Keihin24mm too lean!

Hi All,
Curare, I finally got round to buying 2 new main jets #120 and a #115. Unable to get a #118. and no luck with a pilot jet. They are both genuine Honda parts but still have no numbers. I started with a new sparkplug Champion L95YC. I put in the #120 and took it for a spin . Felt good.Coming down a hill at 85-90 KMPh, I cut the ignition and pulled the clutch. came to stop and checked the plug. It was as white as Columbia’s finest, far too lean. Got home and put the original main jet back in. Visually, it looks the same as the #115 but I can’t be too sure . Did another plug chop and the colour was alittle darker, but I figure its still too lean.

Further thoughts.
My oil mix is 2%, also the "chrome " slide is becoming a bronze/ copper colour. I think because of the wear on the slide my engine has become leaner. Is this possible??
The engine is running very well at the moment, but this always worries me due to the colour of the plug.
Any ideas, advice,comments???


hi grimsey regarding the lean problem your having now if you dont mind me asking have you adjusted your air fuel mixture adjusting screw which is located at the left hand side of basket carb? if so how many turns out from close position was it? btw what modificatin have you done with your scoot if i may ask sir? dlegendslayer. [:dance1:]

Ok I get it a little bit…

Thanks again, I’ll try again at the weekend and let you know.

Oh and I’m still looking for idla jets!

Cheers Grimesy

It’s finally fixed and running great. I installed a pilot jet from a Kawasaki KR 150 ( check them out they are nice!) fiddled around for 30 minutes and picks up perfect. Regarding WOT bogging , I went to a local motorcycle tuning shop . Where the guy promtly revved the tits out of my scoot then removed the air hose installed a K&N type filter rejetted the main jet #115( Curare’s first advice) charged me 90Baht ( 2$, 2 Euros) and said „Bon Voyage“ . How strange!

Anyway ,this morning coming to work touched 90KPH, which is fast for my bike and 5KPH more than normal! With the filter it reminds me of the 80’s ie the woooooopppp noise ( This is not a racist comment,Randall ) All I need now is some Castrol R.

Thanks Curare, Geo and the French speaking Thai man

hi grimsey delorto carb usually when you turn clockwise youre richning the ari fuel mixture so when youre on the lean side as you said you try to turn it clockwise 1/2 turn and take her for spin and do some plug reading again.btw what kind of scoot are tuning? what are the modifications that youve done on already?hope this helps.keep on trying grimsey im sure youll get it right soon.[:bounce:]

Hi Grimesy,
try this:

click Tech on the left, then click on #6. „Tune Keihin and Mikuni-type carbs“
Try again, it is very long and I don’t think this is a place to cut and copy it. Maybe I can mail it to you…
What is it you said in the other topic… was it too bad? I go and check…

Afew more questions.

You said
"for the plug chop to work you have to keep the engine to the WOT long enough if not when you will check the plug it will show how is your carb setting for the average use you are doing.
When you are at WOT (wide open throttle) keep it for 5 sec and then kill the engine.’’

Does this mean that the plug chop I did was incorrect? Ex, screaming down a hill with my engine getting ready to spit the piston through the panel. Are you suggesting that I do another plug chop whilst riding under more normal conditions Ex, a flat road then killing the engine and then pulling the clutch lever in?



hello Grimesy66,

for the plug chop to work you have to keep the engine to the WOT long enough if not when you will check the plug it will show how is your carb setting for the average use you are doing.
When you are at WOT (wide open throttle) keep it for 5 sec and then kill the engine. Start doing it with a main jet you now is at least a little rich. The one you had on as standard should be fine.

To see if your iddle jet is of the correct size do the following:

warm the engine up
turn the iddle up so it will idle about 500 rpm higher then normal.
turn the air screw all the way in. (close it). Then open 1/4 at a time giving 15 sec for the engine to settle.
Continue with this procedure until the engine iddle at its highest.
Then check how many turn of the air screw you have needed to get it.
From 0 to 1 you shold put a bigger iddle jet.
From 1 to 2 you are OK
More then two you need a smaller iddle jet.

This setting will not effect the WOT. So if your are lean at WOT you will remein lean.
The iddle circuit is separated in the carb.

I will try to give you an idea of the different part of the carb and which part are more responsable for each part of its throttle opening.

At iddle: air screw and slow jet

from iddle to 1/8: air screw, slow jet and throttle valve cut away.
(this is the hardest to get right when you are not very familiar with carb set up)

from 1/8 to 1/4 : throttle cut away, needle diameter, needle clip position.

from 1/4 to 1/2 : neddle clip position and needle taper

From 1/2 to 3/4 still needle clip but more needle tapper.

from 3/4 to WOT: Main jet

Hi Grimesy,
maybe you should check this

hope it is of some help, I did with it my fine tuning for a PWK 28.

IMHO at full throttle the main jet should be the only responsible for your white spark…


hi grimsey sorry for the late reply had to checkout for a while,anyways the 40 and 42 are the slow jet or idle jet. as ive said earlier grimsey were you able to check how many turns out you have right now? i really have to know what setting do you have now on your air fuel mixture so that i can analyze the solution. there are 2 screws on the carb one is for the idle which is the one protruding on the carb and the other one is the small one and not protruding and that is the air fuel mixture.try to close it but count yhe turns while closing it.[:bounce:]

Hi, dlengendslayer,
A good point on the fuel air mixture screw. I’ve got know idea. To be honest I’ve been looking around the Net but none the wiser. I think the screw is adjusted differently to a Dellorto?
Any ideas?
Re; engine it’s standard.


Thanks all.
I’m reading and digesting! and working, will reply with more info soon.

Geo’ I can’t get onto ScootRS’ website at all. I’ve been trying for a few weeks now. Was it something I said? see ’ Keihin 24mm sooo smooth" a truly hot topic. Could you copy and post it here?? please.


Hi again.

GT still no luck on your link,I think Randall’s blocked me?Here’s my email address [email protected]

Curare, I’ve followed your great advice and here’s what I’ve got:
The air fuel mixture screw was turned all the way in. I turned the tickover screw/ idle screw in and increased the RPM ( +500???). Then I started turning the air screw out in 1/4 turns, the RPM’s dropped, another two 1/4 turns and the engine stopped. So, going on your advice I need a larger idle jet. Is that correct?
Regarding, being too lean I guess I still need a bigger main jet?I/ll try a #125.

Furthermore, the needle clip position is on the 3rd position from the top. Am I right in thinking that if I lower the needle it will richen the mixture??

Cheers Grimesy

hi grimsey im sure curare have given all the best info regarding on tuning a carb.just step by step and your on it.

thanks so much curare for the expert advised,i check your website and saw your ultimate tuned vespa, it was a dream vespa,im still gathering parts and planning to set up my px200 also.i was inspired more when i saw your vespa.thanks so much hpe to see more picts of your vespa and of the parts you order from sip.cheers[:bounce:]


from your findings, yes the idle jet is too small.

be careful from the opposite effect the needle clip position has on the needle position.

If you lower the needle you lean the mixture in the mid range.
But to lower the needle you have to set the clip in a higher position.

If you put the neele clip higher you lower the needle and you lean the mixture on the mid range.
If you lower the needle clip you lift the needle and you make the mixture richer on the mid range.

Hopefully there is a photo link on this reply. It’s of a Keihin PE carb.

I’ve got 2 questions.
On the exploded view what are numbers 40 and 42?? and what are ideal settings??

btw regretfully,It’s a standard Px 150e non autolube, except for the clutch which is from a Cosa . Or so I’ve been told ( sold!).


finding the right main jet for the engine screaming downhill is trying to find the right main jet not for WOT but for highly over rev.
And you don’t want to find the right setting for over rev but for full rev WOT.
So get it right in normal condition, flat road or on a very, very slight downhill.
In steep dowhill you can reach even 1000rpm more then your engine is capable of when pulling. But at this over rev it is almost or totally out of steam. No pulling power.