning best avoided with the correct part.