Carb n jetting for 172 T5

hi all i know this has been spoken about before by me as well but the thread has gone
and i’m ready to order my carb

going for the kehlin 28/30 whats the best jetting to do for this carb on the following

malossi 172
machined head
casings ported
mikeck exhaust fat muffler versoin
21t clutch
for fast road use motorways looking for good top but not ragging it to death
running 30 dellorto but not sure how well it run before ,it reved out well but the bike was crashed and i decided to rebuild it all 1st

what parts do i need for this kit i want to use the cable choke

thanks in advance


I think to avoid a long explanation you could have a look at
go at Polini 133 install and then it is explained in one of the section, if I remember well.
If for any reason you don’t understand it or you don’t find it ask again.
And for the head: machined to suit it doesn’t mean it is well done. Are you sure about who did it. It is an experienceded tuner?

hi curare
hope your well
i got the pwk carb and finally fitted it with your suggestions and it will only idle for a minute
just wanted to ask your advice on the air screw how many turns out from screwd in to stop

also i had problems gettign it started thinking it was the electrics and i moved the stator plate counter clockwise almost in line with the casing marks, it sits about .7 mm above the casing marks
is it worth moving the stator 1st to see if it will tick over properly

although i’m more worried about the mixture at the moment



start with the air screw 1 1/4 turn out from close.

Are you mounting the 40 slow jet or the 42 ?

You said it iddle for 1min and then stops. Just turn in sligtly the slide screw so it will be a little more accelerated at idle, keep it on iddle for about 5min and then turn the ignition off. Take a look at the plug.

You shouldn’t change the ignition set up if before was ok.

If everything with your engine is ok it shouldn’t take long to set it up.

let me know.

in case you need you can write me an e-mail. you can find my e-mail address clicking on my name. it is on my profile.


head is machined to match kit

timing is about 2.5mm above the mark on casing

going to spend most of tomorrow having a play and tryand sort it out

could someone explain in simple terms how to set the timing with a timing disk
going to have a go with a timing disc to see if i can get more of an idea and learn that way

i dont understand the timing disk set up
how do you set it at 14 degrees etc



the way they give you the carb depends from where you get it,
standard 28 PWKsetting from keihin should be 135 main ,38 slow, JJH needle and slide cut away 3.5
But I say it depends from who you buy it. If it is a genuine keihin or a keihin pattern.
If you call up keihin dealer they generally send you the carb with the set up you ask for.

As far as the timig concern.

Turn the stator plate clockwise so that the mark on the stator plate will be about 3mm on the right of the IT mark of the case.
Then try it check if the carburation is ok…
after you get on this setting you should consider that to find the correct timing is mostly question of feeling, so when you have time try to advance it (turn the stator counter clock wise 1mm so the stator mark will get closer to the IT mark on the case) and see how the engine feels and if doesn’t start beeing flat at high rev that is a synthom of too advance timing.
If it is worse go to the original setting and try from the other way.
Don’t be afraid of trying, it is the only way to get it right.

I know evryone seems to want to upgear every thing nowadays but I run my T5 reedvalve on a 20t. I get no lack of power in 4th and it pulls up to 80mph. Ive done a fair few rallys on it and it gets used as a spare scoot for broken down mates if im on my Lammy. T5s were made to rev high and I think with the mickek you will benifit from going back to 20t.

new update

now running the original 20t primary drive and its fuckin shite

1st ok
2nd ok
3rd starting to struggle takes ages to get going doesn’t seem to rev out
4th revs just drop and bike doesn’t want to pull at all
getting really hacked off with this now

fuel pump is now installed under tank (thanks curare for pics)

jetting as above thread
plug chop on full throttle is light brown fudge color - greyish

any more ideas


It is strange since from your saying it seams that your engine in 4th struggles more with the shorter 20 then the previous 21.

what about your time setting?

Do you have the possibility to try a different head?
A badly shaped head or not well suited can really put your engine down.

And try to be patient. Maybe it will take it longer then you thought, but you will do it.

hi m8
this is the carb i’m going for i hope this is the right one
any ideas as to what needle and slide it has from the factory it says it also comes with 4 jets any idea as too the sizes don’t want to order all these extra peices and find it has them already

also what would you recoomend on timing
you have the 2 marks one on the casind and one on the stator plate but roughly how do the 2 marks look to gether only reason is i’m no good at timing and i’ll try and get it roughly right so i can get it running as it’s only 5 weeks to the i.o.w scooter rally



I would start with this following set up:

main: 128
slow: 40
needle: JJH
clip in 3rd position from top
slide C.A.: 3.5

get a spare 125 and 130 and 42.

If you want to use cable chocke you need a conversion kit that you can find I think here at SIP for the KOSO (keihin pattern).

If you find some problem setting your carb up let us know.
But I think with this you should be preatty close.

yea haaa

well it’s up and running although i have a few problems

1-idling now ok plug has a good carbon layer on it running 42 jet
2-main jet 132 and a nice brown - grey color so it seems ok
3-timing i’ve fiddled with and moved it, about it’s about 2.5 mm above casing mark

pulls like a train in 1st,2nd, into 3rd and it pulls quick to start off with then has a steady build and goes on forever it seems

change to 4th and it drops off quickly starting to struggle and you can hear in the engine tone it struggles

compression doesn’t seem as good as i remember (10 years +ago) since i had a tuned t5
how stiff should it be roughly mine seems to kick very easily
(i’ve put the head gasket on but i can’t remember if i should use it or not)
i’m think of trying it before i try new rings
the head is machine for the 172 kit

my old t5 ran an upjetted standard carb and this bike flew compared to this one

malossi 172 kit
21t clutch
ported casings
malossi reedvalve
28/30 pwk carb
makuni fuel pump
fat muffler lhs mickek

any help appreciated


ok update

now running 172 kit with no head gasket as i now remembber it didn’t have one to start with

now goes well but still drops off in 4th but pulls a lot better

gonna try the 20t back in it and see how it goes from there

let you now soon


Did you build the motor? Had a customers in once and couldn’t work out why it was doing the same. Turned out the previous owner had decided to upgear with a 65t primary from a P2. Otherwise alittle bit baffling.